Image: Skull
The cranium of the newly described hominid, nicknamed Toumai, has the flat face and brow ridge associated with the ancestors of humans.
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The discovery of a fossil skull in a remote Chadian desert could rewrite the scientific saga of human origins, researchers said Wednesday. The skull and other fossil remains have been dated at 6 million to 7 million years old — which would make them the oldest-known relatives of modern humans. If confirmed, the find would dramatically change scientists’ conception of where and when our ancestors arose.

The skull was found almost exactly a year ago by a Chadian student, after years of painstaking research by an international team led by the University of Poitiers’ Michel Brunet. The excavation in northern Chad’s Djurab Desert is more than 1,500 miles (2,500 kilometers) from the current epicenter of research into human ancestors, in east Africa’s Rift Valley.

An analysis of the nearly complete cranium, as well as teeth and jawbone fragments also found at the Chad site, convinced the researchers that they had found a new species, the earliest-known common ancestor of modern-day humans and our now-extinct hominid cousins.

The species was given the scientific name Sahelanthropus tchadensis — referring to the discovery site in Chad, in Africa’s Sahel region — and was nicknamed “Toumai,” a Chadian name meaning “Hope of Life” that is usually given to children born just before the dry season.

MAP: Toros-Menall site
The fossils were found in the Toros-Menalla area of Chad's Djurab Desert. Researchers say a "Mega Lake Chad" extended that far millions of years ago.
Scientists who got an advance peek at the findings, published in Thursday’s issue of the journal Nature, hailed the discovery as “astonishing,” “amazing” and on a scale comparable to the first “man-ape” fossils found in Africa 77 years ago. They marveled that the most ancient hominid was identified in what was virtually virgin territory for anthropologists before Brunet’s group came on the scene.

“This will have the impact of a small nuclear bomb” in the field of human evolution, Harvard anthropologist Daniel Lieberman told Nature. The discovery contradicts the idea that hominids — the earliest creatures who branched off from apes to create the human family tree — got their start in eastern Africa.

Questions remain
But some questions are still hanging over the Chad discovery: The usual radioisotope-dating techniques couldn’t be used on the thin geological strata where the fossils were found. Instead, the researchers used a less reliable method to come up with an age of 6 million to 7 million years, by comparing dozens of animal fossils that were found alongside the skull with similar fossils found in Kenya that were isotope-dated.

“It would have been nice” to use radioisotope dating, George Washington University anthropologist Bernard Wood, who wrote a commentary on the research for Nature, told MSNBC.com. “But these guys didn’t have that luxury. ... They have put forward very strong evidence for the age.”

Also, none of the fossils come from below the neck — which means the researchers could only guess at whether Toumai walked upright. Anthropologists consider that ability, along with larger brain size and characteristic shapes of the teeth, to be what set ancient hominids apart from chimpanzees and other apes.

How it looked
Based on skull measurements, the researchers said Toumai was about as big as a modern chimpanzee.

Image: Jawbone
A CT scan of a fossilized right lower jaw shows the dental wear patterns associated with the ancestors of humans rather than chimpanzees.
Wood said the skull is remarkable because it is a “mosaic” combining the characteristics of chimps and a type of hominid known as an australopithecine.

“Put simply, from the back it looks like a chimpanzee, whereas from the front it could pass for a 1.75-million-year-old advanced australopith,” he wrote in Nature. The face is flatter than a chimp’s face, and the teeth are more humanlike than chimplike, the researchers reported. The canine teeth, for example, are smaller and do not project far above the other teeth, as they do in apes.

Toumai lived in the basin of a “Mega Lake Chad,” lush with vegetation and animal life that ranged from crocodiles and snakes to the ancient ancestors of zebras and antelopes. In the millions of years since then, the lake has dried up to only a fraction of its former extent, leaving behind fossils in the desert, the researchers said.

Evolutionary scientists had considered the period between 5 million and 7 million years ago to be a crucial turning point, when the ancestors of modern humans in east Africa were cut off from their ape cousins in the west and started down a different track. Fragmentary evidence ofhominids dating back 5.2 million to 6 million years has been found in the east African countries of Ethiopia and Kenya.

But if Brunet’s group is correct, hominids must have become distinct from chimpanzees significantly earlier than that, spreading out across central and east Africa in the age before Toumai walked the earth.

The research team — known as the Mission Paleoanthropologique Franco Tchadienne, or the Franco-Chadian Paleoanthropological Mission — says Toumai could be regarded “as the ancestor of all later hominids ... the ancestor of the human lineage.”

But Wood cautioned that the picture isn’t that clear-cut. For example, different groups of apes could have developed humanlike characteristics, only to go extinct.

“Although it’s perfectly possible that this is an ancestor of humans, it’s also possible that it’s the remains of a creature which evolved some human sorts of features because of convergent evolution,” he told MSNBC.com.

Wood said the find doesn’t necessarily pinpoint Chad as the birthplace of humanity, but it does challenge the claim that east Africa was that birthplace.

“The wonderful thing about this discovery,” he said, “is that it shows that east Africa is not the only place where things happen.”

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