5 Skulls That Shook Up the Story of Human Evolution – Discover Magazine

This story appeared in the June 2020 issue as "Head Cases."Subscribeto Discover magazine for more stories like this.

These are heady times for paleoanthropologists. In the opening decades of the 21st century, new discoveries have refined and revised the story of human evolution at an unprecedented rate.Researchers have added four new members to the genus Homo: South Africas Homo naledi, Asias Denisovans, Indonesias hobbit H. floresiensis and, just last year, its neighbor in the Philippines, H. luzonensis. Improvements in extracting and analyzing ancient DNA and preserved proteins have created molecular-level tools capable of determining relationships between both individuals and species.

Much of the new research involves high-tech analysis of fragmentary fossils or genetic code but no single tooth, scrap of finger bone or shiny piece of lab equipment captures our attention quite like a skull.

The head, and especially the face, is the part of a person that we most commonly engage with, and also usually self identify with, says University of Tbingen paleoanthropologist Katerina Harvati, who co authored a 2019 study in Nature Ecology & Evolution on the evolutionary history of the human face.

Fossil skulls, she says, have the ability to convey not only a lot of information about the species to scientists, but also can give an immediate, intuitive impression of what an individual would have been like as a person when alive and can therefore more easily capture the imagination of both scientists and the public.

And that, quite frankly, is where things get messy.

Christoph Zollikofer, an anthropologist at the University of Zurich, cautions that the sheer charisma of fossil skulls of being face-to-face with an individual who lived thousands or even millions of years ago can lead even the professionals astray.

Our mindsets are like shop display windows that separate us from the mannequins: We look at these fossil skulls through our own mirror images and imaginations, Zollikofer says.

When researchers with different visions look at the same ancient skull, often heated debates erupt. Such arguments have become more common in the 21st century with the discovery of each new fossil that challenges conventional thinking about the evolution of hominins humans and our nearest ancestor and kin species. The more fossils paleoanthropologists find, and the more methods they have to study them, the murkier the story of human evolution seems.

When there were few fossils, it was very easy to make a line and show a very linear evolution, says Silvana Condemi, a paleoanthropologist at Aix-Marseille University in France and co-author of A Pocket History of Human Evolution: How We Became Sapiens. Now we have a lot more fossils, and we see its not so simple, it was much more complex, but we dont have enough fossils to understand it.

She adds: We have new tools. We try to be rigorous. We do our best. But a single discovery can still change everything.

Five fossil skull finds, each with its own controversy, provide a glimpse into how much weve learned about our origin story and how much remains uncertain.

(Credit: Didier Descouens/Wikimedia Commons)

Specimen: Touma

Species: Sahelanthropus tchadensis

Age: up to 7 million years old

Found: Chad, Central Africa

First described: 2002

Discovered in 2001 in Northern Chads desert landscape, the find was extraordinary: a collection of bones and bone fragments sitting beside a mostly complete skull. Researchers named the skull Touma, or hope of life in the local language. Its features were a mashup of old and new, a chimp-sized brain but with small canine teeth theyre typically smaller in hominins than in chimps, our nearest living relatives.

It was the fossils age that was even more shocking, however. Touma is between 6 million and 7 million years old. At the time, paleoanthropologists believed that the last common ancestor we share with chimps was at least a million years younger. Touma suggested the split in our lineages occurred much earlier than thought. And, for many paleoanthropologists, one feature in particular suggested that Touma was one of us, the first hominin. The foramen magnum is the opening at the base of the skull where the spinal cord exits. The angle of the opening can reveal if the spine stretched out behind the skull, as it does for four-legged animals, or dropped down, like it does for bipedal hominins.

According to the reconstruction, the foramen magnum is in a position that suggests it was able to walk on two feet, Condemi says of Touma. Bipedalism is one of the essential features of Homo. This species was in that line.

Touma was hailed as the oldest known hominin, giving researchers a look at the very roots of our lineage in the Late Miocene, when our ancestors split from other apes. When the cranium came out, paleoanthropologists looked at it and said, It must be The One, says paleontologist Fred Spoor of the Natural History Museum in London. Then there was pushback from the Miocene ape [research] community saying, Wait a minute.

As more Miocene ape fossils turn up, the overall picture becomes more complex. In 2019, for example, separate reconstructions of hips and torsos of European Miocene apes Danuvius and Rudapithecus definitely not hominins suggested they also might have been at least experimenting with some form of bipedalism.

Its not a given that only hominins were bipedal, says Spoor. We may well end up in a situation where there were bipedal Miocene apes We shouldnt assume that everything in the past has a lineage that continues in the present.

Zollikofer was the lead author on a 2005 reconstruction, published in Nature, of the Touma skull based on high-resolution CT scans. His team concluded that Toumas species, Sahelanthropus, was more closely related to hominins than to apes. But the researchers were less certain about how it moved.

It is clear that its skull shows evidence for some form of upright stance and bipedal locomotion, says Zollikofer. Is Sahelanthropus our ancestor? We will never know! He might have been part of a population of bipedal apes that was an evolutionary dead end.

Spoor sees Touma as a key specimen regardless of whether it belongs in our family tree. The importance of the cranium is immense. Its a 7-million-year-old fossil that is well preserved, he says. The fairest way to describe it is as the earliest possible or potential hominin. If its not a hominin, its likely quite close.

(Credit: ALe Omori/Cleveland Museum of Natural History)

Specimen: MRD

Species: Australopithecus anamensis

Age: 3.8 million years old

Found: Ethiopia, East Africa

First described: 2019

About 4.2 million years ago, the first australopiths, predecessors of our own Homo genus, emerged. Their brains were a little larger than those of a chimp, but not by much, and they were bipedal. The most famous hominin fossil, 3.2-million-year-old Lucy, was a member of Australopithecus afarensis, a later member of the genus. It was A. afarensis, conventional thinking went, that diversified into other australopith species spread across much of Africa.

Many researchers believed that A. afarensis itself evolved about 3.9 million years ago from the first australopith, A. anamensis. Partial fossils of this earlier species had been found at multiple sites in East Africa, but paleoanthropologists just couldnt put a face to the name. Even fragmentary skull fossils from A. anamensis were scarce.

In August, however, researchers revealed a jawdropping find from Ethiopia: a nearly complete skull of A. anamensis.

Anamensis we have known for decades, but this was the first time we had the cranium, says Condemi, who was not involved in the research. Its wonderful to have an idea what it looks like.

We learn the skull was very small, just a little bigger than Sahelanthropus. The face had chimplike features, with a big sagittal crest, she adds, referring to a ridge of bone along the top of the skull that is more pronounced in animals with powerful jaw muscles, which attach to the crest.

Theres just one problem: The skull, called MRD, is 3.8 million years old. Thats about 100,000 years younger than the oldest fossil described as A. afarensis. MRD, according to the researchers who discovered it, nixed the idea that, over time, A. anamensis had evolved into A. afarensis. Instead, the two species appear to have co-existed.

The idea that anamensis led to afarensis has been thrown out the window though not entirely, says Spoor, who was not part of the team.

To the casual observer, the distinction may seem minor, but understanding the course of australopith evolution has direct consequences for charting our own story.

Spoor and other experts focus on the smallest details, such as the angle of projection of the cheekbone, to see the bigger picture of how the hominin family tree grew to include at least six australopith species and, eventually, the genusof Homo.

"To understand how to build the tree, you have to understand what is newly evolved and what is inherited, says Spoor. The new skull gives us the opportunity to think about all that and to reconsider that all these [later] groups originated from anamensis and not afarensis.

He adds an important caveat: The MRD teams conclusions that the two species overlapped are based on the assumption that the oldest fossil classified as A. afarensis a fragment of skull dated to 3.9 million years ago actually belongs to that species.

Zollikofer shares that concern: The interpretation as having two species at the same time suffers from the fact that there are only two specimens. How can we know for sure what is within-group and betweenspecies variation here? We cant.

Until additional skulls of both A. anamensis and A. afarensis turn up, say the researchers, MRD may be just a pretty face.

(Credit: Guram Bumbiashvili/Georgian National Museum)

Specimen: Skull 5

Species: Homo erectus

Age: 1.77 million to 1.85 million years old

Found: Georgia, Caucasus Region of Eurasia

First described: 2013

A day's drive south of the Caucasus Mountains in the country of Georgia, beside a ruined medieval fortress and a small working monastery, sits one of the worlds most important, and confounding, paleoanthropological sites: Dmanisi, home to the oldest hominin fossils outside Africa.

Beginning in the 1980s, researchers unearthed thousands of fossils that are about 1.8 million years old. Among the remains of Etruscan wolves, saber-toothed tigers, deer and other animals are the bones of several hominins, including partial and complete skulls.

A particularly robust lower jaw was found in 2000 and initially described as an entirely new species, H. georgicus. In 2013 in Science, however, researchers announced theyd unearthed the rest of the individuals skull, now known as Skull 5. Having the complete skull led the team to do an about-face, no pun intended. They concluded the Dmanisi hominins were members of H. erectus, the earliest member of our genus found beyond Africa.

What led to the researchers unusual reversal? As Zollikofer puts it, Skull 5 is not alone It has four buddies, and all of them look quite different from each other.

Zollikofer has co-authored several Dmanisi hominin studies, including a 2006 paper in The Anatomical Record Part A on one of the other skulls. That specimen is unique in the entire hominin fossil record: The individual lost its teeth several years before death, leaving it unable to chew. It may have survived with assistance from others, suggesting social behavior otherwise unknown this early in human evolution.

It was the 2013 study on Skull 5, however, for which Zollikofer served as senior author, that ignited an academic firestorm. In addition to reclassifying the Dmanisi hominins as H. erectus, the team went a step further: They suggested that differences between the five Dmanisi skulls offered proof of considerable variation within H. erectus, so much so that other early Homo species, such as Africas H. habilis, could be reclassified as H. erectus.

It was a fresh salvo in one of paleoanthropologys longest-running battles: Was the early evolution of Homo linear, a single species changing over time into a new species? Or was it an unruly tangle of multiple populations, species and subspecies, mixing and mingling, sometimes evolving in isolation and then coming together again to interbreed?

Numerous critics took on the teams conclusions, including Spoor, who authored the provocatively titled Nature commentary Small-brained and big-mouthed.

Spoor appreciates Skull 5s significance It is a beautiful example of a very early Homo erectus but remains opposed to the teams radical proposal to redefine all early Homo species as H. erectus. He notes that their conclusion hinges on the assumption that the five Dmanisi skulls, found in the same general layer of rock, lived at the same time.

One level of excavation can represent 10,000, 20,000 years, Spoor says.

Being able to document the variation between Skull 5 and other Dmanisi hominins may be the most significant thing about the fossils. Exactly what that significance is, however, varies from one researcher to the next.

A reconstruction, based on partial bones that are about 315,000 years old, shows facial features within the range of modern humans. (Credit: Sarah Freidline/MPI Eva Leipzig)

Specimen: Irhoud 10

Species: Homo sapiens

Age: about 315,000 years old

Found: Morocco, North Africa

First described: 2017

For decades, conventional thinking was that H. sapiens emerged no more than 200,000 years ago, and in East Africa. Then a team took another look at a minor fossil site in Morocco.

In 1961, during mining operations, workers digging into a hillside had found an old skull. Subsequent excavations turned up more partial fossils, but the species was as uncertain as their estimated age, which ranged from 40,000 to 160,000 years old.

The most recent round of digging at the site, known as Jebel Irhoud, began in 2004 and included a more rigorous approach to dating the additional fossils found. The results were striking: The hominins, which included a partial face and braincase known as Irhoud 10, were about 315,000 years old.

In 2017 in Nature, researchers announced that Irhoud 10s facial features were within the range of modern humans. The Moroccan hominins were, said the authors, the oldest H. sapiens in the fossil record by more than 100,000 years.

This material represents the very root of our species, lead researcher Jean Jacques Hublin told media at the time.

Other paleoanthropologists saw the teams conclusions as hype.

Paleoanthropologists have an obsession with species, species definitions and ancestors, says Zollikofer, noting that Darwin believed there was far more fluidity among related populations. In Jebel Irhoud you can focus on a set of facial features that create a link to H. sapiens, or focus on other features that create a strong link to earlier humans. Guess which option sells better?

Chief among the archaic features of the Irhoud hominins is the low and elongated braincase, far from the rounded shape thats a hallmark of modern H. sapiens. But others in the field see the Irhoud hominins as an exciting snapshot of evolution in action.

I consider Jebel Irhoud Homo sapiens, says Condemi. What we see in Jebel Irhoud is similar to what we see in the evolution of Neanderthals, in that the Neanderthal we see from 200,000 years ago is not the Neanderthal we see from 50,000 years ago. There is evolution within a lineage.Spoor agrees. Evolution is a continuous event. Different parts of the head evolve at a different tempo. Its neat to see modernity emerge.

While the age of the Irhoud fossils is significant, so is the location. Finding the earliest H. sapiens thousands of miles from East Africa is as unexpected as the Irhoud hominins age.

I think what this evidence shows is that our old model of looking for a specific geographical region where modern humans evolved, a kind of Garden of Eden, so to speak, was probably too simple, says the University of Tbingens Harvati, a co-author of the 2017 paper introducing Irhoud 10. It is much more likely that several closely related populations across Africa contributed to our lineage, at times diverging and coming back together as environmental conditions separated them or brought them back into contact with each other.

I think [Irhoud] means that the cradle of Homo sapiens is not East Africa. Its all of Africa, adds Condemi. It means sapiens is a Pan-African species.

The partial cranium (right) and its reconstruction (left, middle) demonstrate a unique feature of modern humans the rounded shape contrasts sharply with Neanderthals and their ancestors. (Credit: Katerina Harvati/Eberhard Karls University Of Tbingen)

Specimen: Apidima 1

Species: Homo sapiens

Age: about 210,000 years old

Found: Greece, southern Europe

First described: 2019

In 2018, a Partial modern H. sapiens jaw from Israel, known as Misliya-1, pushed back the clock for our first road trip. It was up to 194,000 years old, evidence that our species was venturing out of Africa much earlier than once thought.

Given the general acceptance of Misliya-1, it was perhaps surprising that another fossil, described last July in Nature, met with so much controversy. Known as Apidima 1, the partial skull from Greece had been found more than 40 years earlier but had never been rigorously analyzed. Thats in part because it was discovered within arms reach of another, more complete hominin skull, Apidima 2, a Neanderthal. Found so close to it, Apidima 1 was assumed to be Neanderthal, too.

But then Harvati and her team looked at both skulls and conducted more advanced dating to determine their age. The results surprised even the researchers.

Apidima 2 was about 170,000 years old. But the team concluded that Apidima 1, about 210,000 years old, was H. sapiens: the earliest evidence of our species in Europe by more than 160,000 years.

We thought that Europe was the exclusive realm of the Neanderthals and their ancestors until about 45,000 years ago, says Harvati. However, there is no inherent reason why this should be so. There was no barrier that would have prevented early modern humans already in the Near East to spread further north to Anatolia and southeastern Europe.

And Apidima 1, says Harvati with certainty, is H. sapiens. Even though only a portion of the skull has been preserved, its the back area, which is uniquely rounded in modern humans.

Not everyone shares her confidence.

Says Zollikofer: [Apidima 1] is a conundrum. It could represent a lost aspect of early Neanderthal variation. It could represent a lost human population, without species attribution. It could represent H. sapiens. It is frustrating that it is so badly preserved; on the other hand, just being so badly preserved gives room for imagination.

In fact, just before Nature published Harvatis results, a smaller journal, available only in French, published another study on Apidima 1. Those authors concluded the partial skull belonged to the Neanderthal lineage.

As for the disparity in age between the two fossils, even Harvati first assumed they were contemporaries until results showed otherwise. The partial skulls and other, unidentified bone fragments were preserved in breccia, a mishmash of gravel and random debris washed into and through the cave system and then cemented together over time.

Our current hypothesis is that the specimens both fell into a kind of shaft, which filled with sediments from various parts of the cave, were jumbled together and solidified together, says Harvati.

She plans to return to the cave to conduct fresh excavations. Finding additional fossils may put critics concerns to rest or start new debates.

Spoor, echoing the mindset of many in the field at this thrilling and uncertain time, is ready for the next unexpected fossil find.

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5 Skulls That Shook Up the Story of Human Evolution - Discover Magazine