Human[1] Temporal range: 0.1950Ma Middle Pleistocene Recent An adult human male (left) and female (right) in Northern Thailand. Scientific classification Kingdom: Animalia Phylum: Chordata Clade: Synapsida Class: Mammalia Order: Primates Suborder: Haplorhini Family: Hominidae Tribe: Hominini Genus: Homo Species: H.sapiens Binomial name Homo sapiens Linnaeus, 1758 Subspecies

Homo sapiens idaltu White et al., 2003 Homo sapiens sapiens

Modern humans (Homo sapiens, primarily ssp. Homo sapiens sapiens) are the only extant members of Hominina clade (or human clade), a branch of the taxonomical tribe Hominini belonging to the family of great apes. They are characterized by erect posture and bipedal locomotion; manual dexterity and increased tool use, compared to other animals; and a general trend toward larger, more complex brains and societies.[3][4]

Early homininsparticularly the australopithecines, whose brains and anatomy are in many ways more similar to ancestral non-human apesare less often referred to as "human" than hominins of the genus Homo.[5] Several of these hominins used fire, occupied much of Eurasia, and gave rise to anatomically modern Homo sapiens in Africa about 200,000 years ago.[6][7] They began to exhibit evidence of behavioral modernity around 50,000 years ago. In several waves of migration, anatomically modern humans ventured out of Africa and populated most of the world.[8]

The spread of humans and their large and increasing population has had a profound impact on large areas of the environment and millions of native species worldwide. Advantages that explain this evolutionary success include a relatively larger brain with a particularly well-developed neocortex, prefrontal cortex and temporal lobes, which enable high levels of abstract reasoning, language, problem solving, sociality, and culture through social learning. Humans use tools to a much higher degree than any other animal, are the only extant species known to build fires and cook their food, and are the only extant species to clothe themselves and create and use numerous other technologies and arts.

Humans are uniquely adept at utilizing systems of symbolic communication (such as language and art) for self-expression and the exchange of ideas, and for organizing themselves into purposeful groups. Humans create complex social structures composed of many cooperating and competing groups, from families and kinship networks to political states. Social interactions between humans have established an extremely wide variety of values,[9]social norms, and rituals, which together form the basis of human society. Curiosity and the human desire to understand and influence the environment and to explain and manipulate phenomena (or events) has provided the foundation for developing science, philosophy, mythology, religion, anthropology, and numerous other fields of knowledge.

Though most of human existence has been sustained by hunting and gathering in band societies,[10] increasing numbers of human societies began to practice sedentary agriculture approximately some 10,000 years ago,[11] domesticating plants and animals, thus allowing for the growth of civilization. These human societies subsequently expanded in size, establishing various forms of government, religion, and culture around the world, unifying people within regions to form states and empires. The rapid advancement of scientific and medical understanding in the 19th and 20th centuries led to the development of fuel-driven technologies and increased lifespans, causing the human population to rise exponentially. By February 2016, the global human population had exceeded 7.3 billion.[12]

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In common usage, the word "human" generally refers to the only extant species of the genus Homoanatomically and behaviorally modern Homo sapiens.

In scientific terms, the meanings of "hominid" and "hominin" have changed during the recent decades with advances in the discovery and study of the fossil ancestors of modern humans. The previously clear boundary between humans and apes has blurred, resulting in now acknowledging the hominids as encompassing multiple species, and Homo and close relatives since the split from chimpanzees as the only hominins. There is also a distinction between anatomically modern humans and Archaic Homo sapiens, the earliest fossil members of the species.

The English adjective human is a Middle English loanword from Old French humain, ultimately from Latin hmnus, the adjective form of hom "man." The word's use as a noun (with a plural: humans) dates to the 16th century.[13] The native English term man can refer to the species generally (a synonym for humanity), and could formerly refer to specific individuals of either sex, though this latter use is now obsolete.[14]

The species binomial Homo sapiens was coined by Carl Linnaeus in his 18th century work Systema Naturae.[15] The generic name Homo is a learned 18th century derivation from Latin hom "man," ultimately "earthly being" (Old Latin hem a cognate to Old English guma "man," from PIE demon-, meaning "earth" or "ground").[16] The species-name sapiens means "wise" or "sapient." Note that the Latin word homo refers to humans of either gender, and that sapiens is the singular form (while there is no such word as sapien).[17]

The genus Homo evolved and diverged from other hominins in Africa, after the human clade split from the chimpanzee lineage of the hominids (great apes) branch of the primates. Modern humans, defined as the species Homo sapiens or specifically to the single extant subspecies Homo sapiens sapiens, proceeded to colonize all the continents and larger islands, arriving in Eurasia 125,00060,000 years ago,[18][19]Australia around 40,000 years ago, the Americas around 15,000 years ago, and remote islands such as Hawaii, Easter Island, Madagascar, and New Zealand between the years 300 and 1280.[20][21]

The closest living relatives of humans are chimpanzees (genus Pan) and gorillas (genus Gorilla).[22] With the sequencing of both the human and chimpanzee genome, current estimates of similarity between human and chimpanzee DNA sequences range between 95% and 99%.[22][23][24] By using the technique called a molecular clock which estimates the time required for the number of divergent mutations to accumulate between two lineages, the approximate date for the split between lineages can be calculated. The gibbons (Hylobatidae) and orangutans (genus Pongo) were the first groups to split from the line leading to the humans, then gorillas (genus Gorilla) followed by the chimpanzees (genus Pan). The splitting date between human and chimpanzee lineages is placed around 48 million years ago during the late Miocene epoch.[25][26] During this split, chromosome 2 was formed from two other chromosomes, leaving humans with only 23 pairs of chromosomes, compared to 24 for the other apes.[27][28]

There is little fossil evidence for the divergence of the gorilla, chimpanzee and hominin lineages.[29][30] The earliest fossils that have been proposed as members of the hominin lineage are Sahelanthropus tchadensis dating from 7 million years ago, Orrorin tugenensis dating from 5.7 million years ago, and Ardipithecus kadabba dating to 5.6 million years ago. Each of these species has been argued to be a bipedal ancestor of later hominins, but all such claims are contested. It is also possible that any one of the three is an ancestor of another branch of African apes, or is an ancestor shared between hominins and other African Hominoidea (apes). The question of the relation between these early fossil species and the hominin lineage is still to be resolved. From these early species the australopithecines arose around 4 million years ago diverged into robust (also called Paranthropus) and gracile branches,[31] possibly one of which (such as A. garhi, dating to 2.5 million years ago) is a direct ancestor of the genus Homo.[32]

The earliest members of the genus Homo are Homo habilis which evolved around 2.8 million years ago.[33]Homo habilis has been considered the first species for which there is clear evidence of the use of stone tools. More recently, however, in 2015, stone tools, perhaps predating Homo habilis, have been discovered in northwestern Kenya that have been dated to 3.3 million years old.[34] Nonetheless, the brains of Homo habilis were about the same size as that of a chimpanzee, and their main adaptation was bipedalism as an adaptation to terrestrial living. During the next million years a process of encephalization began, and with the arrival of Homo erectus in the fossil record, cranial capacity had doubled. Homo erectus were the first of the hominina to leave Africa, and these species spread through Africa, Asia, and Europe between 1.3to1.8 million years ago. One population of H. erectus, also sometimes classified as a separate species Homo ergaster, stayed in Africa and evolved into Homo sapiens. It is believed that these species were the first to use fire and complex tools. The earliest transitional fossils between H. ergaster/erectus and archaic humans are from Africa such as Homo rhodesiensis, but seemingly transitional forms are also found at Dmanisi, Georgia. These descendants of African H. erectus spread through Eurasia from ca. 500,000 years ago evolving into H. antecessor, H. heidelbergensis and H. neanderthalensis. The earliest fossils of anatomically modern humans are from the Middle Paleolithic, about 200,000 years ago such as the Omo remains of Ethiopia and the fossils of Herto sometimes classified as Homo sapiens idaltu.[35] Later fossils of archaic Homo sapiens from Skhul in Israel and Southern Europe begin around 90,000 years ago.[36]

Human evolution is characterized by a number of morphological, developmental, physiological, and behavioral changes that have taken place since the split between the last common ancestor of humans and chimpanzees. The most significant of these adaptations are 1. bipedalism, 2. increased brain size, 3. lengthened ontogeny (gestation and infancy), 4. decreased sexual dimorphism (neoteny). The relationship between all these changes is the subject of ongoing debate.[37] Other significant morphological changes included the evolution of a power and precision grip, a change first occurring in H. erectus.[38]

Bipedalism is the basic adaption of the hominin line, and it is considered the main cause behind a suite of skeletal changes shared by all bipedal hominins. The earliest bipedal hominin is considered to be either Sahelanthropus[39] or Orrorin, with Ardipithecus, a full bipedal,[40] coming somewhat later.[citation needed] The knuckle walkers, the gorilla and chimpanzee, diverged around the same time, and either Sahelanthropus or Orrorin may be humans' last shared ancestor with those animals.[citation needed] The early bipedals eventually evolved into the australopithecines and later the genus Homo.[citation needed] There are several theories of the adaptational value of bipedalism. It is possible that bipedalism was favored because it freed up the hands for reaching and carrying food, because it saved energy during locomotion, because it enabled long distance running and hunting, or as a strategy for avoiding hyperthermia by reducing the surface exposed to direct sun.[citation needed]

The human species developed a much larger brain than that of other primates typically 1,330 cm3 in modern humans, over twice the size of that of a chimpanzee or gorilla.[41] The pattern of encephalization started with Homo habilis which at approximately 600cm3 had a brain slightly larger than chimpanzees, and continued with Homo erectus (8001100cm3), and reached a maximum in Neanderthals with an average size of 12001900cm3, larger even than Homo sapiens (but less encephalized).[42] The pattern of human postnatal brain growth differs from that of other apes (heterochrony), and allows for extended periods of social learning and language acquisition in juvenile humans. However, the differences between the structure of human brains and those of other apes may be even more significant than differences in size.[43][44][45][46] The increase in volume over time has affected different areas within the brain unequally the temporal lobes, which contain centers for language processing have increased disproportionately, as has the prefrontal cortex which has been related to complex decision making and moderating social behavior.[41] Encephalization has been tied to an increasing emphasis on meat in the diet,[47][48] or with the development of cooking,[49] and it has been proposed [50] that intelligence increased as a response to an increased necessity for solving social problems as human society became more complex.

The reduced degree of sexual dimorphism is primarily visible in the reduction of the male canine tooth relative to other ape species (except gibbons). Another important physiological change related to sexuality in humans was the evolution of hidden estrus. Humans are the only ape in which the female is fertile year round, and in which no special signals of fertility are produced by the body (such as genital swelling during estrus). Nonetheless humans retain a degree of sexual dimorphism in the distribution of body hair and subcutaneous fat, and in the overall size, males being around 25% larger than females. These changes taken together have been interpreted as a result of an increased emphasis on pair bonding as a possible solution to the requirement for increased parental investment due to the prolonged infancy of offspring.[citation needed]

By the beginning of the Upper Paleolithic period (50,000 BP), full behavioral modernity, including language, music and other cultural universals had developed.[51][52] As modern humans spread out from Africa they encountered other hominids such as Homo neanderthalensis and the so-called Denisovans. The nature of interaction between early humans and these sister species has been a long-standing source of controversy, the question being whether humans replaced these earlier species or whether they were in fact similar enough to interbreed, in which case these earlier populations may have contributed genetic material to modern humans.[53] Recent studies of the human and Neanderthal genomes suggest gene flow between archaic Homo sapiens and Neanderthals and Denisovans.[54][55][56] In March 2016, studies were published that suggest that modern humans bred with hominins, including Denisovans and Neanderthals, on multiple occasions.[57]

This dispersal out of Africa is estimated to have begun about 70,000 years BP from Northeast Africa. Current evidence suggests that there was only one such dispersal and that it only involved a few hundred individuals. The vast majority of humans stayed in Africa and adapted to a diverse array of environments.[58] Modern humans subsequently spread globally, replacing earlier hominins (either through competition or hybridization). They inhabited Eurasia and Oceania by 40,000 years BP, and the Americas at least 14,500 years BP.[59][60]

Until about 10,000 years ago, humans lived as hunter-gatherers. They gradually gained domination over much of the natural environment. They generally lived in small nomadic groups known as band societies, often in caves. The advent of agriculture prompted the Neolithic Revolution, when access to food surplus led to the formation of permanent human settlements, the domestication of animals and the use of metal tools for the first time in history. Agriculture encouraged trade and cooperation, and led to complex society.[citation needed]

The early civilizations of Mesopotamia, Egypt, India, China, Maya, Greece and Rome were some of the cradles of civilization.[61][62][63] The Late Middle Ages and the Early Modern Period saw the rise of revolutionary ideas and technologies. Over the next 500 years, exploration and European colonialism brought great parts of the world under European control, leading to later struggles for independence. The concept of the modern world as distinct from an ancient world is based on a rapid change progress in a brief period of time in many areas.[citation needed] Advances in all areas of human activity prompted new theories such as evolution and psychoanalysis, which changed humanity's views of itself.[citation needed] The Scientific Revolution, Technological Revolution and the Industrial Revolution up until the 19th century resulted in independent discoveries such as imaging technology, major innovations in transport, such as the airplane and automobile; energy development, such as coal and electricity.[64] This correlates with population growth (especially in America)[65] and higher life expectancy, the World population rapidly increased numerous times in the 19th and 20th centuries as nearly 10% of the 100 billion people lived in the past century.[66]

With the advent of the Information Age at the end of the 20th century, modern humans live in a world that has become increasingly globalized and interconnected. As of 2010, almost 2billion humans are able to communicate with each other via the Internet,[67] and 3.3 billion by mobile phone subscriptions.[68] Although interconnection between humans has encouraged the growth of science, art, discussion, and technology, it has also led to culture clashes and the development and use of weapons of mass destruction.[citation needed] Human civilization has led to environmental destruction and pollution significantly contributing to the ongoing mass extinction of other forms of life called the Holocene extinction event,[69] which may be further accelerated by global warming in the future.[70]

Early human settlements were dependent on proximity to water and, depending on the lifestyle, other natural resources used for subsistence, such as populations of animal prey for hunting and arable land for growing crops and grazing livestock. But humans have a great capacity for altering their habitats by means of technology, through irrigation, urban planning, construction, transport, manufacturing goods, deforestation and desertification. Deliberate habitat alteration is often done with the goals of increasing material wealth, increasing thermal comfort, improving the amount of food available, improving aesthetics, or improving ease of access to resources or other human settlements. With the advent of large-scale trade and transport infrastructure, proximity to these resources has become unnecessary, and in many places, these factors are no longer a driving force behind the growth and decline of a population. Nonetheless, the manner in which a habitat is altered is often a major determinant in population change.[citation needed]

Technology has allowed humans to colonize all of the continents and adapt to virtually all climates. Within the last century, humans have explored Antarctica, the ocean depths, and outer space, although large-scale colonization of these environments is not yet feasible. With a population of over seven billion, humans are among the most numerous of the large mammals. Most humans (61%) live in Asia. The remainder live in the Americas (14%), Africa (14%), Europe (11%), and Oceania (0.5%).[71]

Human habitation within closed ecological systems in hostile environments, such as Antarctica and outer space, is expensive, typically limited in duration, and restricted to scientific, military, or industrial expeditions. Life in space has been very sporadic, with no more than thirteen humans in space at any given time.[72] Between 1969 and 1972, two humans at a time spent brief intervals on the Moon. As of January 2017, no other celestial body has been visited by humans, although there has been a continuous human presence in space since the launch of the initial crew to inhabit the International Space Station on October 31, 2000.[73] However, other celestial bodies have been visited by human-made objects.[74][75][76]

Since 1800, the human population has increased from one billion[77] to over seven billion,[78] In 2004, some 2.5 billion out of 6.3 billion people (39.7%) lived in urban areas. In February 2008, the U.N. estimated that half the world's population would live in urban areas by the end of the year.[79] Problems for humans living in cities include various forms of pollution and crime,[80] especially in inner city and suburban slums. Both overall population numbers and the proportion residing in cities are expected to increase significantly in the coming decades.[81]

Humans have had a dramatic effect on the environment. Humans are apex predators, being rarely preyed upon by other species.[82] Currently, through land development, combustion of fossil fuels, and pollution, humans are thought to be the main contributor to global climate change.[83] If this continues at its current rate it is predicted that climate change will wipe out half of all plant and animal species over the next century.[84][85]

Most aspects of human physiology are closely homologous to corresponding aspects of animal physiology. The human body consists of the legs, the torso, the arms, the neck, and the head. An adult human body consists of about 100 trillion (1014) cells. The most commonly defined body systems in humans are the nervous, the cardiovascular, the circulatory, the digestive, the endocrine, the immune, the integumentary, the lymphatic, the muscoskeletal, the reproductive, the respiratory, and the urinary system.[86][87]

Humans, like most of the other apes, lack external tails, have several blood type systems, have opposable thumbs, and are sexually dimorphic. The comparatively minor anatomical differences between humans and chimpanzees are a result of human bipedalism. One difference is that humans have a far faster and more accurate throw than other animals. Humans are also among the best long-distance runners in the animal kingdom, but slower over short distances.[88][89] Humans' thinner body hair and more productive sweat glands help avoid heat exhaustion while running for long distances.[90]

As a consequence of bipedalism, human females have narrower birth canals. The construction of the human pelvis differs from other primates, as do the toes. A trade-off for these advantages of the modern human pelvis is that childbirth is more difficult and dangerous than in most mammals, especially given the larger head size of human babies compared to other primates. This means that human babies must turn around as they pass through the birth canal, which other primates do not do, and it makes humans the only species where females usually require help from their conspecifics (other members of their own species) to reduce the risks of birthing. As a partial evolutionary solution, human fetuses are born less developed and more vulnerable. Chimpanzee babies are cognitively more developed than human babies until the age of six months, when the rapid development of human brains surpasses chimpanzees. Another difference between women and chimpanzee females is that women go through the menopause and become unfertile decades before the end of their lives. All species of non-human apes are capable of giving birth until death. Menopause probably developed as it has provided an evolutionary advantage (more caring time) to young relatives.[89]

Apart from bipedalism, humans differ from chimpanzees mostly in smelling, hearing, digesting proteins, brain size, and the ability of language. Humans' brains are about three times bigger than in chimpanzees. More importantly, the brain to body ratio is much higher in humans than in chimpanzees, and humans have a significantly more developed cerebral cortex, with a larger number of neurons. The mental abilities of humans are remarkable compared to other apes. Humans' ability of speech is unique among primates. Humans are able to create new and complex ideas, and to develop technology, which is unprecedented among other organisms on Earth.[89]

It is estimated that the worldwide average height for an adult human male is about 172cm (5ft 712in),[citation needed] while the worldwide average height for adult human females is about 158cm (5ft 2in).[citation needed] Shrinkage of stature may begin in middle age in some individuals, but tends to be typical in the extremely aged.[91] Through history human populations have universally become taller, probably as a consequence of better nutrition, healthcare, and living conditions.[92] The average mass of an adult human is 5464kg (120140lb) for females and 7683kg (168183lb) for males.[93] Like many other conditions, body weight and body type is influenced by both genetic susceptibility and environment and varies greatly among individuals. (see obesity)[94][95]

Although humans appear hairless compared to other primates, with notable hair growth occurring chiefly on the top of the head, underarms and pubic area, the average human has more hair follicles on his or her body than the average chimpanzee. The main distinction is that human hairs are shorter, finer, and less heavily pigmented than the average chimpanzee's, thus making them harder to see.[96] Humans have about 2 million sweat glands spread over their entire bodies, many more than chimpanzees, whose sweat glands are scarce and are mainly located on the palm of the hand and on the soles of the feet.[97]

The dental formula of humans is: 2.1.2.32.1.2.3. Humans have proportionately shorter palates and much smaller teeth than other primates. They are the only primates to have short, relatively flush canine teeth. Humans have characteristically crowded teeth, with gaps from lost teeth usually closing up quickly in young individuals. Humans are gradually losing their wisdom teeth, with some individuals having them congenitally absent.[98]

Like all mammals, humans are a diploid eukaryotic species. Each somatic cell has two sets of 23 chromosomes, each set received from one parent; gametes have only one set of chromosomes, which is a mixture of the two parental sets. Among the 23 pairs of chromosomes there are 22 pairs of autosomes and one pair of sex chromosomes. Like other mammals, humans have an XY sex-determination system, so that females have the sex chromosomes XX and males have XY.[99]

One human genome was sequenced in full in 2003, and currently efforts are being made to achieve a sample of the genetic diversity of the species (see International HapMap Project). By present estimates, humans have approximately 22,000 genes.[100] The variation in human DNA is very small compared to other species, possibly suggesting a population bottleneck during the Late Pleistocene (around 100,000 years ago), in which the human population was reduced to a small number of breeding pairs.[101][102]Nucleotide diversity is based on single mutations called single nucleotide polymorphisms (SNPs). The nucleotide diversity between humans is about 0.1%, i.e. 1 difference per 1,000 base pairs.[103][104] A difference of 1 in 1,000 nucleotides between two humans chosen at random amounts to about 3 million nucleotide differences, since the human genome has about 3 billion nucleotides. Most of these single nucleotide polymorphisms (SNPs) are neutral but some (about 3 to 5%) are functional and influence phenotypic differences between humans through alleles.[citation needed]

By comparing the parts of the genome that are not under natural selection and which therefore accumulate mutations at a fairly steady rate, it is possible to reconstruct a genetic tree incorporating the entire human species since the last shared ancestor. Each time a certain mutation (SNP) appears in an individual and is passed on to his or her descendants, a haplogroup is formed including all of the descendants of the individual who will also carry that mutation. By comparing mitochondrial DNA, which is inherited only from the mother, geneticists have concluded that the last female common ancestor whose genetic marker is found in all modern humans, the so-called mitochondrial Eve, must have lived around 90,000 to 200,000 years ago.[105][106][107]

Human accelerated regions, first described in August 2006,[108][109] are a set of 49 segments of the human genome that are conserved throughout vertebrate evolution but are strikingly different in humans. They are named according to their degree of difference between humans and their nearest animal relative (chimpanzees) (HAR1 showing the largest degree of human-chimpanzee differences). Found by scanning through genomic databases of multiple species, some of these highly mutated areas may contribute to human-specific traits.[citation needed]

The forces of natural selection have continued to operate on human populations, with evidence that certain regions of the genome display directional selection in the past 15,000 years.[110]

As with other mammals, human reproduction takes place as internal fertilization by sexual intercourse. During this process, the male inserts his erect penis into the female's vagina and ejaculates semen, which contains sperm. The sperm travels through the vagina and cervix into the uterus or Fallopian tubes for fertilization of the ovum. Upon fertilization and implantation, gestation then occurs within the female's uterus.

The zygote divides inside the female's uterus to become an embryo, which over a period of 38 weeks (9 months) of gestation becomes a fetus. After this span of time, the fully grown fetus is birthed from the woman's body and breathes independently as an infant for the first time. At this point, most modern cultures recognize the baby as a person entitled to the full protection of the law, though some jurisdictions extend various levels of personhood earlier to human fetuses while they remain in the uterus.

Compared with other species, human childbirth is dangerous. Painful labors lasting 24 hours or more are not uncommon and sometimes lead to the death of the mother, the child or both.[111] This is because of both the relatively large fetal head circumference and the mother's relatively narrow pelvis.[112][113] The chances of a successful labor increased significantly during the 20th century in wealthier countries with the advent of new medical technologies. In contrast, pregnancy and natural childbirth remain hazardous ordeals in developing regions of the world, with maternal death rates approximately 100 times greater than in developed countries.[114]

In developed countries, infants are typically 34kg (69pounds) in weight and 5060cm (2024inches) in height at birth.[115][not in citation given] However, low birth weight is common in developing countries, and contributes to the high levels of infant mortality in these regions.[116] Helpless at birth, humans continue to grow for some years, typically reaching sexual maturity at 12 to 15years of age. Females continue to develop physically until around the age of 18, whereas male development continues until around age 21. The human life span can be split into a number of stages: infancy, childhood, adolescence, young adulthood, adulthood and old age. The lengths of these stages, however, have varied across cultures and time periods. Compared to other primates, humans experience an unusually rapid growth spurt during adolescence, where the body grows 25% in size. Chimpanzees, for example, grow only 14%, with no pronounced spurt.[117] The presence of the growth spurt is probably necessary to keep children physically small until they are psychologically mature. Humans are one of the few species in which females undergo menopause. It has been proposed that menopause increases a woman's overall reproductive success by allowing her to invest more time and resources in her existing offspring, and in turn their children (the grandmother hypothesis), rather than by continuing to bear children into old age.[118][119]

For various reasons, including biological/genetic causes,[120] women live on average about four years longer than menas of 2013 the global average life expectancy at birth of a girl is estimated at 70.2 years compared to 66.1 for a boy.[121] There are significant geographical variations in human life expectancy, mostly correlated with economic developmentfor example life expectancy at birth in Hong Kong is 84.8years for girls and 78.9 for boys, while in Swaziland, primarily because of AIDS, it is 31.3years for both sexes.[122] The developed world is generally aging, with the median age around 40years. In the developing world the median age is between 15 and 20years. While one in five Europeans is 60years of age or older, only one in twenty Africans is 60years of age or older.[123] The number of centenarians (humans of age 100years or older) in the world was estimated by the United Nations at 210,000 in 2002.[124] At least one person, Jeanne Calment, is known to have reached the age of 122years;[125] higher ages have been claimed but they are not well substantiated.

Humans are omnivorous, capable of consuming a wide variety of plant and animal material.[126][127] Varying with available food sources in regions of habitation, and also varying with cultural and religious norms, human groups have adopted a range of diets, from purely vegetarian to primarily carnivorous. In some cases, dietary restrictions in humans can lead to deficiency diseases; however, stable human groups have adapted to many dietary patterns through both genetic specialization and cultural conventions to use nutritionally balanced food sources.[128] The human diet is prominently reflected in human culture, and has led to the development of food science.

Until the development of agriculture approximately 10,000 years ago, Homo sapiens employed a hunter-gatherer method as their sole means of food collection. This involved combining stationary food sources (such as fruits, grains, tubers, and mushrooms, insect larvae and aquatic mollusks) with wild game, which must be hunted and killed in order to be consumed.[129] It has been proposed that humans have used fire to prepare and cook food since the time of Homo erectus.[130] Around ten thousand years ago, humans developed agriculture,[131] which substantially altered their diet. This change in diet may also have altered human biology; with the spread of dairy farming providing a new and rich source of food, leading to the evolution of the ability to digest lactose in some adults.[132][133] Agriculture led to increased populations, the development of cities, and because of increased population density, the wider spread of infectious diseases. The types of food consumed, and the way in which they are prepared, have varied widely by time, location, and culture.

In general, humans can survive for two to eight weeks without food, depending on stored body fat. Survival without water is usually limited to three or four days. About 36 million humans die every year from causes directly or indirectly related to starvation.[134] Childhood malnutrition is also common and contributes to the global burden of disease.[135] However global food distribution is not even, and obesity among some human populations has increased rapidly, leading to health complications and increased mortality in some developed, and a few developing countries. Worldwide over one billion people are obese,[136] while in the United States 35% of people are obese, leading to this being described as an "obesity epidemic."[137] Obesity is caused by consuming more calories than are expended, so excessive weight gain is usually caused by an energy-dense diet.[136]

No two humansnot even monozygotic twinsare genetically identical. Genes and environment influence human biological variation from visible characteristics to physiology to disease susceptibly to mental abilities. The exact influence of genes and environment on certain traits is not well understood.[138][139]

Most current genetic and archaeological evidence supports a recent single origin of modern humans in East Africa,[140] with first migrations placed at 60,000 years ago. Compared to the great apes, human gene sequenceseven among African populationsare remarkably homogeneous.[141] On average, genetic similarity between any two humans is 99.9%.[142][143] There is about 23 times more genetic diversity within the wild chimpanzee population, than in the entire human gene pool.[144][145][146]

The human body's ability to adapt to different environmental stresses is remarkable, allowing humans to acclimatize to a wide variety of temperatures, humidity, and altitudes. As a result, humans are a cosmopolitan species found in almost all regions of the world, including tropical rainforests, arid desert, extremely cold arctic regions, and heavily polluted cities. Most other species are confined to a few geographical areas by their limited adaptability.[147]

There is biological variation in the human specieswith traits such as blood type, cranial features, eye color, hair color and type, height and build, and skin color varying across the globe. Human body types vary substantially. The typical height of an adult human is between 1.4m and 1.9m (4ft 7 in and 6ft 3 in), although this varies significantly depending, among other things, on sex and ethnic origin.[148][149] Body size is partly determined by genes and is also significantly influenced by environmental factors such as diet, exercise, and sleep patterns, especially as an influence in childhood. Adult height for each sex in a particular ethnic group approximately follows a normal distribution. Those aspects of genetic variation that give clues to human evolutionary history, or are relevant to medical research, have received particular attention. For example, the genes that allow adult humans to digest lactose are present in high frequencies in populations that have long histories of cattle domestication, suggesting natural selection having favored that gene in populations that depend on cow milk. Some hereditary diseases such as sickle cell anemia are frequent in populations where malaria has been endemic throughout historyit is believed that the same gene gives increased resistance to malaria among those who are unaffected carriers of the gene. Similarly, populations that have for a long time inhabited specific climates, such as arctic or tropical regions or high altitudes, tend to have developed specific phenotypes that are beneficial for conserving energy in those environmentsshort stature and stocky build in cold regions, tall and lanky in hot regions, and with high lung capacities at high altitudes. Similarly, skin color varies clinally with darker skin around the equatorwhere the added protection from the sun's ultraviolet radiation is thought to give an evolutionary advantageand lighter skin tones closer to the poles.[150][151][152][153]

The hue of human skin and hair is determined by the presence of pigments called melanins. Human skin color can range from darkest brown to lightest peach, or even nearly white or colorless in cases of albinism.[146] Human hair ranges in color from white to red to blond to brown to black, which is most frequent.[154] Hair color depends on the amount of melanin (an effective sun blocking pigment) in the skin and hair, with hair melanin concentrations in hair fading with increased age, leading to grey or even white hair. Most researchers believe that skin darkening is an adaptation that evolved as protection against ultraviolet solar radiation, which also helps balancing folate, which is destroyed by ultraviolet radiation. Light skin pigmentation protects against depletion of vitamin D, which requires sunlight to make.[155] Skin pigmentation of contemporary humans is clinally distributed across the planet, and in general correlates with the level of ultraviolet radiation in a particular geographic area. Human skin also has a capacity to darken (tan) in response to exposure to ultraviolet radiation.[156][157][158]

Within the human species, the greatest degree of genetic variation exists between males and females. While the nucleotide genetic variation of individuals of the same sex across global populations is no greater than 0.1%, the genetic difference between males and females is between 1% and 2%. Although different in nature[clarification needed], this approaches the genetic differentiation between men and male chimpanzees or women and female chimpanzees. The genetic difference between sexes contributes to anatomical, hormonal, neural, and physiological differences between men and women, although the exact degree and nature of social and environmental influences on sexes are not completely understood. Males on average are 15% heavier and 15cm taller than females. There is a difference between body types, body organs and systems, hormonal levels, sensory systems, and muscle mass between sexes. On average, there is a difference of about 4050% in upper body strength and 2030% in lower body strength between men and women. Women generally have a higher body fat percentage than men. Women have lighter skin than men of the same population; this has been explained by a higher need for vitamin D (which is synthesized by sunlight) in females during pregnancy and lactation. As there are chromosomal differences between females and males, some X and Y chromosome related conditions and disorders only affect either men or women. Other conditional differences between males and females are not related to sex chromosomes. Even after allowing for body weight and volume, the male voice is usually an octave deeper than the female voice. Women have a longer life span in almost every population around the world.[160][161][162][163][164][165][166][167][168]

Males typically have larger tracheae and branching bronchi, with about 30% greater lung volume per unit body mass. They have larger hearts, 10% higher red blood cell count, and higher hemoglobin, hence greater oxygen-carrying capacity. They also have higher circulating clotting factors (vitamin K, prothrombin and platelets). These differences lead to faster healing of wounds and higher peripheral pain tolerance.[169] Females typically have more white blood cells (stored and circulating), more granulocytes and B and T lymphocytes. Additionally, they produce more antibodies at a faster rate than males. Hence they develop fewer infectious diseases and these continue for shorter periods.[169]Ethologists argue that females, interacting with other females and multiple offspring in social groups, have experienced such traits as a selective advantage.[170][171][172][173][174] According to Daly and Wilson, "The sexes differ more in human beings than in monogamous mammals, but much less than in extremely polygamous mammals."[175] But given that sexual dimorphism in the closest relatives of humans is much greater than among humans, the human clade must be considered to be characterized by decreasing sexual dimorphism, probably due to less competitive mating patterns. One proposed explanation is that human sexuality has developed more in common with its close relative the bonobo, which exhibits similar sexual dimorphism, is polygynandrous and uses recreational sex to reinforce social bonds and reduce aggression.[176]

Humans of the same sex are 99.9% genetically identical. There is extremely little variation between human geographical populations, and most of the variation that does occur is at the personal level within local areas, and not between populations.[146][177][178] Of the 0.1% of human genetic differentiation, 85% exists within any randomly chosen local population, be they Italians, Koreans, or Kurds. Two randomly chosen Koreans may be genetically as different as a Korean and an Italian. Any ethnic group contains 85% of the human genetic diversity of the world. Genetic data shows that no matter how population groups are defined, two people from the same population group are about as different from each other as two people from any two different population groups.[146][179][180][181]

Current genetic research has demonstrated that humans on the African continent are the most genetically diverse.[182] There is more human genetic diversity in Africa than anywhere else on Earth. The genetic structure of Africans was traced to 14 ancestral population clusters. Human genetic diversity decreases in native populations with migratory distance from Africa and this is thought to be the result of bottlenecks during human migration.[183][184] Humans have lived in Africa for the longest time, which has allowed accumulation of a higher diversity of genetic mutations in these populations. Only part of Africa's population migrated out of the continent, bringing just part of the original African genetic variety with them. African populations harbor genetic alleles that are not found in other places of the world. All the common alleles found in populations outside of Africa are found on the African continent.[146]

Geographical distribution of human variation is complex and constantly shifts through time which reflects complicated human evolutionary history. Most human biological variation is clinally distributed and blends gradually from one area to the next. Groups of people around the world have different frequencies of polymorphic genes. Furthermore, different traits are non-concordant and each have different clinal distribution. Adaptability varies both from person to person and from population to population. The most efficient adaptive responses are found in geographical populations where the environmental stimuli are the strongest (e.g. Tibetans are highly adapted to high altitudes). The clinal geographic genetic variation is further complicated by the migration and mixing between human populations which has been occurring since prehistoric times.[146][185][186][187][188][189]

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