Category Archives: Evolution

Human evolution

Human evolution is the lengthy process of change by which people originated from apelike ancestors. Scientific evidence shows that the physical and behavioral traits shared by all people originated from apelike ancestors and evolved over a period of approximately six million years.

One of the earliest defining human traits, bipedalism -- the ability to walk on two legs -- evolved over 4 million years ago. Other important human characteristics -- such as a large and complex brain, the ability to make and use tools, and the capacity for language -- developed more recently. Many advanced traits -- including complex symbolic expression, art, and elaborate cultural diversity -- emerged mainly during the past 100,000 years.

Humans are primates. Physical and genetic similarities show that the modern human species, Homo sapiens, has a very close relationship to another group of primate species, the apes. Humans and the great apes (large apes) of Africa -- chimpanzees (including bonobos, or so-called pygmy chimpanzees) and gorillas -- share a common ancestor that lived between 8 and 6 million years ago. Humans first evolved in Africa, and much of human evolution occurred on that continent. The fossils of early humans who lived between 6 and 2 million years ago come entirely from Africa.

Most scientists currently recognize some 15 to 20 different species of early humans. Scientists do not all agree, however, about how these species are related or which ones simply died out. Many early human species -- certainly the majority of them left no living descendants. Scientists also debate over how to identify and classify particular species of early humans, and about what factors influenced the evolution and extinction of each species.

Early humans first migrated out of Africa into Asia probably between 2 million and 1.8 million years ago. They entered Europe somewhat later, between 1.5 million and 1 million years. Species of modern humans populated many parts of the world much later. For instance, people first came to Australia probably within the past 60,000 years and to the Americas within the past 30,000 years or so. The beginnings of agriculture and the rise of the first civilizations occurred within the past 12,000 years.

Paleoanthropology is the scientific study of human evolution. Paleoanthropology is a subfield of anthropology, the study of human culture, society, and biology. The field involves an understanding of the similarities and differences between humans and other species in their genes, body form, physiology, and behavior. Paleoanthropologists search for the roots of human physical traits and behavior. They seek to discover how evolution has shaped the potentials, tendencies, and limitations of all people. For many people, paleoanthropology is an exciting scientific field because it investigates the origin, over millions of years, of the universal and defining traits of our species. However, some people find the concept of human evolution troubling because it can seem not to fit with religious and other traditional beliefs about how people, other living things, and the world came to be. Nevertheless, many people have come to reconcile their beliefs with the scientific evidence.

Early human fossils and archeological remains offer the most important clues about this ancient past. These remains include bones, tools and any other evidence (such as footprints, evidence of hearths, or butchery marks on animal bones) left by earlier people. Usually, the remains were buried and preserved naturally. They are then found either on the surface (exposed by rain, rivers, and wind erosion) or by digging in the ground. By studying fossilized bones, scientists learn about the physical appearance of earlier humans and how it changed. Bone size, shape, and markings left by muscles tell us how those predecessors moved around, held tools, and how the size of their brains changed over a long time. Archeological evidence refers to the things earlier people made and the places where scientists find them. By studying this type of evidence, archeologists can understand how early humans made and used tools and lived in their environments.

The process of evolution involves a series of natural changes that cause species (populations of different organisms) to arise, adapt to the environment, and become extinct. All species or organisms have originated through the process of biological evolution. In animals that reproduce sexually, including humans, the term species refers to a group whose adult members regularly interbreed, resulting in fertile offspring -- that is, offspring themselves capable of reproducing. Scientists classify each species with a unique, two-part scientific name. In this system, modern humans are classified as Homo sapiens.

Evolution occurs when there is change in the genetic material -- the chemical molecule, DNA -- which is inherited from the parents, and especially in the proportions of different genes in a population. Genes represent the segments of DNA that provide the chemical code for producing proteins. Information contained in the DNA can change by a process known as mutation. The way particular genes are expressed that is, how they influence the body or behavior of an organism -- can also change. Genes affect how the body and behavior of an organism develop during its life, and this is why genetically inherited characteristics can influence the likelihood of an organisms survival and reproduction.

Evolution does not change any single individual. Instead, it changes the inherited means of growth and development that typify a population (a group of individuals of the same species living in a particular habitat). Parents pass adaptive genetic changes to their offspring, and ultimately these changes become common throughout a population. As a result, the offspring inherit those genetic characteristics that enhance their chances of survival and ability to give birth, which may work well until the environment changes. Over time, genetic change can alter a species' overall way of life, such as what it eats, how it grows, and where it can live. Human evolution took place as new genetic variations in early ancestor populations favored new abilities to adapt to environmental change and so altered the human way of life.

Dr. Rick Potts provides a video short introduction to some of the evidence for human evolution, in the form of fossils and artifacts.

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Evolution (2001) - IMDb

Evolution is a scientific theory used by biologists. It explains how living things change over a long time, and how they have come to be the way they are.[1]

The Earth has been around for a very long time.[2][3] By doing research on the layers of rock, we can find out about its past. That kind of research is called historical geology.

We know that living things have changed over time, because we can see their remains in the rocks. These remains are called 'fossils'. So we know that the animals and plants of today are different from those of long ago. And the further we go back, the more different the fossils are.[4] How has this come about? Evolution has taken place. That evolution has taken place is a fact, because it is overwhelmingly supported by many lines of evidence.[5][6][7] At the same time, evolutionary questions are still being actively researched by biologists.

Comparison of DNA sequences allows organisms to be grouped by how similar their sequences are. In 2010 an analysis compared sequences to phylogenetic trees, and supported the idea of common descent. There is now "strong quantitative support, by a formal test",[8] for the unity of life.[9]

The theory of evolution is the basis of modern biology. "Nothing in biology makes sense except in the light of evolution".[10]

The evidence for evolution is given in a number of books.[11][12][13][14] Some of this evidence is discussed here.

The realization that some rocks contain fossils was a landmark in natural history. There are three parts to this story:

The most convincing evidence for the occurrence of evolution is the discovery of extinct organisms in older geological strata... The older the strata are...the more different the fossil will be from living representatives... that is to be expected if the fauna and flora of the earlier strata had gradually evolved into their descendants.

Ernst Mayr [1]p13

The evolution of the horse family (Equidae) is a good example of the way that evolution works. The oldest fossil of a horse is about 52 million years old. It was a small animal with five toes on the front feet and four on the hind feet. At that time, there were more forests in the world than today. This horse lived in woodland, eating leaves, nuts and fruit with its simple teeth. It was only about as big as a fox.[19]

About 30 million years ago the world started to become cooler and drier. Forests shrank; grassland expanded, and horses changed. They ate grass, they grew larger, and they ran faster because they had to escape faster predators. Because grass wears teeth out, horses with longer-lasting teeth had an advantage.

For most of this long period of time, there were a number of horse types (genera). Now, however, only one genus exists: the modern horse, Equus. It has teeth which grow all its life, hooves on single toes, great long legs for running, and the animal is big and strong enough to survive in the open plain.[19] Horses lived in western Canada until 12,000 years ago,[20] but all horses in North America became extinct about 11,000 years ago. The causes of this extinction are not yet clear. Climate change and over-hunting by humans are suggested.

So, scientists can see that changes have happened. They have happened slowly over a long time. How these changes have come about is explained by the theory of evolution.

This is a topic which fascinated both Charles Darwin and Alfred Russel Wallace.[21][22][23] When new species occur, usually by the splitting of older species, this takes place in one place in the world. Once it is established, a new species may spread to some places and not others.

Australasia has been separated from other continents for many millions of years. In the main part of the continent, Australia, 83% of mammals, 89% of reptiles, 90% of fish and insects and 93% of amphibians are endemic.[24] Its native mammals are mostly marsupials like kangaroos, bandicoots, and quolls.[25] By contrast, marsupials are today totally absent from Africa and form a small portion of the mammalian fauna of South America, where opossums, shrew opossums, and the monito del monte occur (see the Great American Interchange).

The only living representatives of primitive egg-laying mammals (monotremes) are the echidnas and the platypus. They are only found in Australasia, which includes Tasmania, New Guinea, and Kangaroo Island. These monotremes are totally absent in the rest of the world.[26] On the other hand, Australia is missing many groups of placental mammals that are common on other continents (carnivora, artiodactyls, shrews, squirrels, lagomorphs), although it does have indigenous bats and rodents, which arrived later.[27]

The evolutionary story is that placental mammals evolved in Eurasia, and wiped out the marsupials and monotremes wherever they spread. They did not reach Australasia until more recently. That is the simple reason why Australia has most of the world's marsupials and all the world's monotremes.

In about 6,500sqmi (17,000km2), the Hawaiian Islands have the most diverse collection of Drosophila flies in the world, living from rainforests to mountain meadows. About 800 Hawaiian drosophilid species are known.

Genetic evidence shows that all the native drosophilid species in Hawaii have descended from a single ancestral species that colonized the islands, about 20 million years ago. The subsequent adaptive radiation was spurred by a lack of competition and a wide variety of vacant niches. Although it would be possible for a single pregnant female to colonise an island, it is more likely to have been a group from the same species.[28][29][30][31]

The combination of continental drift and evolution can explain what is found in the fossil record. Glossopteris is an extinct species of seed fern plants from the Permian period on the ancient supercontinent of Gondwana.[32]

Glossopteris fossils are found in Permian strata in southeast South America, southeast Africa, all of Madagascar, northern India, all of Australia, all of New Zealand, and scattered on the southern and northern edges of Antarctica.

During the Permian, these continents were connected as Gondwana. This is known from magnetic striping in the rocks, other fossil distributions, and glacial scratches pointing away from the temperate climate of the South Pole during the Permian.[13]p103[33]

When biologists look at living things, they see that animals and plants belong to groups which have something in common. Charles Darwin explained that this followed naturally if "we admit the common parentage of allied forms, together with their modification through variation and natural selection".[21]p402[11]p456

For example, all insects are related. They share a basic body plan, whose development is controlled by master regulatory genes.[34] They have six legs; they have hard parts on the outside of the body (an exoskeleton); they have eyes formed of many separate chambers, and so on. Biologists explain this with evolution. All insects are the descendants of a group of animals who lived a long time ago. They still keep the basic plan (six legs and so on) but the details change. They look different now because they changed in different ways: this is evolution.[35]

It was Darwin who first suggested that all life on Earth had a single origin, and from that beginning "endless forms most beautiful and most wonderful have been, and are being, evolved".[11]p490[21] Evidence from molecular biology in recent years has supported the idea that all life is related by common descent.[36]

Strong evidence for common descent comes from vestigial structures.[21]p397 The useless wings of flightless beetles are sealed under fused wing covers. This can be simply explained by their descent from ancestral beetles which had wings that worked.[14]p49

Rudimentary body parts, those that are smaller and simpler in structure than corresponding parts in ancestral species, are called vestigial organs. Those organs are functional in the ancestral species but are now either nonfunctional or re-adapted to a new function. Examples are the pelvic girdles of whales, halteres (hind wings) of flies, wings of flightless birds, and the leaves of some xerophytes (e.g. cactus) and parasitic plants (e.g. dodder).

However, vestigial structures may have their original function replaced with another. For example, the halteres in flies help balance the insect while in flight, and the wings of ostriches are used in mating rituals, and in aggressive display. The ear ossicles in mammals are former bones of the lower jaw.

In 1893, Robert Wiedersheim published a book on human anatomy and its relevance to man's evolutionary history. This book contained a list of 86 human organs that he considered vestigial.[37] This list included examples such as the appendix and the 3rd molar teeth (wisdom teeth).

The strong grip of a baby is another example.[38] It is a vestigial reflex, a remnant of the past when pre-human babies clung to their mothers' hair as the mothers swung through the trees. This is borne out by the babies' feet, which curl up when it is sitting down (primate babies grip with the feet as well). All primates except modern man have thick body hair to which an infant can cling, unlike modern humans. The grasp reflex allows the mother to escape danger by climbing a tree using both hands and feet.[13][39]

Vestigial organs often have some selection against them. The original organs took resources, sometimes huge resources. If they no longer have a function, reducing their size improves fitness. And there is direct evidence of selection. Some cave crustacea reproduce more successfully with smaller eyes than do those with larger eyes. This may be because the nervous tissue dealing with sight now becomes available to handle other sensory input.[40]p310

From the eighteenth century it was known that embryos of different species were much more similar than the adults. In particular, some parts of embryos reflect their evolutionary past. For example, the embryos of land vertebrates develop gill slits like fish embryos. Of course, this is only a temporary stage, which gives rise to many structures in the neck of reptiles, birds and mammals. The proto-gill slits are part of a complicated system of development: that is why they persisted.[34]

Another example are the embryonic teeth of baleen whales.[41] They are later lost. The baleen filter is developed from different tissue, called keratin. Early fossil baleen whales did actually have teeth as well as the baleen.[42]

A good example is the barnacle. It took many centuries before natural historians discovered that barnacles were crustacea. Their adults look so unlike other crustacea, but their larvae are very similar to those of other crustacea.[43]

Charles Darwin lived in a world where animal husbandry and domesticated crops were vitally important. In both cases farmers selected for breeding individuals with special properties, and prevented the breeding of individuals with less desirable characteristics. The eighteenth and early nineteenth century saw a growth in scientific agriculture, and artificial breeding was part of this.

Darwin discussed artificial selection as a model for natural selection in the 1859 first edition of his work On the Origin of Species, in Chapter IV: Natural selection:

Nikolai Vavilov showed that rye, originally a weed, came to be a crop plant by unintentional selection. Rye is a tougher plant than wheat: it survives in harsher conditions. Having become a crop like the wheat, rye was able to become a crop plant in harsh areas, such as hills and mountains.[45][46]

There is no real difference in the genetic processes underlying artificial and natural selection, and the concept of artificial selection was used by Charles Darwin as an illustration of the wider process of natural selection. There are practical differences. Experimental studies of artificial selection show that "the rate of evolution in selection experiments is at least two orders of magnitude (that is 100 times) greater than any rate seen in nature or the fossil record".[47]p157

Some have thought that artificial selection could not produce new species. It now seems that it can.

New species have been created by domesticated animal husbandry, but the details are not known or not clear. For example, domestic sheep were created by hybridisation, and no longer produce viable offspring with Ovis orientalis, one species from which they are descended.[48] Domestic cattle, on the other hand, can be considered the same species as several varieties of wild ox, gaur, yak, etc., as they readily produce fertile offspring with them.[49]

The best-documented new species came from laboratory experiments in the late 1980s. William Rice and G.W. Salt bred fruit flies, Drosophila melanogaster, using a maze with three different choices of habitat such as light/dark and wet/dry. Each generation was put into the maze, and the groups of flies that came out of two of the eight exits were set apart to breed with each other in their respective groups.

After thirty-five generations, the two groups and their offspring were isolated reproductively because of their strong habitat preferences: they mated only within the areas they preferred, and so did not mate with flies that preferred the other areas.[50][51]

Diane Dodd was also able to show how reproductive isolation can develop from mating preferences in Drosophila pseudoobscura fruit flies after only eight generations using different food types, starch and maltose.[52]

Dodd's experiment has been easy for others to repeat. It has also been done with other fruit flies and foods.[53]

Some biologists say that evolution has happened when a trait that is caused by genetics becomes more or less common in a group of organisms.[54] Others call it evolution when new species appear.

Changes can happen quickly in the smaller, simpler organisms. For example, many bacteria that cause disease can no longer be killed with some of the antibiotic medicines. These medicines have only been in use about eighty years, and at first worked extremely well. The bacteria have evolved so that they are no longer affected by antibiotics anymore.[55] The drugs killed off all the bacteria except a few which had some resistance. These few resistant bacteria produced the next generation.

The Colorado beetle is famous for its ability to resist pesticides. Over the last 50 years it has become resistant to 52 chemical compounds used in insecticides, including cyanide.[56] This is natural selection speeded up by the artificial conditions. However, not every population is resistant to every chemical.[57] The populations only become resistant to chemicals used in their area.

Although there were a number of natural historians in the 18th century who had some idea of evolution, the first well-formed ideas came in the 19th century. Three biologists are most important.

Jean-Baptiste de Lamarck (17441829), a French biologist, claimed that animals changed according to natural laws. He said that animals could pass on traits they had acquired during their lifetime to their offspring, using inheritance. Today, his theory is known as Lamarckism. Its main purpose is to explain adaptations by natural means.[58] He proposed a tendency for organisms to become more complex, moving up a ladder of progress, plus use and disuse.

Lamarck's idea was that a giraffe's neck grew longer because it tried to reach higher up. This idea failed because it cannot be reconciled with heredity (Mendel's work). Mendel made his discoveries about half a century after Lamarck's work.

Charles Darwin (18091882) wrote his On the Origin of Species in 1859. In this book, he put forward much evidence that evolution had occurred. He also proposed natural selection as the way evolution had taken place. But Darwin did not understand about genetics and how traits were actually passed on. He could not accurately explain what made children look like their parents.

Nevertheless, Darwin's explanation of evolution was fundamentally correct. In contrast to Lamarck, Darwin's idea was that the giraffe's neck became longer because those with longer necks survived better.[21]p177/8 These survivors passed their genes on, and in time the whole race got longer necks.

An Austrian monk called Gregor Mendel (18221884) bred plants. In the mid-19th century, he discovered how traits were passed on from one generation to the next.

He used peas for his experiments: some peas have white flowers and others have red ones. Some peas have green seeds and others have yellow seeds. Mendel used artificial pollination to breed the peas. His results are discussed further in Mendelian inheritance. Darwin thought that the inheritance from both parents blended together. Mendel proved that the genes from the two parents stay separate, and may be passed on unchanged to later generations.

Mendel published his results in a journal that was not well-known, and his discoveries were overlooked. Around 1900, his work was rediscovered.[59][60]Genes are bits of information made of DNA which work like a set of instructions. A set of genes are in every living cell. Together, genes organise the way an egg develops into an adult. With mammals, and many other living things, a copy of each gene comes from the father and another copy from the mother. Some living organisms, including some plants, only have one parent, so get all their genes from them. These genes produce the genetic differences which evolution acts on.

Darwin's On the Origin of Species has two themes: the evidence for evolution, and his ideas on how evolution took place. This section deals with the second issue.

The first two chapters of the Origin deal with variation in domesticated plants and animals, and variation in nature.

All living things show variation. Every population which has been studied shows that animal and plants vary as much as humans do.[61][62]p90 This is a great fact of nature, and without it evolution would not occur. Darwin said that, just as man selects what he wants in his farm animals, so in nature the variations allow natural selection to work.[63]

The features of an individual are influenced by two things, heredity and environment. First, development is controlled by genes inherited from the parents. Second, living brings its own influences. Some things are entirely inherited, others partly, and some not inherited at all.

The colour of eyes is entirely inherited; they are a genetic trait. Height or weight is only partly inherited, and the language is not at all inherited. Just to be clear: the fact that humans can speak is inherited, but what language is spoken depends on where a person lives and what they are taught. Another example: a person inherits a brain of somewhat variable capacity. What happens after birth depends on many things such as home environment, education and other experiences. When a person is adult, their brain is what their inheritance and life experience have made it.

Evolution only concerns the traits which can be inherited, wholly or partly. The hereditary traits are passed on from one generation to the next through the genes. A person's genes contain all the traits which they inherit from their parents. The accidents of life are not passed on. Also, of course, each person lives a somewhat different life: that increases the differences.

Organisms in any population vary in reproductive success.[64]p81 From the point of view of evolution, 'reproductive success' means the total number of offspring which live to breed and leave offspring themselves.

Variation can only affect future generations if it is inherited. Because of the work of Gregor Mendel, we know that much variation is inherited. Mendel's 'factors' are now called genes. Research has shown that almost every individual in a sexually reproducing species is genetically unique.[65]p204

Genetic variation is increased by gene mutations. DNA does not always reproduce exactly. Rare changes occur, and these changes can be inherited. Many changes in DNA cause faults; some are neutral or even advantageous. This gives rise to genetic variation, which is the seed-corn of evolution. Sexual reproduction, by the crossing over of chromosomes during meiosis, spreads variation through the population. Other events, like natural selection and drift, reduce variation. So a population in the wild always has variation, but the details are always changing.[62]p90

Evolution mainly works by natural selection. What does this mean? Animals and plants which are best suited to their environment will, on average, survive better. There is a struggle for existence. Those who survive will produce the next generation. Their genes will be passed on, and the genes of those who did not reproduce will not. This is the basic mechanism which changes a population and causes evolution.

Natural selection explains why living organisms change over time to have the anatomy, the functions and behaviour that they have. It works like this:

There are now many cases where natural selection has been proved to occur in wild populations.[5][67][68] Almost every case investigated of camouflage, mimicry and polymorphism has shown strong effects of selection.[69]

The force of selection can be much stronger than was thought by the early population geneticists. The resistance to pesticides has grown quickly. Resistance to warfarin in Norway rats (Rattus norvegicus) grew rapidly because those that survived made up more and more of the population. Research showed that, in the absence of warfarin, the resistant homozygote was at a 54% disadvantage to the normal wild type homozygote.[62]p182[70] This great disadvantage was quickly overcome by the selection for warfarin resistance.

Mammals normally cannot drink milk as adults, but humans are an exception. Milk is digested by the enzyme lactase, which switches off as mammals stop taking milk from their mothers. The human ability to drink milk during adult life is supported by a lactase mutation which prevents this switch-off. Human populations have a high proportion of this mutation wherever milk is important in the diet. The spread of this 'milk tolerance' is promoted by natural selection, because it helps people survive where milk is available. Genetic studies suggest that the oldest mutations causing lactase persistence only reached high levels in human populations in the last ten thousand years.[71][72] Therefore, lactase persistence is often cited as an example of recent human evolution.[73][74] As lactase persistence is genetic, but animal husbandry a cultural trait, this is geneculture coevolution.[75]

Adaptation is one of the basic phenomena of biology.[76] Through the process of adaptation, an organism becomes better suited to its habitat.[77]

Adaptation is one of the two main processes that explain the diverse species we see in biology. The other is speciation (species-splitting or cladogenesis).[78][79] A favourite example used today to study the interplay of adaptation and speciation is the evolution of cichlid fish in African rivers and lakes.[80][81]

When people speak about adaptation they often mean something which helps an animal or plant survive. One of the most widespread adaptations in animals is the evolution of the eye. Another example is the adaptation of horses' teeth to grinding grass. Camouflage is another adaptation; so is mimicry. The better adapted animals are the most likely to survive, and to reproduce successfully (natural selection).

An internal parasite (such as a fluke) is a good example: it has a very simple bodily structure, but still the organism is highly adapted to its particular environment. From this we see that adaptation is not just a matter of visible traits: in such parasites critical adaptations take place in the life cycle, which is often quite complex.[82]

Not all features of an organism are adaptations.[62]p251 Adaptations tend to reflect the past life of a species. If a species has recently changed its life style, a once valuable adaptation may become useless, and eventually become a dwindling vestige.

Adaptations are never perfect. There are always tradeoffs between the various functions and structures in a body. It is the organism as a whole which lives and reproduces, therefore it is the complete set of adaptations which gets passed on to future generations.

In populations, there are forces which add variation to the population (such as mutation), and forces which remove it. Genetic drift is the name given to random changes which remove variation from a population. Genetic drift gets rid of variation at the rate of 1/(2N) where N = population size.[47]p29 It is therefore "a very weak evolutionary force in large populations".[47]p55

Genetic drift explains how random chance can affect evolution in surprisingly big ways, but only when populations are quite small. Overall, its action is to make the individuals more similar to each other, and hence more vulnerable to disease or to chance events in their environment.

How species form is a major part of evolutionary biology. Darwin interpreted 'evolution' (a word he did not use at first) as being about speciation. That is why he called his famous book On the Origin of Species.

Darwin thought most species arose directly from pre-existing species. This is called anagenesis: new species by older species changing. Now we think most species arise by previous species splitting: cladogenesis.[87][88]

Two groups that start the same can also become very different if they live in different places. When a species gets split into two geographical regions, a process starts. Each adapts to its own situation. After a while, individuals from one group can no longer reproduce with the other group. Two good species have evolved from one.

A German explorer, Moritz Wagner, during his three years in Algeria in the 1830s, studied flightless beetles. Each species is confined to a stretch of the north coast between rivers which descend from the Atlas mountains to the Mediterranean. As soon as one crosses a river, a different but closely related species appears.[89] He wrote later:

This was an early account of the importance of geographical separation. Another biologist who thought geographical separation was critical was Ernst Mayr.[91]

One example of natural speciation is the three-spined stickleback, a sea fish that, after the last ice age, invaded freshwater, and set up colonies in isolated lakes and streams. Over about 10,000 generations, the sticklebacks show great differences, including variations in fins, changes in the number or size of their bony plates, variable jaw structure, and color differences.[92]

The wombats of Australia fall into two main groups, Common wombats and Hairy-nosed wombats. The two types look very similar, apart from the hairiness of their noses. However, they are adapted to different environments. Common wombats live in forested areas and eat mostly green food with lots of moisture. They often feed in the daytime. Hairy-nosed wombats live on hot dry plains where they eat dry grass with very little water or goodness in it. Their metabolic system is slow and they sleep most of the day underground.

When two groups that started the same become different enough, then they become two different species. Part of the theory of evolution is that all living things started off the same, but then split off into different groups over billions of years.[93]

This was an important movement in evolutionary biology, which started in the 1930s and finished in the 1950s.[94][95] It has been updated regularly ever since. The synthesis explains how the ideas of Charles Darwin fit with the discoveries of Gregor Mendel, who found out how we inherit our genes. The modern synthesis brought Darwin's idea up to date. It bridged the gap between different types of biologists: geneticists, naturalists, and palaeontologists.

When the theory of evolution was developed, it was not clear that natural selection and genetics worked together. But Ronald Fisher showed that natural selection would work to change species.[96]Sewall Wright explained genetic drift in 1931.[97]

Co-evolution is where the existence of one species is tightly bound up with the life of one or more other species.

New or 'improved' adaptations which occur in one species are often followed by the appearance and spread of related features in the other species. The life and death of living things is intimately connected, not just with the physical environment, but with the life of other species.

These relationships may continue for millions of years, as it has in the pollination of flowering plants by insects. The gut contents, wing structures, and mouthparts of fossilized beetles and flies suggest that they acted as early pollinators. The association between beetles and angiosperms during the Lower Cretaceous period led to parallel radiations of angiosperms and insects into the late Cretaceous. The evolution of nectaries in Upper Cretaceous flowers signals the beginning of the mutualism between hymenoptera and angiosperms.[102]

Charles Darwin was the first to use this metaphor in biology. The evolutionary tree shows the relationships among various biological groups. It includes data from DNA, RNA and protein analysis. Tree of life work is a product of traditional comparative anatomy, and modern molecular evolution and molecular clock research.

The major figure in this work is Carl Woese, who defined the Archaea, the third domain (or kingdom) of life.[103] Below is a simplified version of present-day understanding.[104]

Macroevolution: the study of changes above the species level, and how they take place. The basic data for such a study are fossils (palaeontology) and the reconstruction of ancient environments. Some subjects whose study falls within the realm of macroevolution:

It is a term of convenience: for most biologists it does not suggest any change in the process of evolution.[5][105][106]p87 For some palaeontologists, what they see in the fossil record cannot be explained just by the gradualist evolutionary synthesis.[107] They are in the minority.

Altruism the willingness of some to sacrifice themselves for others is widespread in social animals. As explained above, the next generation can only come from those who survive and reproduce. Some biologists have thought that this meant altruism could not evolve by the normal process of selection. Instead a process called "group selection" was proposed.[108][109] Group selection refers to the idea that alleles can become fixed or spread in a population because of the benefits they bestow on groups, regardless of the alleles' effect on the fitness of individuals within that group.

For several decades, critiques cast serious doubt on group selection as a major mechanism of evolution.[110][111][112][113]

In simple cases it can be seen at once that traditional selection suffices. For example, if one sibling sacrifices itself for three siblings, the genetic disposition for the act will be increased. This is because siblings share on average 50% of their genetic inheritance, and the sacrificial act has led to greater representation of the genes in the next generation.

Altruism is now generally seen as emerging from standard selection.[114][115][116][117][118] The warning note from Ernst Mayr, and the work of William Hamilton are both important to this discussion.[119][120]

Hamilton's equation describes whether or not a gene for altruistic behaviour will spread in a population. The gene will spread if rxb is greater than c:

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Evolution - Simple English Wikipedia, the free encyclopedia

You are here: Science >> Darwin's Theory Of Evolution

Darwin's Theory of Evolution - The Premise Darwin's Theory of Evolution is the widely held notion that all life is related and has descended from a common ancestor: the birds and the bananas, the fishes and the flowers -- all related. Darwin's general theory presumes the development of life from non-life and stresses a purely naturalistic (undirected) "descent with modification". That is, complex creatures evolve from more simplistic ancestors naturally over time. In a nutshell, as random genetic mutations occur within an organism's genetic code, the beneficial mutations are preserved because they aid survival -- a process known as "natural selection." These beneficial mutations are passed on to the next generation. Over time, beneficial mutations accumulate and the result is an entirely different organism (not just a variation of the original, but an entirely different creature).

Darwin's Theory of Evolution - Natural Selection While Darwin's Theory of Evolution is a relatively young archetype, the evolutionary worldview itself is as old as antiquity. Ancient Greek philosophers such as Anaximander postulated the development of life from non-life and the evolutionary descent of man from animal. Charles Darwin simply brought something new to the old philosophy -- a plausible mechanism called "natural selection." Natural selection acts to preserve and accumulate minor advantageous genetic mutations. Suppose a member of a species developed a functional advantage (it grew wings and learned to fly). Its offspring would inherit that advantage and pass it on to their offspring. The inferior (disadvantaged) members of the same species would gradually die out, leaving only the superior (advantaged) members of the species. Natural selection is the preservation of a functional advantage that enables a species to compete better in the wild. Natural selection is the naturalistic equivalent to domestic breeding. Over the centuries, human breeders have produced dramatic changes in domestic animal populations by selecting individuals to breed. Breeders eliminate undesirable traits gradually over time. Similarly, natural selection eliminates inferior species gradually over time.

Darwin's Theory of Evolution - Slowly But Surely... Darwin's Theory of Evolution is a slow gradual process. Darwin wrote, "Natural selection acts only by taking advantage of slight successive variations; she can never take a great and sudden leap, but must advance by short and sure, though slow steps." [1] Thus, Darwin conceded that, "If it could be demonstrated that any complex organ existed, which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down." [2] Such a complex organ would be known as an "irreducibly complex system". An irreducibly complex system is one composed of multiple parts, all of which are necessary for the system to function. If even one part is missing, the entire system will fail to function. Every individual part is integral. [3] Thus, such a system could not have evolved slowly, piece by piece. The common mousetrap is an everyday non-biological example of irreducible complexity. It is composed of five basic parts: a catch (to hold the bait), a powerful spring, a thin rod called "the hammer," a holding bar to secure the hammer in place, and a platform to mount the trap. If any one of these parts is missing, the mechanism will not work. Each individual part is integral. The mousetrap is irreducibly complex. [4]

Darwin's Theory of Evolution - A Theory In Crisis Darwin's Theory of Evolution is a theory in crisis in light of the tremendous advances we've made in molecular biology, biochemistry and genetics over the past fifty years. We now know that there are in fact tens of thousands of irreducibly complex systems on the cellular level. Specified complexity pervades the microscopic biological world. Molecular biologist Michael Denton wrote, "Although the tiniest bacterial cells are incredibly small, weighing less than 10-12 grams, each is in effect a veritable micro-miniaturized factory containing thousands of exquisitely designed pieces of intricate molecular machinery, made up altogether of one hundred thousand million atoms, far more complicated than any machinery built by man and absolutely without parallel in the non-living world." [5]

And we don't need a microscope to observe irreducible complexity. The eye, the ear and the heart are all examples of irreducible complexity, though they were not recognized as such in Darwin's day. Nevertheless, Darwin confessed, "To suppose that the eye with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest degree." [6]

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Darwin's Theory Of Evolution

The word "evolution" in its broadest sense refers to change or growth that occurs in a particular order. Although this broad version of the term would include astronomical evolution and the evolution of computer design, this article focuses on the evolution of biological organisms. That use of the term dates back to the ancient Greeks, but today the word is more often used to refer to Darwin's theory of evolution by natural selection. This theory is sometimes crudely referred to as the theory of "survival of the fittest." It was proposed by CharlesDarwin in On the Origin of Species in 1859 and, independently, by Alfred Wallace in 1858although Wallace, unlike Darwin, said the human soul is not the product of evolution.

Greek and medieval references to "evolution" use it as a descriptive term for a state of nature, in which everything in nature has a certain order or purpose. This is a teleological view of nature. For example, Aristotle classified all living organisms hierarchically in his great scala naturae or Great Chain of Being, with plants at the bottom, moving through lesser animals, and on to humans at the pinnacle of creation, each becoming progressively more perfect in form. It was the medieval philosophers, such as Augustine, who began to incorporate teleological views of nature with religion: God is the designer of all creatures, and everything has a purpose and a place as ordained by Him.

In current times, to some, the terms "evolution" and "God" may look like unlikely bed fellows (see the discussion on teleology). This is due primarily to today's rejection by biologists of a teleological view of evolution in favor of a more mechanistic one. The process of rejection is commonly considered to have begun with Descartes and to have culminated in Darwins theory of evolution by natural selection.

Fundamental to natural selection is the idea of change by common descent. This implies that all living organisms are related to each other; for any two species, if we look back far enough we will find that they are descended from a common ancestor. This is a radically different view than Aristotles Great Chain of Being, in which each species is formed individually with its own purpose and place in nature and where no species evolves into a new species. Evolution by natural selection is a purely mechanistic theory of change that does not appeal to any sense of purpose or a designer. There is no foresight or purpose in nature, and there is no implication that one species is more perfect than another. There is only change driven by selection pressures from the environment. Although the modern theory of biological evolution by natural selection is well accepted among professional biologists, there is still controversy about whether natural selection selects for fit genes or fit organisms or fit species.

Evolution by natural selection is a theory about the process of change. Although Darwin's original theory did not specify that genes account for an organism's heritable traits, that is now universally accepted among modern evolutionists. In a given population, natural selection occurs when genetically-based traits that promote survival in one's environment are passed onto future generations and become more frequent in later generations. Organisms develop different survival and reproduction enhancing traits in response to their different environments (with abundance or shortage of food, presence or absence of predators, and so forth) and, given enough time and environmental changes, these small changes can accumulate to form a whole new species. Thus for Darwin there is no sharp distinction between a new variation and a new species. This theory accounts for the diversity of Earth's organisms better than theological design theories or competing scientific theories such as Lamarck's theory that an organism can pass on to its offspringcharacteristics that it acquired during its lifetime.

Evolution by natural selection works on three principles: variation (within a given generation there will be variation in traits, some that aid survival and reproduction and some that dont, and some that have a genetic basis and some that dont); competition (there will be limited resources that individuals must compete for, and traits that aid survival and reproduction will help in competition); and heritability (only traits that aid survival and reproduction and have a genetic basis can passed onto future generations).

Evolution is not so much a modern discovery as some of its advocates would have us believe. It made its appearance early in Greek philosophy, and maintained its position more or less, with the most diverse modifications, and frequently confused with the idea of emanation, until the close of ancient thought. The Greeks had, it is true, no term exactly equivalent to " evolution"; but when Thales asserts that all things originated from water; when Anaximenes calls air the principle of all things, regarding the subsequent process as a thinning or thickening, they must have considered individual beings and the phenomenal world as, a result of evolution, even if they did not carry the process out in detail. Anaximander is often regarded as a precursor of the modem theory of development. He deduces living beings, in a gradual development, from moisture under the influence of warmth, and suggests the view that men originated from animals of another sort, since if they had come into existence as human beings, needing fostering care for a long time, they would not have been able to maintain their existence. In Empedocles, as in Epicurus and Lucretius, who follow in Hs footsteps, there are rudimentary suggestions of the Darwinian theory in its broader sense; and here too, as with Darwin, the mechanical principle comes in; the process is adapted to a certain end by a sort of natural selection, without regarding nature as deliberately forming its results for these ends.

If the mechanical view is to be found in these philosophers, the teleological occurs in Heraclitus, who conceives the process as a rational development, in accordance with the Logos and names steps of the process, as from igneous air to water, and thence to earth. The Stoics followed Heraclitus in the main lines of their physics. The primal principle is, as with him, igneous air. only that this is named God by them with much greater definiteness. The Godhead has life in itself, and develops into the universe, differentiating primarily into two kinds of elements the finer or active, and the coarser or passive. Formation or development goes on continuously, under the impulse of the formative principle, by whatever name it is known, until all is once more dissolved by the ekpyrosis into the fundamental principle, and the whole process begins over again. Their conception of the process as analogous to the development of the seed finds special expression in their term of logos spermatikos. In one point the Stoics differ essentially from Heraclitus. With them the whole process is accomplished according to certain ends indwelling in the Godhead, which is a provident, careful intelligence, while no providence is assumed in Heraclitus.

Empedocles asserts definitely that the sphairos, as the full reconciliation of opposites, is opposed, as the superior, to the individual beings brought into existence by hatred, which are then once more united by love to the primal essence, the interchange of world-periods thus continuing indefinitely. Development is to be found also in the atomistic philosopher Democritus; in a purely mechanical manner without any purpose, bodies come into existence out of atoms, and
ultimately entire worlds appear and disappear from and to eternity. Like his predecessors, Deinocritus, deduces organic beings from what is inorganic-moist earth or slime.

Development, as well as the process of becoming, in general, was denied by the Eleatic philosophers. Their doctrine, diametrically opposed to the older thoroughgoing evolutionism, had its influence in determining the acceptance of unchangeable ideas, or forms, by Plato and Aristotle. Though Plato reproduces the doctrine of Heraclitus as to the flux of all things in the phenomenal world, he denies any continuous change in the world of ideas. Change is permanent only in so far as the eternal forms stamp themselves upon individual objects. Though this, as a rule, takes place but imperfectly, the stubborn mass is so far affected that all works out as far as possible for the best. The demiurge willed that all should become as far as possible like himself; and so the world finally becomes beautiful and perfect. Here we have a development, though the principle which has the most real existence does not change; the forms, or archetypal ideas, remain eternally what they are.

In Aristotle also the forms are the real existences, working in matter but eternally remaining the same, at once the motive cause and the effectual end of all things. Here the idea of evolution is clearer than in Plato, especially for the physical world, which is wholly dominated by purpose. The transition from lifeless to living matter is a gradual one, so that the dividing-line between them is scarcely perceptible. Next to lifeless matter comes the vegetable kingdom, which seems, compared with the inorganic, to have life, but appears lifeless compared with the organic. The transition from plants to animals is again a gradual one. The lowest organisms originate from the primeval slime, or from animal differentiation; there is a continual progression from simple, undeveloped types to the higher and more perfect. As the highest stage, the end and aim of the whole process, man appears; all lower forms are merely unsuccessful attempts to produce him. The ape is a transitional stage between man and other viviparous animals. If development has so important a work in Aristotle's physics, it is not less important in his metaphysics. The whole transition from potentiality to actuality (from dynamis toentelecheia) is nothing but a transition from the lower to the higher, everything striving to assimilate itself to the absolutely perfect, to the Divine. Thus Aristotle, like Plato, regards the entire order of the universe as a sort of deification. But the part played in the development by the Godhead, the absolutely immaterial form, is less than that of the forms which operate in matter, since, being already everything,, it is incapable of becoming anything else. Thus Aristotle, despite his evolutionistic notions, does not take the view of a thoroughgoing evolutionist as regards the universe; nor do the Neoplatonists, whose highest principle remains wholly unchanged, though all things emanate from it.

The idea of evolution was not particularly dominant in patristic and scholastic theology and philosophy, both on account of the dualism which runs through them as an echo of Plato and Aristotle, and on account of the generally accepted Christian theory of creation. However, evolution is not generally denied; and with Augustine (De civitate dei, xv. 1) it is taken as the basis for a philosophy of history. Erigena and some of his followers seem to teach a sort of evolution. The issue of finite beings from God is called analysis or resolution in contrast to the reverse or deification the return to God, who once more assimilates all things. God himself, although denominated the beginning, middle, and end, all in all remains unmixed in his own essence, transcendent though immanent in the world. The teaching of. Nicholas of Cusa is similar to Erigena's, though a certain amount of Pythagoreanism comes in here. The world exhibits explicitly what the Godhead implicitly contains; the world is an animated, ordered whole, in which God is everywhere present. Since God embraces all things in himself, he unites all opposites: he is the complicatio omnium contradictoriorum. The idea of evolution thus appears in Nicholas in a rather pantheistic form, but it is not developed.

In spite of some obscurities in his conception of the world Giordano Bruno is a little clearer. According to him God is the immanent first cause in the universe; there is no difference between matter and form; matter, which includes in itself forms and ends, is the source of all becoming and of all actuality. The infinite ether which fills infinite space conceals within itself the nucleus of all things, and they proceed from it according to determinate laws, yet in a teleological manner. Thus the worlds originate not by an arbitrary act, but by an inner necessity of the divine nature. They are natura naturata, as distinguished from the operative nature of God, natitra naturans, which is present in all thin-S as the being- of all that is, the beauty of all that is fair. As in the Stoic teaching, with which Bruno's philosophy has much in common, the conception of evolution comes out clearly both for physics and metaphysics.

Leibniz attempted to reconcile the mechanical-physical and the teleological views, after Descartes, in his Principia philosophitce, excluding all purpose, had explained nature both lifeless and living, as mere mechanism. It is right, however, to point out that Descartes had a metaphysics above his physics, in which the conception of God took an important place, and that thus the mechanical notion of evolution did not really include everything. In Leibnitz the principles of mechanics and physics are dependent upon the direction of a supreme intelligence, without which they would be inexplicable to us. Only by such a preliminary assumption are we able to recognize that one ordered thing follows upon another continuously. It is in this sense that the law of continuity is to be understood, which is of such great importance in Leibnitz. At bottom it is the same as the law of ordered development. The genera of all beings follow continuously one upon another, and between the main classes, as between animals and vegetables, there must be a continuous sequence of intermediate beings. Here again, however, evolution is not taught in its most thorough form, since the divine monad, of God, does not come into the world but transcends it.

Among the German philosophers of the eighteenth century Herder must be mentioned first of the pioneers of modern evolutionism. He lays down the doctrine of a continuous development in the unity of nature from inorganic to organic, from the stone to the plant, from the plant to the animal, and from the animal to man. As nature develops according to fixed laws and natural conditions, so does history, which is only a continuation of the process of nature. Both nature and history labor to educate man in perfect humanity; but as this is seldom attained, a future life is suggested. Lessing had dwelt on the education of the human race as a development to the higher and more perfect. It is only recently that the significance of Herder, in regard to the conception and treatment of historic development, has been adequately recognized. Goethe also followed out the idea of evolution in his zoological and botanical investigations, with his theory of the metamorphosis of plants and his endeavor to discover unity in different organisms.

Kant is also often mentioned as having been an early teacher of the modern theory of descent. It is true he considers the analogy of the forms which he fin
ds in various classes of organisms a ground for supposing that they may have come originally from a common source. He calls the hypothesis that specifically different being have originated one from the other "a daring adventure of the reason." But he entertains the thought that in a later epoch "an orang-outang or a chimpanzee may develop the organs which serve for walking, grasping objects, and speaking-in short, that lie may evolve the structure of man, with an organ for the use of reason, which shall gradually develop itself by social culture." Here, indeed, important ideas of Darwin were anticipated; but Kant's critical system was such that development could have no predominant place in it.

The idea of evolution came out more strongly in his German idealistic successors, especially in Schelling, who regarded nature as a preliminary stage to mind, and the process of physical development as continuing in history. The unconscious productions of nature are only unsuccessful attempts to reflect itself; lifeless nature is an immature intelligence, so that in its phenomena an intelligent character appears only unconsciously. Its highest aim, that, of becoming an object to itself, is only attained in the highest and last reflection-in man, or in what we call reason, through which for the first time nature returns perfectly upon itself. All stages of nature are connected by a common life, and show in their development a conclusive unity. The course of history as a whole must be conceived as offering a gradually progressive revelation of the Absolute. For this he names three periods-that of fate, that of nature, and that of providence, of which we are now in the second. Schelling's followers carried the idea of development somewhat further than their master. This is true especially of Oken, who conceives natural science as the science of the eternal transformation of God into the world, of the dissolution of the Absolute into plurality, and of its continuous further operation in this plurality. The development is continued through the vegetable and animal kingdoms up to man, who in his art and science and polity completely establishes the will of nature. Oken, it is true, conceived man as the sole object of all animal development, so that the lower stages are only abortive attempts to produce him-a theory afterward controverted by Ernst von Baer and Cuvier, the former of whom, standing somewhat in opposition to Darwin, is of great interest to the student of the history of the theory of evolution.

Some evolutionistic ideas are found in Krause and Schleiermacher; but Hegel, with his absolute idealism, is a more notable representative of them. In his system philosophy is the science of the Absolute, of the absolute reason developing or unfolding itself. Reason develops itself first in the abstract element of thought, then expresses itself externally in nature, and finally returns from this externalization into itself in mind. As Heraclitus had taught eternal becoming, so Hegel, who avowedly accepted all the propositions of the Ephesian philosopher in his logic, taught eternal proceeding. The difference between the Greek and the German was that the former believed in the flux of matter, of fire transmuting itself by degrees into all things, and in nature as the sole existence, outside of which there was nothing; while the latter conceived the abstract idea or reason as that which really is or becomes, and nature as only a necessary but transient phase in the process of development. With Heraclitus evolution meant the return of all things into the primal principle followed by a new world-development; with Hegel it was an eternal process of thought, giving no answer to the question as to the end of historical development.

While Heraclitus had laid down his doctrine of eternal becoming rather by intuition than on the ground of experience, and the entire evolutionary process of Hegel had been expressly conceived as based on pure thought, Darwin's and Wallace's epoch-making doctrine rested upon a vast mass of ascertained facts. He was, of course, not the first to lay down the origin of species one from another as a formal doctrine. Besides those predecessors of his to whom allusion has already been made, two others may be mentioned here: his grandfather, Erasmus Darwin, who emphasized organic variability; and still more Lamarck, who denied the immutability of species and forms, and claimed to have demonstrated by observation the gradual development of the animal kingdom. What is new in Charles Darwin is not his theory of descent, but its confirmation by the theory of natural selection and the survival of the fittest in the struggle for existence. Thus a result is brought about which corresponds as far as possible to a rational end in a purely mechanical process, without any cooperation of teleological principles, without any innate tendency in the organisms to proceed to a higher stage. This theory postulates in the later organisms deviations from the earlier ones; and that these deviations, in so far as they are improvements, perpetuate themselves and become generic marks of differentiation. This, however, imports a difficulty, since the origin of the first of these deviations is inexplicable. The differentia of mankind, whom Darwin, led by the force of analogy, deduces from a species of apes, consists in intellect and moral qualities, but comes into existence only by degrees. The moral sensibilities develop from the original social impulse innate in man; this impulse is an effort to secure not so much individual happiness as the general welfare.

It would be impossible to name here all those who, in different countries, have followed in Darwin's footsteps, first in the biological field and then in those of psychology, ethics, sociology, and religion. They have carried his teaching further in several directions, modifying it to some extent and making it fruitful, while positivism has not seldom come into alliance with it. In Germany Ernst Haeckel must be mentioned with his biogenetic law, according to which the development of the individual is an epitome of the history of the race, and with his less securely grounded notion of the world-ether as a creative deity. In France Alfred Fouillee worked out a theory of idea-forces, a combination of Platonic idealism with English (though not specifically Darwinian) evolutionism. Marie-Jean Guyau understood by evolution a life led according to the fundamental law that the most intensive life is also the most extensive. He develops his ethics altogether from the facts of the social existence of mankind, and his religion is a universal sociomorphism, the feeling of the unity of man with the entire cosmos.

The most careful and thorough development of the whole system took place in England. For a long time it was represented principally by the work of Herbert Spencer, who had come out for the principle of evolution even before the publication of Darwin's Origin of Species. He carries the idea through the whole range of philosophy in his great System of Synthetic Philosophy and undertakes to show that development is the highest law of all nature, not merely of the organic. As the foundation of ill that exists, though itself unknowable and only revealing itself in material and mental forms, he places a power, the Absolute, of which we have but an indefinite conception. The individual processes of the world of phenomena are classed under the head of evolution, or extension of movement, with which integration of matter, union into a single whole, is connected, and dissolution or absorption of movement, which includes disintegration of matter, the breaking of connection. Both processes go on
simultaneously, and include the history of every existence which we can perceive. In the course of their development the organisms incorporate matter with themselves; the plant grows by taking into itself elements which have previously existed in the form of gases, and the animal by assimilating elements found in plants and in other animals. The same sort of integration is observed in social organisms, as when nomadic families unite into a tribe, or subjects under a prince, and princes under a king. In like manner integration is evident in the development of language, of art, and of science, especially philosophy. But as the individuals unite into a whole, a strongly marked differentiation goes on at the same time, as in the distinction between the surface and the interior of the earth, or between various climates. Natural selection is not considered necessary to account for varying species, but gradual conditions of life create them. The aim of the development is to show a condition of perfect balance in the whole; when this is attained, the development, in virtue of the continuous operation of external powers, passes into dissolution. Those epochs of development and of dissolution follow alternately upon each other. This view of Spencer suggests the hodos ano and hodos kato of Heraclitus, and his flowing back of individual things into the primal principle.

Similar principles are carried out not only for organic phenomena but also for mental and social; and on the basis of the theory of evolution a remarkable combination of intuitionism and empiricism is achieved. In his principles of sociology Spencer lays down the laws of hyperorganic evolution, and gives the various stages of human customs and especially of religious ideas, deducing all religion much too one-sidedly from ancestor-worship. The belief in an immortal " second self " is explained by such phenomena as shadows and echoes. The notion of gods is suppose to arise from the idea of a ghostly life after death. In his Principles of Ethics he attempts a similar compromise between intuitionism and empiricism, deducing the consciousness of duty from innumerable accumulated experiences. The compelling element in moral actions, originally arising from fear of religious, civil, or social punishment, disappears with the development of true morality. There is no permanent opposition between egoism and altruism, but the latter develops simultaneously with the former.

Spencer's ethical principles were fruitfully modified, especially by Sir Leslie Stephen and S. Alexander, though with constant adherence to the idea of development. While the doctrine of evolution in Huxley and Tyndall is associated with agnosticism, and thus freed from all connection with metaphysics, as indeed was the case with Spencer, in spite of his recognition of the Absolute as the necessary basis for religion and for thought, in another direction an attempt was made to combine evolutionism closely with a metaphysics in which the idea of God was prominent. Thus the evolution theory of Clifford and Romanes led them to a thoroughgoing monism, and that of J. M. F. Schiller to pluralism. According to the last-named a personal deity, limited in power, exists side by side with a multitude of intellectual beings, who existed before the formation of the world in a chaotic state as absolutely isolated individuals. The process of world formation begins with the decision of the divine Spirit to bring a harmony of the cosmos out of these many existences. Though Spencer's influence in philosophical development was not so great in Germany as in England, the idea of development has continued in recent years to exert no little power. Space forbids more than a mention of Lotze's teleological idealism; Von Harttmann's absolute monism, in which the goal of the teleological development of the universe is the reversion of the will into not-willing; Wundt's metaphysics of the will, according to which the world is a development, an eternal becoming, in which nature is a preliminary stage to mind; and Nietzsche's individualism, the final point of which is the development of the superman.

The author of this article is anonymous. The IEP is actively seeking an author who will write a replacement article.

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History of Evolution | Internet Encyclopedia of Philosophy

The theory of evolution is a naturalistic theory of the history of life on earth (this refers to the theory of evolution which employs methodological naturalism and is taught in schools and universities). Merriam-Webster's dictionary gives the following definition of evolution: "a theory that the various types of animals and plants have their origin in other preexisting types and that the distinguishable differences are due to modifications in successive generations..."[2] Currently, there are several theories of evolution.

Since World War II a majority of the most prominent and vocal defenders of the evolutionary position which employs methodological naturalism have been atheists and agnostics.[3] In 2007, "Discovery Institute's Center for Science and Culture...announced that over 700 scientists from around the world have now signed a statement expressing their skepticism about the contemporary theory of Darwinian evolution."[4]

In 2011, the results of a study was published indicating that most United States high school biology teachers are reluctant to endorse the theory of evolution in class. [5] In addition, in 2011, eight anti-evolution bills were introduced into state legislatures within the United States encouraging students to employ critical thinking skills when examining the evolutionary paradigm. In 2009, there were seven states which required critical analysis skills be employed when examining evolutionary material within schools.[6]

A 2005 poll by the Louis Finkelstein Institute for Social and Religious Research found that 60% of American medical doctors reject Darwinism, stating that they do not believe man evolved through natural processes alone.[7] Thirty-eight percent of the American medical doctors polled agreed with the statement that "Humans evolved naturally with no supernatural involvement." [8] The study also reported that 1/3 of all medical doctors favor the theory of intelligent design over evolution.[9] In 2010, the Gallup organization reported that 40% of Americans believe in young earth creationism.[10] In January 2006, the BBC reported concerning Britain:

Furthermore, more than 40% of those questioned believe that creationism or intelligent design (ID) should be taught in school science lessons.[11]

Picture above was taken at Johns Hopkins University

Johns Hopkins University Press reported in 2014: "Over the past forty years, creationism has spread swiftly among European Catholics, Protestants, Jews, Hindus, and Muslims, even as anti-creationists sought to smother its flames."[12] In addition, China has the world's largest atheist population and the rapid growth of biblical creationism/Evangelical Christianity in China may have a significant impact on the number of individuals in the world who believe in evolution and also on global atheism (see: China and biblical creationism and Asian atheism).

The theory of evolution posits a process of transformation from simple life forms to more complex life forms, which has never been observed or duplicated in a laboratory.[13][14] Although not a creation scientist, Swedish geneticist Dr. Nils Heribert-Nilsson, Professor of Botany at the University of Lund in Sweden and a member of the Royal Swedish Academy of Sciences, stated: "My attempts to demonstrate Evolution by an experiment carried on for more than 40 years have completely failed. At least, I should hardly be accused of having started from a preconceived antievolutionary standpoint."[15][16]

The fossil record is often used as evidence in the creation versus evolution controversy. The fossil record does not support the theory of evolution and is one of the flaws in the theory of evolution.[17] In 1981, there were at least a hundred million fossils that were catalogued and identified in the world's museums.[18] Despite the large number of fossils available to scientists in 1981, evolutionist Mark Ridley, who currently serves as a professor of zoology at Oxford University, was forced to confess: "In any case, no real evolutionist, whether gradualist or punctuationist, uses the fossil record as evidence in favour of the theory of evolution as opposed to special creation."[19]

In addition to the evolutionary position lacking evidential support and being counterevidential, the great intellectuals in history such as Archimedes, Aristotle, St. Augustine, Francis Bacon, Isaac Newton, and Lord Kelvin did not propose an evolutionary process for a species to transform into a more complex version. Even after the theory of evolution was proposed and promoted heavily in England and Germany, most leading scientists were against the theory of evolution.[20]

The theory of evolution was published by naturalist Charles Darwin in his book On The Origin of Species by Means of Natural Selection or The Preservation of Favored Races in the Struggle for Life, in 1859. In a letter to Asa Gray, Darwin confided: "...I am quite conscious that my speculations run quite beyond the bounds of true science."[21]Prior to publishing the book, Darwin wrote in his private notebooks that he was a materialist, which is a type of atheist.[22] Darwin was a weak atheist/agnostic (see: religious views of Charles Darwin) .[23] Charles Darwins casual mentioning of a creator in earlier editions of The Origin of Species appears to have been a merely a ploy to downplay the implications of his materialistic theory.[24] The amount of credit Darwin actually deserves for the theory is disputed. [25] Darwin's theory attempted to explain the origin of the various kinds of plants and animals via the process of natural selection or "survival of the fittest".

The basic principle behind natural selection is that in the struggle for life some organisms in a given population will be better suited to their particular environment and thus have a reproductive advantage which increases the representation of their particular traits over time. Many years before Charles Darwin, there were several other individuals who published articles on the topic of natural selection.[26]

Darwin did not first propose in his book Origin of Species that man had descended from non-human ancestors. Darwin's theory of evolution incorporated that later in Darwin's book entitled Descent of Man.

As far as the history of the theory of evolution, although Darwin is well known when it comes to the early advocacy of the evolutionary position in the Western world, evolutionary ideas were taught by the ancient Greeks as early as the 7th century B.C.[27] The concept of naturalistic evolution differs from the concept of theistic evolution in that it states God does not guide the posited process of macroevolution.[28]

In 2012, the science news website Livescience.com published a news article entitled Belief in Evolution Boils Down to a Gut Feeling which indicated that research suggests that gut feelings trumped facts when it comes to evolutionists believing in evolution.[29] In January of 2012, the Journal of Research in Science Teaching published a study indicating that evolutionary belief is significantly based on gut feelings.[30][31] The January 20, 2012 article entitled Belief in Evolution Boils Down to a Gut Feeling published by the website Live Science wrote of the research: "They found that intuition had a significant impact on what the students accepted, no matter how much they knew and regardless of their religious beliefs."[32]

In response to evolutionary indoctrination and the uncritical acceptance of evolution by many evolutionists, the scientists at the organization
Creation Ministries International created a Question evolution! campaign which poses 15 questions for evolutionists. In addition, leading creationist organizations have created lists of poor arguments that evolutionists should not use.[33] See also: Causes of evolutionary belief

See also: Theories of evolution

Evolutionist Theodosius Dobzhansky wrote concerning the theory of evolution: "The process of mutation is the only known source of the new materials of genetic variability, and hence of evolution."[34] Concerning various theories of evolution, most evolutionists believe that the processes of mutation, genetic drift and natural selection created every species of life that we see on earth today after life first came about on earth although there is little consensus on how this process is allegedly to have occurred.[35]

Pierre-Paul Grass, who served as Chair of evolutionary biology at Sorbonne University for thirty years and was ex-president of the French Academy of Sciences, stated the following: "Some contemporary biologists, as soon as they observe a mutation, talk about evolution. They are implicitly supporting the following syllogism: mutations are the only evolutionary variations, all living beings undergo mutations, therefore all living beings evolve....No matter how numerous they may be, mutations do not produce any kind of evolution." Grass pointed out that bacteria which are the subject of study of many geneticists and molecular biologists are organisms which produce the most mutants.[36] Grasse then points that bacteria are considered to have "stabilized".[37] Grass regards the "unceasing mutations" to be "merely hereditary fluctuations around a median position; a swing to the right, a swing to the left, but no final evolutionary effect."[38]

In addition, Harvard biologist Ernst Mayr wrote: "It must be admitted, however, that it is a considerable strain on ones credulity to assume that finely balanced systems such as certain sense organs (the eye of vertebrates, or the birds feather) could be improved by random mutations."[39]

Creation scientists believe that mutations, natural selection, and genetic drift would not cause macroevolution.[40] Furthermore, creation scientists assert that the life sciences as a whole support the creation model and do not support the theory of evolution.[41]Homology involves the theory that macroevolutionary relationships can be demonstrated by the similarity in the anatomy and physiology of different organisms.[42] An example of a homology argument is that DNA similarities between human and other living organisms is evidence for the theory of evolution.[43] Creation scientists provide sound reasons why the homology argument is not a valid argument. Both evolutionary scientists and young earth creation scientists believe that speciation occurs, however, young earth creation scientists state that speciation generally occurs at a much faster rate than evolutionist believe is the case.[44]

Critics of the theory of evolution state that many of today's proponents of the evolutionary position have diluted the meaning of the term "evolution" to the point where it defined as or the definition includes change over time in the gene pool of a population over time through such processes as mutation, natural selection, and genetic drift.[45] Dr. Jonathan Sarfati of Creation Ministries International declares concerning the diluted definition of the word "evolution":

See also: Atheism and equivocation

Dr. Jonathan Sarfati wrote:

All (sexually reproducing) organisms contain their genetic information in paired form. Each offspring inherits half its genetic information from its mother, and half from its father. So there are two genes at a given position (locus, plural loci) coding for a particular characteristic. An organism can be heterozygous at a given locus, meaning it carries different forms (alleles) of this gene... So there is no problem for creationists explaining that the original created kinds could each give rise to many different varieties. In fact, the original created kinds would have had much more heterozygosity than their modern, more specialized descendants. No wonder Ayala pointed out that most of the variation in populations arises from reshuffling of previously existing genes, not from mutations. Many varieties can arise simply by two previously hidden recessive alleles coming together. However, Ayala believes the genetic information came ultimately from mutations, not creation. His belief is contrary to information theory, as shown in chapter 9 on Design.[48]

Dr. Don Batten of Creation Ministries International has pointed out that prominent evolutionists, such as PZ Myers and Nick Matzke, have indicated that a naturalistic postulation of the origin of life (often called abiogenesis), is part of the evolutionary model.[49] This poses a very serious problem for the evolutionary position as the evidence clearly points life being a product of design and not through naturalistic processes.[50]

The genetic entropy theory by Cornell University Professor Dr. John Sanford on eroding genomes of all living organisms due to mutations inherited from one generation to the next is declared to be one of the major challenges to evolutionary theory. The central part of Sanfords argument is that mutations, represented by spelling mistakes in DNA, are accumulating so quickly in some creatures (and particularly in people) that natural selection cannot stop the functional degradation of the genome, let alone drive an evolutionary process that could lead for example, from apes into people.[51]

Sanford's book Genetic Entropy and the Mystery of the Genome explains why human DNA is inexorably deteriorating at an alarming rate, thus cannot be millions of years old.[52]

The evolutionist Michael Lynch wrote in the Proceedings of the National Academy of Sciences of the United States of America in a December 3, 2009 article entitled: Rate, molecular spectrum, and consequences of human mutation (taken from the abstract):

Creation scientists and intelligent design advocates point out that the genetic code (DNA code), genetic programs, and biological information argue for an intelligent cause in regards the origins question and assert it is one of the many problems of the theory of evolution.[55][56]

Dr. Walt Brown states the genetic material that controls the biological processes of life is coded information and that human experience tells us that codes are created only by the result of intelligence and not merely by processes of nature.[55] Dr. Brown also asserts that the "information stored in the genetic material of all life is a complex program. Therefore, it appears that an unfathomable intelligence created these genetic programs."[55]

To support his view regarding the divine origin of genetic programs Dr. Walt Brown cites the work of David Abel and Professor Jack Trevors who wrote the following:

In the peer reviewed biology journal Proceedings of the Biological Society of Washington Dr. Stephen Meyer argues that no current materialistic theory of evolution can account for the origin of the information necessary to build novel animal forms and proposed an intelligent cause as the best explanation for the origin of biological information and the higher taxa.[58] The editor of the Proceedings of the Biological Society of Washington, Dr. Richard Sternberg, came under intense scrutiny
and persecution for the aforementioned article published by Dr. Meyer.

See also: Theory of evolution and little consensus and Theories of evolution

There is little scientific consensus on how macroevolution is said to have happened and the claimed mechanisms of evolutionary change, as can be seen in the following quotes:

Pierre-Paul Grass, who served as Chair of Evolution at Sorbonne University for thirty years and was ex-president of the French Academy of Sciences, stated the following:

Today, our duty is to destroy the myth of evolution, considered as a simple, understood, and explained phenomenon which keeps rapidly unfolding before us. Biologists must be encouraged to think about the weaknesses of the interpretations and extrapolations that theoreticians put forward or lay down as established truths. The deceit is sometimes unconscious, but not always, since some people, owing to their sectarianism, purposely overlook reality and refuse to acknowledge the inadequacies and the falsity of their beliefs. - Pierre-Paul Grass - Evolution of Living Organisms (1977), pages 6 and 8[62]

See: Modern evolutionary synthesis and Theories of evolution

A notable case of a scientists using fraudulent material to promote the theory of evolution was the work of German scientist and atheist Ernst Haeckel. Noted evolutionist and Stephen Gould, who held a agnostic worldview[63] and promoted the notion of non-overlapping magesteria, wrote the following regarding Ernst Haeckel's work in a March 2000 issue of Natural History:

An irony of history is that the March 9, 1907 edition of the NY Times refers to Ernst Haeckel as the "celebrated Darwinian and founder of the Association for the Propagation of Ethical Atheism."[65]

Stephen Gould continues by quoting Michael Richardson of the St. Georges Hospital Medical School in London, who stated: "I know of at least fifty recent biology texts which use the drawings uncritically".[64]

See also: Evolution and the fossil record

As alluded to earlier, today there are over one hundred million identified and cataloged fossils in the world's museums.[66] If the evolutionary position was valid, then there should be "transitional forms" in the fossil record reflecting the intermediate life forms. Another term for these "transitional forms" is "missing links".

Charles Darwin admitted that his theory required the existence of "transitional forms." Darwin wrote: "So that the number of intermediate and transitional links, between all living and extinct species, must have been inconceivably great. But assuredly, if this theory be true, such have lived upon the earth."[68] However, Darwin wrote: "Why then is not every geological formation and every strata full of such intermediate links? Geology assuredly does not reveal any such finely-graduated organic chain; and this perhaps, is the most obvious and serious objection which can be urged against my theory."[69] Darwin thought the lack of transitional links in his time was because "only a small portion of the surface of the earth has been geologically explored and no part with sufficient care...".[70] As Charles Darwin grew older he became increasingly concerned about the lack of evidence for the theory of evolution in terms of the existence of transitional forms. Darwin wrote, "When we descend to details, we cannot prove that a single species has changed; nor can we prove that the supposed changes are beneficial, which is the groundwork of the theory.[71]

Scientist Dr. Michael Denton wrote regarding the fossil record:

Creationists assert that evolutionists have had over 140 years to find a transitional fossil and nothing approaching a conclusive transitional form has ever been found and that only a handful of highly doubtful examples of transitional fossils exist.[73] Distinguished anthropologist Sir Edmund R. Leach declared, "Missing links in the sequence of fossil evidence were a worry to Darwin. He felt sure they would eventually turn up, but they are still missing and seem likely to remain so."[74]

David B. Kitts of the School of Geology and Geophysics at the University of Oklahoma wrote that "Evolution requires intermediate forms between species and paleontology does not provide them".[75]

David Raup, who was the curator of geology at the museum holding the world's largest fossil collection, the Field Museum of Natural History in Chicago, observed:

One of the most famous proponents of the theory of evolution was the late Harvard paleontologist Stephen Jay Gould. But Gould admitted:

For more information please see:

Creationists can cite quotations which assert that no solid fossil evidence for the theory of evolution position exists:

For more fossil record quotes please see: Fossil record quotes and Additional fossil record quotes

For more information please see: Paleoanthropology and Human evolution

Paleoanthropology is an interdisciplinary branch of anthropology that concerns itself with the origins of early humans and it examines and evaluates items such as fossils and artifacts.[82] Dr. David Pilbeam is a paleoanthropologist who received his Ph.D. at Yale University and Dr. Pilbeam is presently Professor of Social Sciences at Harvard University and Curator of Paleontology at the Peabody Museum of Archaeology and Ethnology. In addition, Dr. Pilbeam served as an advisor for the Kenya government regarding the creation of an international institute for the study of human origins.[83]

Dr. Pilbeam wrote a review of Richard Leakey's book Origins in the journal American Scientist:

Dr. Pilbeam wrote the following regarding the theory of evolution and paleoanthropology:

Evolutionist and Harvard professor Richard Lewontin wrote in 1995 that "Despite the excited and optimistic claims that have been made by some paleontologists, no fossil hominid species can be established as our direct ancestor...."[85] In the September 2005 issue of National Geographic, Joel Achenbach asserted that human evolution is a "fact" but he also candidly admitted that the field of paleoanthropology "has again become a rather glorious mess."[86][87] In the same National Geographic article Harvard paleoanthropologist Dan Lieberman states, "We're not doing a very good job of being honest about what we don't know...".[87]

Concerning pictures of the supposed ancestors of man featured in science journals and the news media Boyce Rensberger wrote in the journal Science the following regarding their highly speculative nature:

Creation scientists concur with Dr. Pilbeam regarding the speculative nature of the field of paleoanthropology and assert there is no compelling evidence in the field of paleoanthropology for the various theories of human evolution.[90]

In 2011, Dr. Grady S. McMurtry declared:

It is acknowledged that the Laws of Genetics are conservative, they are not creative. Genetics only copies or rearranges the previously existing information and passes it on to the next generation. When copying information, you have only two choices; you can only copy it perfectly or imperfectly, you cannot copy something more perfectly. Mutations do not build one upon another beneficially. Mutations do not create new organs; they only modify existing organs and structures. Mutations overwhelmingly lose information; they do not gain it; therefore, mutations cause changes which are contrary
of evolutionary philosophy.

As a follow on, the addition of excess undirected energy will destroy the previously existing system. Indeed, you will never get an increase in the specifications on the DNA to create new organs without the input from a greater intelligence.

Mutations affect and are affected by many genes and other intergenic information acting in combination with one another. The addition of the accidental duplication of previously existing information is detrimental to any organism.

Mutations do produce microevolution, however, this term is far better understood as merely lateral adaptation, which is only variation within a kind, a mathematical shifting of gene frequency within a gene pool. The shifting of gene frequencies and a loss of information cannot produce macroevolution.

As Dr. Roger Lewin commented after the 1980 University of Chicago conference entitled Macroevolution:

The central question of the Chicago conference was whether the mechanisms underlying microevolution can be extrapolated to explain the phenomena of macroevolution. At the risk of doing violence to the positions of some of the people at the meeting, the answer can be given as a clear, No. [Emphasis added]

Dr. Roger Lewin, Evolution Theory under Fire, Science. Vol. 210, 21 November 1980. p. 883-887.[91]

In 1988, the prominent Harvard University biologist Ernst Mayr wrote in his essay Does Microevolution Explain Macroevolution?:

...In this respect, indeed, macroevolution as a field of study is completely decoupled from microevolution.[92]

See also: Creation Ministries International on the second law of thermodynamics and evolution

Creation Ministries International has a great wealth of information on why the second law of thermodynamics is incompatible with the evolutionary paradigm.

Some of their key resources on this matter are:

See also: Theories of evolution

Because the fossil record is characterized by the abrupt appearance of species and stasis in the fossil record the theory of punctuated equilibrium was developed and its chief proponents were Stephen Gould, Niles Eldridge, and Steven Stanley. According to the American Museum of Natural History the theory of Punctuated Equilibrium "asserts that evolution occurs in dramatic spurts interspersed with long periods of stasis".[93] Because Stephen Gould was the leading proponent of the theory of punctuated equilibrium much of the criticism of the theory has been directed towards Gould.[94][95] The development of a new evolutionary school of thought occurring due to the fossil record not supporting the evolutionary position was not unprecedented. In 1930, Austin H. Clark, an American evolutionary zoologist who wrote 630 articles and books in six languages, came up with an evolutionary hypothesis called zoogenesis which postulated that each of the major types of life forms evolved separately and independently from all the others.[96] Prior to publishing his work entitled The New Evolution: Zoogenesis, Clark wrote in a journal article published in the Quarterly Review of Biology that "so far as concerns the major groups of animals, the creationists seem to have the better of the argument. There is not the slightest evidence that any one of the major groups arose from any other."[97]

In 1995, there was an essay in the New York Review of Books by the late John Maynard Smith, a noted evolutionary biologist who was considered the dean of British neo-Darwinists, and Smith wrote the following regarding Gould's work in respect to the theory of evolution:

Noted journalist and author Robert Wright , wrote in 1996 that, among top-flight evolutionary biologists, Gould is considered a pestnot just a lightweight, but an actively muddled man who has warped the public's understanding of Darwinism.[100][101]

Creation scientist Dr. Jonathan Sarfati wrote regarding the implausibility of the theory of punctuated equilibrium and the implausibility of the idea of gradual evolution the following:

Individuals who are against the evolutionary position assert that evolutionary scientists employ extremely implausible "just so stories" to support their position and have done this since at least the time of Charles Darwin.[104][105]

A well known example of a "just so story" is when Darwin, in his Origin of the Species, wrote a chapter entitled "Difficulties on Theory" in which he stated:

Even the prominent evolutionist and geneticist Professor Richard Lewontin admitted the following:

Dr. Sarfati wrote regarding the theory of evolution the following:

Opponents to the theory of evolution commonly point to the following in nature as being implausibly created through evolutionary processes:

Lastly, biochemist Michael Behe wrote the following:

Phillip E. Johnson cites Francis Crick in order to illustrate the fact that the biological world has the strong appearance of being designed:

Stephen C. Meyer offers the following statement regarding the design of the biological world:

The Stanford Encyclopedia of Philosophy states regarding a candid admission of Charles Darwin:

In the course of that conversation I said to Mr. Darwin, with reference to some of his own remarkable works on the Fertilisation of Orchids, and upon The Earthworms, and various other observations he made of the wonderful contrivances for certain purposes in natureI said it was impossible to look at these without seeing that they were the effect and the expression of Mind. I shall never forget Mr. Darwin's answer. He looked at me very hard and said, Well, that often comes over me with overwhelming force; but at other times, and he shook his head vaguely, adding, it seems to go away.(Argyll 1885, 244][127]

Research and historical data indicate that a significant portion of atheists/agnostics often see the their lives and the world as being the product of purposeful design (see: Atheism and purpose).[128]

See: Argument from beauty

Advocates of the theory of evolution have often claimed that those who oppose the theory of evolution don't publish their opposition to the theory of evolution in the appropriate scientific literature (creationist scientists have peer reviewed journals which favor the creationist position).[129][130][131] Recently, there has been articles which were favorable to the intelligent design position in scientific journals which traditionally have favored the theory of evolution.[132]

Karl Popper, a leading philosopher of science and originator of the falsifiability as a criterion of demarcation of science from nonscience,[133] stated that Darwinism is "not a testable scientific theory, but a metaphysical research programme."[134] Leading Darwinist and philosopher of science, Michael Ruse declared the concerning Popper's statement and the actions he took after making that statement: "Since making this claim, Popper himself has modified his position somewhat; but, disclaimers aside, I suspect that even now he does not really believe that Darwinism in its modern form is genuinely falsifiable."[135]

The issue of the falsifiability of the evolutionary position is very important issue and although offering a poor cure to the problem that Karl Popper described,
committed evolutionists Louis Charles Birch & Paul R. Ehrlich stated in the journal Nature:

The Swedish cytogeneticist, Antonio Lima-De-Faria, who has been knighted by the king of Sweden for his scientific achievements, noted that "there has never been a theory of evolution".[137][138]

See also: Suppression of alternatives to evolution and Atheism and the suppression of science

Many of the leaders of the atheist movement, such as the evolutionist and the new atheist Richard Dawkins, argue for atheism and evolution with a religious fervor (See also: Atheism and evolution).

Daniel Smartt has identified seven dimensions which make up religion: narrative, experiential, social, ethical, doctrinal, ritual and material. It is not necessary in Smartt's model for every one of these to be present in order for something to be a religion.[139]. However, it can be argued that all seven are present in the case of atheism.[140][141] Please see: Atheism: A religionand Atheism and Atheism is a religion.

See also: Atheism is a religion and Atheism and evolution

Atheism is a religion and naturalistic notions concerning origins are religious in nature and both have legal implications as far as evolution being taught in public schools.[143][144][145]

John Calvert, a lawyer and intelligent design proponent wrote:

See also:

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Evolution - Conservapedia

You are here: Science >> Darwin's Theory Of Evolution

Darwin's Theory of Evolution - The Premise Darwin's Theory of Evolution is the widely held notion that all life is related and has descended from a common ancestor: the birds and the bananas, the fishes and the flowers -- all related. Darwin's general theory presumes the development of life from non-life and stresses a purely naturalistic (undirected) "descent with modification". That is, complex creatures evolve from more simplistic ancestors naturally over time. In a nutshell, as random genetic mutations occur within an organism's genetic code, the beneficial mutations are preserved because they aid survival -- a process known as "natural selection." These beneficial mutations are passed on to the next generation. Over time, beneficial mutations accumulate and the result is an entirely different organism (not just a variation of the original, but an entirely different creature).

Darwin's Theory of Evolution - Natural Selection While Darwin's Theory of Evolution is a relatively young archetype, the evolutionary worldview itself is as old as antiquity. Ancient Greek philosophers such as Anaximander postulated the development of life from non-life and the evolutionary descent of man from animal. Charles Darwin simply brought something new to the old philosophy -- a plausible mechanism called "natural selection." Natural selection acts to preserve and accumulate minor advantageous genetic mutations. Suppose a member of a species developed a functional advantage (it grew wings and learned to fly). Its offspring would inherit that advantage and pass it on to their offspring. The inferior (disadvantaged) members of the same species would gradually die out, leaving only the superior (advantaged) members of the species. Natural selection is the preservation of a functional advantage that enables a species to compete better in the wild. Natural selection is the naturalistic equivalent to domestic breeding. Over the centuries, human breeders have produced dramatic changes in domestic animal populations by selecting individuals to breed. Breeders eliminate undesirable traits gradually over time. Similarly, natural selection eliminates inferior species gradually over time.

Darwin's Theory of Evolution - Slowly But Surely... Darwin's Theory of Evolution is a slow gradual process. Darwin wrote, "Natural selection acts only by taking advantage of slight successive variations; she can never take a great and sudden leap, but must advance by short and sure, though slow steps." [1] Thus, Darwin conceded that, "If it could be demonstrated that any complex organ existed, which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down." [2] Such a complex organ would be known as an "irreducibly complex system". An irreducibly complex system is one composed of multiple parts, all of which are necessary for the system to function. If even one part is missing, the entire system will fail to function. Every individual part is integral. [3] Thus, such a system could not have evolved slowly, piece by piece. The common mousetrap is an everyday non-biological example of irreducible complexity. It is composed of five basic parts: a catch (to hold the bait), a powerful spring, a thin rod called "the hammer," a holding bar to secure the hammer in place, and a platform to mount the trap. If any one of these parts is missing, the mechanism will not work. Each individual part is integral. The mousetrap is irreducibly complex. [4]

Darwin's Theory of Evolution - A Theory In Crisis Darwin's Theory of Evolution is a theory in crisis in light of the tremendous advances we've made in molecular biology, biochemistry and genetics over the past fifty years. We now know that there are in fact tens of thousands of irreducibly complex systems on the cellular level. Specified complexity pervades the microscopic biological world. Molecular biologist Michael Denton wrote, "Although the tiniest bacterial cells are incredibly small, weighing less than 10-12 grams, each is in effect a veritable micro-miniaturized factory containing thousands of exquisitely designed pieces of intricate molecular machinery, made up altogether of one hundred thousand million atoms, far more complicated than any machinery built by man and absolutely without parallel in the non-living world." [5]

And we don't need a microscope to observe irreducible complexity. The eye, the ear and the heart are all examples of irreducible complexity, though they were not recognized as such in Darwin's day. Nevertheless, Darwin confessed, "To suppose that the eye with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest degree." [6]

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Darwin's Theory Of Evolution

Evolution is change in the heritable traits of biological populations over successive generations.[1][2] Evolutionary processes give rise to diversity at every level of biological organisation, including the levels of species, individual organisms, and molecules.[3]

All life on Earth shares a common ancestor known as the last universal ancestor,[4][5][6] which lived approximately 3.53.8 billion years ago,[7] although a study in 2015 found "remains of biotic life" from 4.1 billion years ago in ancient rocks in Western Australia.[8][9]

Repeated formation of new species (speciation), change within species (anagenesis), and loss of species (extinction) throughout the evolutionary history of life on Earth are demonstrated by shared sets of morphological and biochemical traits, including shared DNA sequences.[10] These shared traits are more similar among species that share a more recent common ancestor, and can be used to reconstruct a biological "tree of life" based on evolutionary relationships (phylogenetics), using both existing species and fossils. The fossil record includes a progression from early biogenic graphite,[11] to microbial mat fossils,[12][13][14] to fossilized multicellular organisms. Existing patterns of biodiversity have been shaped both by speciation and by extinction.[15] More than 99 percent of all species that ever lived on Earth are estimated to be extinct.[16][17] Estimates of Earth's current species range from 10 to 14 million,[18] of which about 1.2 million have been documented.[19]

In the mid-19th century, Charles Darwin formulated the scientific theory of evolution by natural selection, published in his book On the Origin of Species (1859). Evolution by natural selection is a process demonstrated by the observation that more offspring are produced than can possibly survive, along with three facts about populations: 1) traits vary among individuals with respect to morphology, physiology, and behaviour (phenotypic variation), 2) different traits confer different rates of survival and reproduction (differential fitness), and 3) traits can be passed from generation to generation (heritability of fitness).[20] Thus, in successive generations members of a population are replaced by progeny of parents better adapted to survive and reproduce in the biophysical environment in which natural selection takes place. This teleonomy is the quality whereby the process of natural selection creates and preserves traits that are seemingly fitted for the functional roles they perform.[21] Natural selection is the only known cause of adaptation but not the only known cause of evolution. Other, nonadaptive causes of microevolution include mutation and genetic drift.[22]

In the early 20th century the modern evolutionary synthesis integrated classical genetics with Darwin's theory of evolution by natural selection through the discipline of population genetics. The importance of natural selection as a cause of evolution was accepted into other branches of biology. Moreover, previously held notions about evolution, such as orthogenesis, evolutionism, and other beliefs about innate "progress" within the largest-scale trends in evolution, became obsolete scientific theories.[23] Scientists continue to study various aspects of evolutionary biology by forming and testing hypotheses, constructing mathematical models of theoretical biology and biological theories, using observational data, and performing experiments in both the field and the laboratory.

In terms of practical application, an understanding of evolution has been instrumental to developments in numerous scientific and industrial fields, including agriculture, human and veterinary medicine, and the life sciences in general.[24][25][26] Discoveries in evolutionary biology have made a significant impact not just in the traditional branches of biology but also in other academic disciplines, including biological anthropology, and evolutionary psychology.[27][28]Evolutionary Computation, a sub-field of Artificial Intelligence, is the result of the application of Darwinian principles to problems in Computer Science.

The proposal that one type of organism could descend from another type goes back to some of the first pre-Socratic Greek philosophers, such as Anaximander and Empedocles.[30] Such proposals survived into Roman times. The poet and philosopher Lucretius followed Empedocles in his masterwork De rerum natura (On the Nature of Things).[31][32] In contrast to these materialistic views, Aristotle understood all natural things, not only living things, as being imperfect actualisations of different fixed natural possibilities, known as "forms," "ideas," or (in Latin translations) "species."[33][34] This was part of his teleological understanding of nature in which all things have an intended role to play in a divine cosmic order. Variations of this idea became the standard understanding of the Middle Ages and were integrated into Christian learning, but Aristotle did not demand that real types of organisms always correspond one-for-one with exact metaphysical forms and specifically gave examples of how new types of living things could come to be.[35]

In the 17th century, the new method of modern science rejected Aristotle's approach. It sought explanations of natural phenomena in terms of physical laws that were the same for all visible things and that did not require the existence of any fixed natural categories or divine cosmic order. However, this new approach was slow to take root in the biological sciences, the last bastion of the concept of fixed natural types. John Ray applied one of the previously more general terms for fixed natural types, "species," to plant and animal types, but he strictly identified each type of living thing as a species and proposed that each species could be defined by the features that perpetuated themselves generation after generation.[36] These species were designed by God, but showed differences caused by local conditions. The biological classification introduced by Carl Linnaeus in 1735 explicitly recognized the hierarchical nature of species relationships, but still viewed species as fixed according to a divine plan.[37]

Other naturalists of this time speculated on the evolutionary change of species over time according to natural laws. In 1751, Pierre Louis Maupertuis wrote of natural modifications occurring during reproduction and accumulating over many generations to produce new species.[38]Georges-Louis Leclerc, Comte de Buffon suggested that species could degenerate into different organisms, and Erasmus Darwin proposed that all warm-blooded animals could have descended from a single microorganism (or "filament").[39] The first full-fledged evolutionary scheme was Jean-Baptiste Lamarck's "transmutation" theory of 1809,[40] which envisaged spontaneous generation continually producing simple forms of life that developed greater complexity in parallel lineages with an inherent progressive tendency, and postulated that on a local level these lineages adapted to the environment by inheriting changes caused by their use or disuse in parents.[41][42] (The latter process was later called Lamarckism.)[41][43][44][45] These ideas were condemned by established naturalists as speculation lacking empirical support. In particular, Georges Cuvier insisted that species were unrelated and fixed, their similarities reflecting divine design for functional needs. In the meantime, Ray's ideas of benevolent design had been developed by William Paley into the Natural Theology or Evidences of the Existence and Attributes of the Deity (1802)
, which proposed complex adaptations as evidence of divine design and which was admired by Charles Darwin.[46][47][48]

The crucial break from the concept of constant typological classes or types in biology came with the theory of evolution through natural selection, which was formulated by Charles Darwin in terms of variable populations. Partly influenced by An Essay on the Principle of Population (1798) by Thomas Robert Malthus, Darwin noted that population growth would lead to a "struggle for existence" in which favorable variations prevailed as others perished. In each generation, many offspring fail to survive to an age of reproduction because of limited resources. This could explain the diversity of plants and animals from a common ancestry through the working of natural laws in the same way for all types of organism.[49][50][51][52] Darwin developed his theory of "natural selection" from 1838 onwards and was writing up his "big book" on the subject when Alfred Russel Wallace sent him a version of virtually the same theory in 1858. Their separate papers were presented together at a 1858 meeting of the Linnean Society of London.[53] At the end of 1859, Darwin's publication of his "abstract" as On the Origin of Species explained natural selection in detail and in a way that led to an increasingly wide acceptance of concepts of evolution. Thomas Henry Huxley applied Darwin's ideas to humans, using paleontology and comparative anatomy to provide strong evidence that humans and apes shared a common ancestry. Some were disturbed by this since it implied that humans did not have a special place in the universe.[54]

Precise mechanisms of reproductive heritability and the origin of new traits remained a mystery. Towards this end, Darwin developed his provisional theory of pangenesis.[55] In 1865, Gregor Mendel reported that traits were inherited in a predictable manner through the independent assortment and segregation of elements (later known as genes). Mendel's laws of inheritance eventually supplanted most of Darwin's pangenesis theory.[56]August Weismann made the important distinction between germ cells that give rise to gametes (such as sperm and egg cells) and the somatic cells of the body, demonstrating that heredity passes through the germ line only. Hugo de Vries connected Darwin's pangenesis theory to Weismann's germ/soma cell distinction and proposed that Darwin's pangenes were concentrated in the cell nucleus and when expressed they could move into the cytoplasm to change the cells structure. De Vries was also one of the researchers who made Mendel's work well-known, believing that Mendelian traits corresponded to the transfer of heritable variations along the germline.[57] To explain how new variants originate, de Vries developed a mutation theory that led to a temporary rift between those who accepted Darwinian evolution and biometricians who allied with de Vries.[42][58][59] In the 1930s, pioneers in the field of population genetics, such as Ronald Fisher, Sewall Wright and J. B. S. Haldane set the foundations of evolution onto a robust statistical philosophy. The false contradiction between Darwin's theory, genetic mutations, and Mendelian inheritance was thus reconciled.[60]

In the 1920s and 1930s a modern evolutionary synthesis connected natural selection, mutation theory, and Mendelian inheritance into a unified theory that applied generally to any branch of biology. The modern synthesis was able to explain patterns observed across species in populations, through fossil transitions in palaeontology, and even complex cellular mechanisms in developmental biology.[42][61] The publication of the structure of DNA by James Watson and Francis Crick in 1953 demonstrated a physical mechanism for inheritance.[62]Molecular biology improved our understanding of the relationship between genotype and phenotype. Advancements were also made in phylogenetic systematics, mapping the transition of traits into a comparative and testable framework through the publication and use of evolutionary trees.[63][64] In 1973, evolutionary biologist Theodosius Dobzhansky penned that "nothing in biology makes sense except in the light of evolution," because it has brought to light the relations of what first seemed disjointed facts in natural history into a coherent explanatory body of knowledge that describes and predicts many observable facts about life on this planet.[65]

Since then, the modern synthesis has been further extended to explain biological phenomena across the full and integrative scale of the biological hierarchy, from genes to species. This extension, known as evolutionary developmental biology and informally called "evo-devo," emphasises how changes between generations (evolution) acts on patterns of change within individual organisms (development).[66][67][68]

Evolution in organisms occurs through changes in heritable traitsthe inherited characteristics of an organism. In humans, for example, eye colour is an inherited characteristic and an individual might inherit the "brown-eye trait" from one of their parents.[69] Inherited traits are controlled by genes and the complete set of genes within an organism's genome (genetic material) is called its genotype.[70]

The complete set of observable traits that make up the structure and behaviour of an organism is called its phenotype. These traits come from the interaction of its genotype with the environment.[71] As a result, many aspects of an organism's phenotype are not inherited. For example, suntanned skin comes from the interaction between a person's genotype and sunlight; thus, suntans are not passed on to people's children. However, some people tan more easily than others, due to differences in genotypic variation; a striking example are people with the inherited trait of albinism, who do not tan at all and are very sensitive to sunburn.[72]

Heritable traits are passed from one generation to the next via DNA, a molecule that encodes genetic information.[70] DNA is a long biopolymer composed of four types of bases. The sequence of bases along a particular DNA molecule specify the genetic information, in a manner similar to a sequence of letters spelling out a sentence. Before a cell divides, the DNA is copied, so that each of the resulting two cells will inherit the DNA sequence. Portions of a DNA molecule that specify a single functional unit are called genes; different genes have different sequences of bases. Within cells, the long strands of DNA form condensed structures called chromosomes. The specific location of a DNA sequence within a chromosome is known as a locus. If the DNA sequence at a locus varies between individuals, the different forms of this sequence are called alleles. DNA sequences can change through mutations, producing new alleles. If a mutation occurs within a gene, the new allele may affect the trait that the gene controls, altering the phenotype of the organism.[73] However, while this simple correspondence between an allele and a trait works in some cases, most traits are more complex and are controlled by quantitative trait loci (multiple interacting genes).[74][75]

Recent findings have confirmed important examples of heritable changes that cannot be explained by changes to the sequence of nucleotides in the DNA. These phenomena are classed as epigenetic inheritance systems.[76]DNA methylation marking chromatin, self-sustaining metabolic loops, gene silencing by RNA interference and the three-dimensional conformation of proteins (such as prions) are areas where epigenetic inheritance systems have been discovered at the organismic level.[77][78] De
velopmental biologists suggest that complex interactions in genetic networks and communication among cells can lead to heritable variations that may underlay some of the mechanics in developmental plasticity and canalisation.[79] Heritability may also occur at even larger scales. For example, ecological inheritance through the process of niche construction is defined by the regular and repeated activities of organisms in their environment. This generates a legacy of effects that modify and feed back into the selection regime of subsequent generations. Descendants inherit genes plus environmental characteristics generated by the ecological actions of ancestors.[80] Other examples of heritability in evolution that are not under the direct control of genes include the inheritance of cultural traits and symbiogenesis.[81][82]

An individual organism's phenotype results from both its genotype and the influence from the environment it has lived in. A substantial part of the phenotypic variation in a population is caused by genotypic variation.[75] The modern evolutionary synthesis defines evolution as the change over time in this genetic variation. The frequency of one particular allele will become more or less prevalent relative to other forms of that gene. Variation disappears when a new allele reaches the point of fixationwhen it either disappears from the population or replaces the ancestral allele entirely.[83]

Natural selection will only cause evolution if there is enough genetic variation in a population. Before the discovery of Mendelian genetics, one common hypothesis was blending inheritance. But with blending inheritance, genetic variance would be rapidly lost, making evolution by natural selection implausible. The HardyWeinberg principle provides the solution to how variation is maintained in a population with Mendelian inheritance. The frequencies of alleles (variations in a gene) will remain constant in the absence of selection, mutation, migration and genetic drift.[84]

Variation comes from mutations in the genome, reshuffling of genes through sexual reproduction and migration between populations (gene flow). Despite the constant introduction of new variation through mutation and gene flow, most of the genome of a species is identical in all individuals of that species.[85] However, even relatively small differences in genotype can lead to dramatic differences in phenotype: for example, chimpanzees and humans differ in only about 5% of their genomes.[86]

Mutations are changes in the DNA sequence of a cell's genome. When mutations occur, they may alter the product of a gene, or prevent the gene from functioning, or have no effect. Based on studies in the fly Drosophila melanogaster, it has been suggested that if a mutation changes a protein produced by a gene, this will probably be harmful, with about 70% of these mutations having damaging effects, and the remainder being either neutral or weakly beneficial.[87]

Mutations can involve large sections of a chromosome becoming duplicated (usually by genetic recombination), which can introduce extra copies of a gene into a genome.[88] Extra copies of genes are a major source of the raw material needed for new genes to evolve.[89] This is important because most new genes evolve within gene families from pre-existing genes that share common ancestors.[90] For example, the human eye uses four genes to make structures that sense light: three for colour vision and one for night vision; all four are descended from a single ancestral gene.[91]

New genes can be generated from an ancestral gene when a duplicate copy mutates and acquires a new function. This process is easier once a gene has been duplicated because it increases the redundancy of the system; one gene in the pair can acquire a new function while the other copy continues to perform its original function.[92][93] Other types of mutations can even generate entirely new genes from previously noncoding DNA.[94][95]

The generation of new genes can also involve small parts of several genes being duplicated, with these fragments then recombining to form new combinations with new functions.[96][97] When new genes are assembled from shuffling pre-existing parts, domains act as modules with simple independent functions, which can be mixed together to produce new combinations with new and complex functions.[98] For example, polyketide synthases are large enzymes that make antibiotics; they contain up to one hundred independent domains that each catalyse one step in the overall process, like a step in an assembly line.[99]

In asexual organisms, genes are inherited together, or linked, as they cannot mix with genes of other organisms during reproduction. In contrast, the offspring of sexual organisms contain random mixtures of their parents' chromosomes that are produced through independent assortment. In a related process called homologous recombination, sexual organisms exchange DNA between two matching chromosomes.[100] Recombination and reassortment do not alter allele frequencies, but instead change which alleles are associated with each other, producing offspring with new combinations of alleles.[101] Sex usually increases genetic variation and may increase the rate of evolution.[102][103]

The two-fold cost of sex was first described by John Maynard Smith.[104] The first cost is that only one of the two sexes can bear young.[clarification needed] (This cost does not apply to hermaphroditic species, like most plants and many invertebrates.) The second cost is that any individual who reproduces sexually can only pass on 50% of its genes to any individual offspring, with even less passed on as each new generation passes.[105] (Again, this applies mostly to the evolution of sexual dimorphism, which occurred long after the evolution of sex itself.) Yet sexual reproduction is the more common means of reproduction among eukaryotes and multicellular organisms (although more common than sexual dimorphism). The Red Queen hypothesis has been used to explain the significance of sexual reproduction as a means to enable continual evolution and adaptation in response to coevolution with other species in an ever-changing environment.[105][106][107][108]

Gene flow is the exchange of genes between populations and between species.[109] It can therefore be a source of variation that is new to a population or to a species. Gene flow can be caused by the movement of individuals between separate populations of organisms, as might be caused by the movement of mice between inland and coastal populations, or the movement of pollen between heavy metal tolerant and heavy metal sensitive populations of grasses.

Gene transfer between species includes the formation of hybrid organisms and horizontal gene transfer. Horizontal gene transfer is the transfer of genetic material from one organism to another organism that is not its offspring; this is most common among bacteria.[110] In medicine, this contributes to the spread of antibiotic resistance, as when one bacteria acquires resistance genes it can rapidly transfer them to other species.[111] Horizontal transfer of genes from bacteria to eukaryotes such as the yeast Saccharomyces cerevisiae and the adzuki bean weevil Callosobruchus chinensis has occurred.[112][113] An example of larger-scale transfers are the eukaryotic bdelloid rotifers, which have received a range of genes from bacteria, fungi and plants.[114]Viruses can also carry DNA between organisms, allowing transfer of genes even across biological domains.[115]

Large-scale gene transfer has also occurred between the ancestors of eu
karyotic cells and bacteria, during the acquisition of chloroplasts and mitochondria. It is possible that eukaryotes themselves originated from horizontal gene transfers between bacteria and archaea.[116]

From a Neo-Darwinian perspective, evolution occurs when there are changes in the frequencies of alleles within a population of interbreeding organisms.[84] For example, the allele for black colour in a population of moths becoming more common. Mechanisms that can lead to changes in allele frequencies include natural selection, genetic drift, genetic hitchhiking, mutation and gene flow.

Evolution by means of natural selection is the process by which traits that enhance survival and reproduction become more common in successive generations of a population. It has often been called a "self-evident" mechanism because it necessarily follows from three simple facts:[20]

More offspring are produced than can possibly survive, and these conditions produce competition between organisms for survival and reproduction. Consequently, organisms with traits that give them an advantage over their competitors are more likely to pass on their traits to the next generation than those with traits that do not confer an advantage.[117]

The central concept of natural selection is the evolutionary fitness of an organism.[118] Fitness is measured by an organism's ability to survive and reproduce, which determines the size of its genetic contribution to the next generation.[118] However, fitness is not the same as the total number of offspring: instead fitness is indicated by the proportion of subsequent generations that carry an organism's genes.[119] For example, if an organism could survive well and reproduce rapidly, but its offspring were all too small and weak to survive, this organism would make little genetic contribution to future generations and would thus have low fitness.[118]

If an allele increases fitness more than the other alleles of that gene, then with each generation this allele will become more common within the population. These traits are said to be "selected for." Examples of traits that can increase fitness are enhanced survival and increased fecundity. Conversely, the lower fitness caused by having a less beneficial or deleterious allele results in this allele becoming rarerthey are "selected against."[120] Importantly, the fitness of an allele is not a fixed characteristic; if the environment changes, previously neutral or harmful traits may become beneficial and previously beneficial traits become harmful.[73] However, even if the direction of selection does reverse in this way, traits that were lost in the past may not re-evolve in an identical form (see Dollo's law).[121][122]

Natural selection within a population for a trait that can vary across a range of values, such as height, can be categorised into three different types. The first is directional selection, which is a shift in the average value of a trait over timefor example, organisms slowly getting taller.[123] Secondly, disruptive selection is selection for extreme trait values and often results in two different values becoming most common, with selection against the average value. This would be when either short or tall organisms had an advantage, but not those of medium height. Finally, in stabilising selection there is selection against extreme trait values on both ends, which causes a decrease in variance around the average value and less diversity.[117][124] This would, for example, cause organisms to slowly become all the same height.

A special case of natural selection is sexual selection, which is selection for any trait that increases mating success by increasing the attractiveness of an organism to potential mates.[125] Traits that evolved through sexual selection are particularly prominent among males of several animal species. Although sexually favoured, traits such as cumbersome antlers, mating calls, large body size and bright colours often attract predation, which compromises the survival of individual males.[126][127] This survival disadvantage is balanced by higher reproductive success in males that show these hard-to-fake, sexually selected traits.[128]

Natural selection most generally makes nature the measure against which individuals and individual traits, are more or less likely to survive. "Nature" in this sense refers to an ecosystem, that is, a system in which organisms interact with every other element, physical as well as biological, in their local environment. Eugene Odum, a founder of ecology, defined an ecosystem as: "Any unit that includes all of the organisms...in a given area interacting with the physical environment so that a flow of energy leads to clearly defined trophic structure, biotic diversity and material cycles (ie: exchange of materials between living and nonliving parts) within the system."[129] Each population within an ecosystem occupies a distinct niche, or position, with distinct relationships to other parts of the system. These relationships involve the life history of the organism, its position in the food chain and its geographic range. This broad understanding of nature enables scientists to delineate specific forces which, together, comprise natural selection.

Natural selection can act at different levels of organisation, such as genes, cells, individual organisms, groups of organisms and species.[130][131][132] Selection can act at multiple levels simultaneously.[133] An example of selection occurring below the level of the individual organism are genes called transposons, which can replicate and spread throughout a genome.[134] Selection at a level above the individual, such as group selection, may allow the evolution of cooperation, as discussed below.[135]

In addition to being a major source of variation, mutation may also function as a mechanism of evolution when there are different probabilities at the molecular level for different mutations to occur, a process known as mutation bias.[136] If two genotypes, for example one with the nucleotide G and another with the nucleotide A in the same position, have the same fitness, but mutation from G to A happens more often than mutation from A to G, then genotypes with A will tend to evolve.[137] Different insertion vs. deletion mutation biases in different taxa can lead to the evolution of different genome sizes.[138][139] Developmental or mutational biases have also been observed in morphological evolution.[140][141] For example, according to the phenotype-first theory of evolution, mutations can eventually cause the genetic assimilation of traits that were previously induced by the environment.[142][143]

Mutation bias effects are superimposed on other processes. If selection would favor either one out of two mutations, but there is no extra advantage to having both, then the mutation that occurs the most frequently is the one that is most likely to become fixed in a population.[144][145] Mutations leading to the loss of function of a gene are much more common than mutations that produce a new, fully functional gene. Most loss of function mutations are selected against. But when selection is weak, mutation bias towards loss of function can affect evolution.[146] For example, pigments are no longer useful when animals live in the darkness of caves, and tend to be lost.[147] This kind of loss of function can occur because of mutation bias, and/or because the function had a cost, and once the benefit of the function disappeared, natural selection leads to the loss. Loss of sporulation ability in Bacillus subtilis during laboratory evolution appears to have been caused by
mutation bias, rather than natural selection against the cost of maintaining sporulation ability.[148] When there is no selection for loss of function, the speed at which loss evolves depends more on the mutation rate than it does on the effective population size,[149] indicating that it is driven more by mutation bias than by genetic drift. In parasitic organisms, mutation bias leads to selection pressures as seen in Ehrlichia. Mutations are biased towards antigenic variants in outer-membrane proteins.

Genetic drift is the change in allele frequency from one generation to the next that occurs because alleles are subject to sampling error.[150] As a result, when selective forces are absent or relatively weak, allele frequencies tend to "drift" upward or downward randomly (in a random walk). This drift halts when an allele eventually becomes fixed, either by disappearing from the population, or replacing the other alleles entirely. Genetic drift may therefore eliminate some alleles from a population due to chance alone. Even in the absence of selective forces, genetic drift can cause two separate populations that began with the same genetic structure to drift apart into two divergent populations with different sets of alleles.[151]

It is usually difficult to measure the relative importance of selection and neutral processes, including drift.[152] The comparative importance of adaptive and non-adaptive forces in driving evolutionary change is an area of current research.[153]

The neutral theory of molecular evolution proposed that most evolutionary changes are the result of the fixation of neutral mutations by genetic drift.[22] Hence, in this model, most genetic changes in a population are the result of constant mutation pressure and genetic drift.[154] This form of the neutral theory is now largely abandoned, since it does not seem to fit the genetic variation seen in nature.[155][156] However, a more recent and better-supported version of this model is the nearly neutral theory, where a mutation that would be effectively neutral in a small population is not necessarily neutral in a large population.[117] Other alternative theories propose that genetic drift is dwarfed by other stochastic forces in evolution, such as genetic hitchhiking, also known as genetic draft.[150][157][158]

The time for a neutral allele to become fixed by genetic drift depends on population size, with fixation occurring more rapidly in smaller populations.[159] The number of individuals in a population is not critical, but instead a measure known as the effective population size.[160] The effective population is usually smaller than the total population since it takes into account factors such as the level of inbreeding and the stage of the lifecycle in which the population is the smallest.[160] The effective population size may not be the same for every gene in the same population.[161]

Recombination allows alleles on the same strand of DNA to become separated. However, the rate of recombination is low (approximately two events per chromosome per generation). As a result, genes close together on a chromosome may not always be shuffled away from each other and genes that are close together tend to be inherited together, a phenomenon known as linkage.[162] This tendency is measured by finding how often two alleles occur together on a single chromosome compared to expectations, which is called their linkage disequilibrium. A set of alleles that is usually inherited in a group is called a haplotype. This can be important when one allele in a particular haplotype is strongly beneficial: natural selection can drive a selective sweep that will also cause the other alleles in the haplotype to become more common in the population; this effect is called genetic hitchhiking or genetic draft.[163] Genetic draft caused by the fact that some neutral genes are genetically linked to others that are under selection can be partially captured by an appropriate effective population size.[157]

Gene flow involves the exchange of genes between populations and between species.[109] The presence or absence of gene flow fundamentally changes the course of evolution. Due to the complexity of organisms, any two completely isolated populations will eventually evolve genetic incompatibilities through neutral processes, as in the Bateson-Dobzhansky-Muller model, even if both populations remain essentially identical in terms of their adaptation to the environment.

If genetic differentiation between populations develops, gene flow between populations can introduce traits or alleles which are disadvantageous in the local population and this may lead to organisms within these populations evolving mechanisms that prevent mating with genetically distant populations, eventually resulting in the appearance of new species. Thus, exchange of genetic information between individuals is fundamentally important for the development of the biological species concept.

During the development of the modern synthesis, Sewall Wright developed his shifting balance theory, which regarded gene flow between partially isolated populations as an important aspect of adaptive evolution.[164] However, recently there has been substantial criticism of the importance of the shifting balance theory.[165]

Evolution influences every aspect of the form and behaviour of organisms. Most prominent are the specific behavioural and physical adaptations that are the outcome of natural selection. These adaptations increase fitness by aiding activities such as finding food, avoiding predators or attracting mates. Organisms can also respond to selection by cooperating with each other, usually by aiding their relatives or engaging in mutually beneficial symbiosis. In the longer term, evolution produces new species through splitting ancestral populations of organisms into new groups that cannot or will not interbreed.

These outcomes of evolution are distinguished based on time scale as macroevolution versus microevolution. Macroevolution refers to evolution that occurs at or above the level of species, in particular speciation and extinction; whereas microevolution refers to smaller evolutionary changes within a species or population, in particular shifts in gene frequency and adaptation.[166] In general, macroevolution is regarded as the outcome of long periods of microevolution.[167] Thus, the distinction between micro- and macroevolution is not a fundamental onethe difference is simply the time involved.[168] However, in macroevolution, the traits of the entire species may be important. For instance, a large amount of variation among individuals allows a species to rapidly adapt to new habitats, lessening the chance of it going extinct, while a wide geographic range increases the chance of speciation, by making it more likely that part of the population will become isolated. In this sense, microevolution and macroevolution might involve selection at different levelswith microevolution acting on genes and organisms, versus macroevolutionary processes such as species selection acting on entire species and affecting their rates of speciation and extinction.[170][171]

A common misconception is that evolution has goals, long-term plans, or an innate tendency for "progress," as expressed in beliefs such as orthogenesis and evolutionism; realistically however, evolution has no long-term goal and does not necessarily produce greater complexity.[172][173][174] Although complex species have evolved, they occur as a side effect of the overall number of organisms increasing and simple forms of life still remain more common in the
biosphere.[175] For example, the overwhelming majority of species are microscopic prokaryotes, which form about half the world's biomass despite their small size,[176] and constitute the vast majority of Earth's biodiversity.[177] Simple organisms have therefore been the dominant form of life on Earth throughout its history and continue to be the main form of life up to the present day, with complex life only appearing more diverse because it is more noticeable.[178] Indeed, the evolution of microorganisms is particularly important to modern evolutionary research, since their rapid reproduction allows the study of experimental evolution and the observation of evolution and adaptation in real time.[179][180]

Adaptation is the process that makes organisms better suited to their habitat.[181][182] Also, the term adaptation may refer to a trait that is important for an organism's survival. For example, the adaptation of horses' teeth to the grinding of grass. By using the term adaptation for the evolutionary process and adaptive trait for the product (the bodily part or function), the two senses of the word may be distinguished. Adaptations are produced by natural selection.[183] The following definitions are due to Theodosius Dobzhansky:

Adaptation may cause either the gain of a new feature, or the loss of an ancestral feature. An example that shows both types of change is bacterial adaptation to antibiotic selection, with genetic changes causing antibiotic resistance by both modifying the target of the drug, or increasing the activity of transporters that pump the drug out of the cell.[187] Other striking examples are the bacteria Escherichia coli evolving the ability to use citric acid as a nutrient in a long-term laboratory experiment,[188]Flavobacterium evolving a novel enzyme that allows these bacteria to grow on the by-products of nylon manufacturing,[189][190] and the soil bacterium Sphingobium evolving an entirely new metabolic pathway that degrades the synthetic pesticide pentachlorophenol.[191][192] An interesting but still controversial idea is that some adaptations might increase the ability of organisms to generate genetic diversity and adapt by natural selection (increasing organisms' evolvability).[193][194][195][196][197]

Adaptation occurs through the gradual modification of existing structures. Consequently, structures with similar internal organisation may have different functions in related organisms. This is the result of a single ancestral structure being adapted to function in different ways. The bones within bat wings, for example, are very similar to those in mice feet and primate hands, due to the descent of all these structures from a common mammalian ancestor.[199] However, since all living organisms are related to some extent,[200] even organs that appear to have little or no structural similarity, such as arthropod, squid and vertebrate eyes, or the limbs and wings of arthropods and vertebrates, can depend on a common set of homologous genes that control their assembly and function; this is called deep homology.[201][202]

During evolution, some structures may lose their original function and become vestigial structures.[203] Such structures may have little or no function in a current species, yet have a clear function in ancestral species, or other closely related species. Examples include pseudogenes,[204] the non-functional remains of eyes in blind cave-dwelling fish,[205] wings in flightless birds,[206] the presence of hip bones in whales and snakes,[198] and sexual traits in organisms that reproduce via asexual reproduction.[207] Examples of vestigial structures in humans include wisdom teeth,[208] the coccyx,[203] the vermiform appendix,[203] and other behavioural vestiges such as goose bumps[209][210] and primitive reflexes.[211][212][213]

However, many traits that appear to be simple adaptations are in fact exaptations: structures originally adapted for one function, but which coincidentally became somewhat useful for some other function in the process. One example is the African lizard Holaspis guentheri, which developed an extremely flat head for hiding in crevices, as can be seen by looking at its near relatives. However, in this species, the head has become so flattened that it assists in gliding from tree to treean exaptation. Within cells, molecular machines such as the bacterial flagella[215] and protein sorting machinery[216] evolved by the recruitment of several pre-existing proteins that previously had different functions.[166] Another example is the recruitment of enzymes from glycolysis and xenobiotic metabolism to serve as structural proteins called crystallins within the lenses of organisms' eyes.[217][218]

An area of current investigation in evolutionary developmental biology is the developmental basis of adaptations and exaptations.[219] This research addresses the origin and evolution of embryonic development and how modifications of development and developmental processes produce novel features.[220] These studies have shown that evolution can alter development to produce new structures, such as embryonic bone structures that develop into the jaw in other animals instead forming part of the middle ear in mammals.[221] It is also possible for structures that have been lost in evolution to reappear due to changes in developmental genes, such as a mutation in chickens causing embryos to grow teeth similar to those of crocodiles.[222] It is now becoming clear that most alterations in the form of organisms are due to changes in a small set of conserved genes.[223]

Interactions between organisms can produce both conflict and cooperation. When the interaction is between pairs of species, such as a pathogen and a host, or a predator and its prey, these species can develop matched sets of adaptations. Here, the evolution of one species causes adaptations in a second species. These changes in the second species then, in turn, cause new adaptations in the first species. This cycle of selection and response is called coevolution.[224] An example is the production of tetrodotoxin in the rough-skinned newt and the evolution of tetrodotoxin resistance in its predator, the common garter snake. In this predator-prey pair, an evolutionary arms race has produced high levels of toxin in the newt and correspondingly high levels of toxin resistance in the snake.[225]

Not all co-evolved interactions between species involve conflict.[226] Many cases of mutually beneficial interactions have evolved. For instance, an extreme cooperation exists between plants and the mycorrhizal fungi that grow on their roots and aid the plant in absorbing nutrients from the soil.[227] This is a reciprocal relationship as the plants provide the fungi with sugars from photosynthesis. Here, the fungi actually grow inside plant cells, allowing them to exchange nutrients with their hosts, while sending signals that suppress the plant immune system.[228]

Coalitions between organisms of the same species have also evolved. An extreme case is the eusociality found in social insects, such as bees, termites and ants, where sterile insects feed and guard the small number of organisms in a colony that are able to reproduce. On an even smaller scale, the somatic cells that make up the body of an animal limit their reproduction so they can maintain a stable organism, which then supports a small number of the animal's germ cells to produce offspring. Here, somatic cells respond to specific signals that instruct them whether to grow, remain as they are, or die. If cells ignore these signals and multiply inappropriately, their uncontrolled growth
causes cancer.[229]

Such cooperation within species may have evolved through the process of kin selection, which is where one organism acts to help raise a relative's offspring.[230] This activity is selected for because if the helping individual contains alleles which promote the helping activity, it is likely that its kin will also contain these alleles and thus those alleles will be passed on.[231] Other processes that may promote cooperation include group selection, where cooperation provides benefits to a group of organisms.[232]

Speciation is the process where a species diverges into two or more descendant species.[233]

There are multiple ways to define the concept of "species." The choice of definition is dependent on the particularities of the species concerned.[234] For example, some species concepts apply more readily toward sexually reproducing organisms while others lend themselves better toward asexual organisms. Despite the diversity of various species concepts, these various concepts can be placed into one of three broad philosophical approaches: interbreeding, ecological and phylogenetic.[235] The Biological Species Concept (BSC) is a classic example of the interbreeding approach. Defined by Ernst Mayr in 1942, the BSC states that "species are groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups."[236] Despite its wide and long-term use, the BSC like others is not without controversy, for example because these concepts cannot be applied to prokaryotes,[237] and this is called the species problem.[234] Some researchers have attempted a unifying monistic definition of species, while others adopt a pluralistic approach and suggest that there may be different ways to logically interpret the definition of a species.[234][235]

Barriers to reproduction between two diverging sexual populations are required for the populations to become new species. Gene flow may slow this process by spreading the new genetic variants also to the other populations. Depending on how far two species have diverged since their most recent common ancestor, it may still be possible for them to produce offspring, as with horses and donkeys mating to produce mules.[238] Such hybrids are generally infertile. In this case, closely related species may regularly interbreed, but hybrids will be selected against and the species will remain distinct. However, viable hybrids are occasionally formed and these new species can either have properties intermediate between their parent species, or possess a totally new phenotype.[239] The importance of hybridisation in producing new species of animals is unclear, although cases have been seen in many types of animals,[240] with the gray tree frog being a particularly well-studied example.[241]

Speciation has been observed multiple times under both controlled laboratory conditions and in nature.[242] In sexually reproducing organisms, speciation results from reproductive isolation followed by genealogical divergence. There are four mechanisms for speciation. The most common in animals is allopatric speciation, which occurs in populations initially isolated geographically, such as by habitat fragmentation or migration. Selection under these conditions can produce very rapid changes in the appearance and behaviour of organisms.[243][244] As selection and drift act independently on populations isolated from the rest of their species, separation may eventually produce organisms that cannot interbreed.[245]

The second mechanism of speciation is peripatric speciation, which occurs when small populations of organisms become isolated in a new environment. This differs from allopatric speciation in that the isolated populations are numerically much smaller than the parental population. Here, the founder effect causes rapid speciation after an increase in inbreeding increases selection on homozygotes, leading to rapid genetic change.[246]

The third mechanism of speciation is parapatric speciation. This is similar to peripatric speciation in that a small population enters a new habitat, but differs in that there is no physical separation between these two populations. Instead, speciation results from the evolution of mechanisms that reduce gene flow between the two populations.[233] Generally this occurs when there has been a drastic change in the environment within the parental species' habitat. One example is the grass Anthoxanthum odoratum, which can undergo parapatric speciation in response to localised metal pollution from mines.[247] Here, plants evolve that have resistance to high levels of metals in the soil. Selection against interbreeding with the metal-sensitive parental population produced a gradual change in the flowering time of the metal-resistant plants, which eventually produced complete reproductive isolation. Selection against hybrids between the two populations may cause reinforcement, which is the evolution of traits that promote mating within a species, as well as character displacement, which is when two species become more distinct in appearance.[248]

Finally, in sympatric speciation species diverge without geographic isolation or changes in habitat. This form is rare since even a small amount of gene flow may remove genetic differences between parts of a population.[249] Generally, sympatric speciation in animals requires the evolution of both genetic differences and non-random mating, to allow reproductive isolation to evolve.[250]

One type of sympatric speciation involves crossbreeding of two related species to produce a new hybrid species. This is not common in animals as animal hybrids are usually sterile. This is because during meiosis the homologous chromosomes from each parent are from different species and cannot successfully pair. However, it is more common in plants because plants often double their number of chromosomes, to form polyploids.[251] This allows the chromosomes from each parental species to form matching pairs during meiosis, since each parent's chromosomes are represented by a pair already.[252] An example of such a speciation event is when the plant species Arabidopsis thaliana and Arabidopsis arenosa crossbred to give the new species Arabidopsis suecica.[253] This happened about 20,000 years ago,[254] and the speciation process has been repeated in the laboratory, which allows the study of the genetic mechanisms involved in this process.[255] Indeed, chromosome doubling within a species may be a common cause of reproductive isolation, as half the doubled chromosomes will be unmatched when breeding with undoubled organisms.[256]

Speciation events are important in the theory of punctuated equilibrium, which accounts for the pattern in the fossil record of short "bursts" of evolution interspersed with relatively long periods of stasis, where species remain relatively unchanged.[257] In this theory, speciation and rapid evolution are linked, with natural selection and genetic drift acting most strongly on organisms undergoing speciation in novel habitats or small populations. As a result, the periods of stasis in the fossil record correspond to the parental population and the organisms undergoing speciation and rapid evolution are found in small populations or geographically restricted habitats and therefore rarely being preserved as fossils.[170]

Extinction is the disappearance of an entire species. Extinction is not an unusual event, as species regularly appear through speciation and disappear through extinction.[258] Nearly all animal and plant species that have lived on Earth are now extinct,[259] and extinction appear
s to be the ultimate fate of all species.[260] These extinctions have happened continuously throughout the history of life, although the rate of extinction spikes in occasional mass extinction events.[261] The CretaceousPaleogene extinction event, during which the non-avian dinosaurs became extinct, is the most well-known, but the earlier PermianTriassic extinction event was even more severe, with approximately 96% of all marine species driven to extinction.[261] The Holocene extinction event is an ongoing mass extinction associated with humanity's expansion across the globe over the past few thousand years. Present-day extinction rates are 1001000 times greater than the background rate and up to 30% of current species may be extinct by the mid 21st century.[262] Human activities are now the primary cause of the ongoing extinction event;[263]global warming may further accelerate it in the future.[264]

The role of extinction in evolution is not very well understood and may depend on which type of extinction is considered.[261] The causes of the continuous "low-level" extinction events, which form the majority of extinctions, may be the result of competition between species for limited resources (the competitive exclusion principle).[66] If one species can out-compete another, this could produce species selection, with the fitter species surviving and the other species being driven to extinction.[131] The intermittent mass extinctions are also important, but instead of acting as a selective force, they drastically reduce diversity in a nonspecific manner and promote bursts of rapid evolution and speciation in survivors.[265]

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The Earth is about 4.54 billion years old.[266][267][268] The earliest undisputed evidence of life on Earth dates from at least 3.5 billion years ago,[7][269] during the Eoarchean Era after a geological crust started to solidify following the earlier molten Hadean Eon. Microbial mat fossils have been found in 3.48 billion-year-old sandstone in Western Australia.[12][13][14] Other early physical evidence of a biogenic substance is graphite in 3.7 billion-year-old metasedimentary rocks discovered in Western Greenland[11] as well as "remains of biotic life" found in 4.1 billion-year-old rocks in Western Australia.[8][9] According to one of the researchers, "If life arose relatively quickly on Earth then it could be common in the universe."[8]

More than 99 percent of all species, amounting to over five billion species,[270] that ever lived on Earth are estimated to be extinct.[16][17] Estimates on the number of Earth's current species range from 10 million to 14 million,[18] of which about 1.2 million have been documented and over 86 percent have not yet been described.[19]

Highly energetic chemistry is thought to have produced a self-replicating molecule around 4 billion years ago, and half a billion years later the last common ancestor of all life existed.[5] The current scientific consensus is that the complex biochemistry that makes up life came from simpler chemical reactions.[271] The beginning of life may have included self-replicating molecules such as RNA[272] and the assembly of simple cells.[273]

All organisms on Earth are descended from a common ancestor or ancestral gene pool.[200][274] Current species are a stage in the process of evolution, with their diversity the product of a long series of speciation and extinction events.[275] The common descent of organisms was first deduced from four simple facts about organisms: First, they have geographic distributions that cannot be explained by local adaptation. Second, the diversity of life is not a set of completely unique organisms, but organisms that share morphological similarities. Third, vestigial traits with no clear purpose resemble functional ancestral traits and finally, that organisms can be classified using these similarities into a hierarchy of nested groupssimilar to a family tree.[276] However, modern research has suggested that, due to horizontal gene transfer, this "tree of life" may be more complicated than a simple branching tree since some genes have spread independently between distantly related species.[277][278]

Past species have also left records of their evolutionary history. Fossils, along with the comparative anatomy of present-day organisms, constitute the morphological, or anatomical, record.[279] By comparing the anatomies of both modern and extinct species, paleontologists can infer the lineages of those species. However, this approach is most successful for organisms that had hard body parts, such as shells, bones or teeth. Further, as prokaryotes such as bacteria and archaea share a limited set of common morphologies, their fossils do not provide information on their ancestry.

More recently, evidence for common descent has come from the study of biochemical similarities between organisms. For example, all living cells use the same basic set of nucleotides and amino acids.[280] The development of molecular genetics has revealed the record of evolution left in organisms' genomes: dating when species diverged through the molecular clock produced by mutations.[281] For example, these DNA sequence comparisons have revealed that humans and chimpanzees share 98% of their genomes and analysing the few areas where they differ helps shed light on when the common ancestor of these species existed.[282]

Prokaryotes inhabited the Earth from approximately 34 billion years ago.[284][285] No obvious changes in morphology or cellular organisation occurred in these organisms over the next few billion years.[286] The eukaryotic cells emerged between 1.62.7 billion years ago. The next major change in cell structure came when bacteria were engulfed by eukaryotic cells, in a cooperative association called endosymbiosis.[287][288] The engulfed bacteria and the host cell then underwent coevolution, with the bacteria evolving into either mitochondria or hydrogenosomes.[289] Another engulfment of cyanobacterial-like organisms led to the formation of chloroplasts in algae and plants.[290]

The history of life was that of the unicellular eukaryotes, prokaryotes and archaea until about 610 million years ago when multicellular organisms began to appear in the oceans in the Ediacaran period.[284][291] The evolution of multicellularity occurred in multiple independent events, in organisms as diverse as sponges, brown algae, cyanobacteria, slime moulds and myxobacteria.[292] In January 2016, scientists reported that, about 800 million years ago, a minor genetic change in a single molecule called GK-PID may have allowed organisms to go from a single cell organism to one of many cells.[293]

Soon after the emergence of these first multicellular organisms, a remarkable amount of biological diversity appeared over approximately 10 million years, in an event called the Cambrian explosion. Here, the majority of types of modern animals appeared in the fossil record, as well as unique lineages that subsequently became extinct.[294] Various triggers for the Cambrian explosion have been proposed, including the accumulation of oxygen in the atmosphere from photosynthesis.[295]

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