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A single new fossil can change our thinking about human origins, but its discovery – deep in a cave or buried in rock – remains a daunting battle for paleoanthropologists armed with hammers.
"It can take years and luck to find the right one," said Aurélien Mounier, paleoanthropologist at the National Museum of Natural History.
Researchers like Mounier use computers and mathematical techniques to reconstruct the appearance of fossils they have not yet found. On Tuesday, Dr. Mounier and Marta Mirazón Lahr, paleoanthropologist at the University of Cambridge in Britain, unveiled a virtual skull belonging to the last common ancestor of all modern humans, who lived in Africa about 300,000 years ago.
The rendering of this ancestral skull, described in the journal Nature Research, is strikingly similar to fossils of the same age as found in East Africa and South Africa. Scientists suggest that modern humanity is born from the fusion of the populations of these two regions.
"We are starting to look at the paleontological record in a different way," said Dr. Mounier. "We are more aware of diversity and complexity."
The ancestry of all living humans can be attributed to Africa. DNA studies indicate that our common ancestors lived somewhere on the continent between 260,000 and 350,000 years ago.
But how these early humans evolved is a persistent puzzle. The fossil record of this era in Africa does not offer easy answers. Over the decades, researchers have found only a few remains, with a strange mixture of traits.
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In 1986, for example, paleoanthropologists discovered a fossil in Kenya, 270,000 to 300,000 years old. They called it "Homo sapiens archaic". Other experts have claimed that it belonged to another species. And others simply raised their hands.
Two years ago, a team of scientists working in Morocco provided a new major clue. They discovered a set of remains dating back about 315,000 years, belonging to the Homo sapiens – the oldest remnant of our species to date.
But these humans were different from modern humans in some important ways. They lacked chins, for example, and had a long and weak puzzle.
Doctors Mounier and Lahr sought to understand the connection between Africa's enigmatic fossils and modern humans. Researchers have developed mathematical techniques to compare the structure of skulls, search for evolutionary links.
The first challenge was the fact that today, people do not share perfectly identical skulls. From person to person, there is a lot of variation. Populations have slightly different skull shapes on average, but these averages can be misleading.
"We know that in a population, there can be much more variation than between two populations," said Dr. Mounier. "We are all very similar, yet we are all very diverse."
No skull can replace that of everyone. Doctors Mounier and Lahr have thus retreated from this modern diversity to what they believed to be the skull of a common ancestor.
They performed CT scans of 260 skulls of people from a wide range of populations – from the people of the African rainforests to the Pacific islands to the coast of Greenland. They also scanned 100,000-year-old skulls found in Israel that are clearly similar to living human beings.
The researchers also selected a selection of extinct human relatives, such as Neanderthals, to study the same way.
Next, the scientists placed all these living and extinct individuals on an evolutionary tree. In doing so, they were able to follow the evolution of the skulls along each of the branches, to arrive at an image of the skull of the common ancestor of living beings.
"More or less, it's a very modern human," said Dr. Mounier about the skull. "But that does not really fit the current population, it's something different."
The rendering of this ancestral skull shows the same vaulted puzzle that we have today. But it also has heavier brow ridges and a prominent underside.
Doctors Mounier and Lahr compared their ancestral skull to real African fossil skulls of the same period. The researchers found a number of differences – so much so that they think that fossils do not belong to a single population, but to three.
The Moroccan fossil belongs to a population. Another fossil, found in Tanzania, represents the second. The third population includes two fossils from two sites several thousand kilometers apart: South Africa and Kenya. The researchers concluded that this third population most closely resembles the ancestor of modern man.
Doctors Mounier and Lahr speculate on the evolutionary lineage that gave birth to modern humans and gave birth to a number of populations in Africa about 350,000 years ago. These humans all had big brains and were making more and more sophisticated tools.
But there were clear differences in their anatomy. In Morocco, for example, the first Homo sapiens had a very Neanderthal aspect. "This is clearly not the closest candidate to play a role in the evolution of modern man," said Dr. Mounier.
The populations from which the Moroccan and Tanzanian fossils originate may have disappeared without contributing to the genetic pool of living humans.
But other groups may have come into contact from time to time and have crossed each other. This may be what happened to the ancient humans of East and South Africa. "The idea is that they merged to finally form our species," said Dr. Mounier.
Katerina Harvati, a paleoanthropologist from the University of Tübingen in Germany who did not participate in the new study, termed it as "very good way to test hypotheses about the fossil record".
But she warned that any reconstruction of our common ancestor depends on skull researchers. With the fossils of Israel, she would like to see other modern human fossils added to the analysis.
The additional data could alter the virtual skull – and maybe even theories about our origins.
Mounier sees the new study as a framework for studying human origins, not the last word. "We can do a lot of things, even without new fossils," he said.
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