For decades scientists have worked to connect the dots between dozens of fossil discoveries in East and southern Africa in hopes of constructing an accurate picture of human origins. Now, a new find in western Central Africa suggests the picture may be radically different than widely assumed.
A team of researchers excavating in northern Chad has unearthed the well-preserved skull and other fossilized remains of what they believe was a previously unknown hominid, or early human precursor, that lived six to seven million years ago. That date would make it the oldest known ancestor of humans.
The finding has excited the scientific community especially because it opens a window onto a period near the time when humans and apes diverged from a common ancestor. Virtually nothing about that period is known, as most human fossils are considerably younger.
Various aspects of the new fossils could force scientists to rethink some basic theories about human origins, according to several scientists who were not part of the research team.
In a statement issued by Nature, which reported the discovery in its July 11 (2002) issue, anthropology professor Daniel Lieberman of Harvard University said the new find "will have the impact of a small nuclear bomb."
"One of the most important things this skull tells us is how much we don't know," he said in a phone interview. "It suggests how diverse hominids might have been in Africa, and shows that lots of things were going on in Africa that we can't imagine."
Lieberman saw the skull and, like some other observers, said he was particularly intrigued by the creature's unusual mix of both primitive and advanced traits. The braincase is chimp-like, for example, but the face, teeth, and somewhat flattened head resemble those of humans.
"What's most astonishing is that the facial features are like those that we don't see until 1.8 million years ago in the genus Homo. It is more Homo than australopithecine," he said, referring to the best-known group of hominids, which appeared in East Africa three to four million years ago and whose fossils have provided most of what we know about the earliest human ancestors.
So, is the new skull fossil a hominid—perhaps our earliest known ancestor?
"It's very hard to be sure, but I think it's a hominid," said Lieberman. "But whether it was the earliest hominid or the earliest ancestor of anyone living today, we can't tell."
Michel Brunet of the University of Poitiers in France headed the international team of more than three dozen researchers. They found the fossils—an intact cranium, two lower jaw fragments, and several teeth—at a site in the Djurab Desert where the group has been excavating since the mid-1990s.
The researchers compared the ancient skull and related fossils with the fossils of many other known hominids and primates. Based on characteristics such as the tooth type and the thickness of the enamel, the shape and positioning of the head, and the facial features, the team concluded that the creature represented a new genus and species of hominid.
They officially named it Sahelanthropus tchadensis. Its nickname is "Toumaë," a Goran-language word meaning "hope of life"; in the Djurab Desert, the name is given to babies born just before the dry season.
Brunet's team found the ancient skull last year and hoped to keep it secret until after the findings underwent scientific review. But rumors and brief news reports about the discovery in European newspapers set off a buzz of excitement in the scientific community.
The discovery follows several other hominid fossil claims in recent years that have pushed the search for our human ancestors much further back in time. Some of those fossils are nearly as old as Sahelanthropus.
Nonetheless, scientists say the skull found in Chad stands apart as remarkable for a number of reasons, including its age and completeness.
Chris Stringer, head of the Human Origins Group at the Natural History Museum in London, said the skull promises new insight into a period, the Upper Miocene, that has largely been a blank for paleoanthropologists.
"It's the only complete skull we have for this time period," he said. "We have ape skulls from Europe and Asia from eight to nine million years ago, and in Africa we've found skulls of human relatives who lived three to four million years ago. But there's been no good skull material in between—this is the only really complete skull in that five-million-year gap."
Bernard Wood, Henry R. Luce Professor of Human Origins in the Department of Anthropology at George Washington University in Washington, D.C., said the finding "opens the door to that extinct world of six to seven million years ago that's critical to our understanding of what sort of creatures connect us to the rest of the tree of life."
The desert site in northern Chad where Sahelanthropus was found is 1,500 miles (2,500 kilometers) west of Africa's Rift Valley. The east side of the Rift Valley has long been regarded as the "cradle of humanity" because of the abundant hominid fossils recovered there.
Stringer thinks much more interesting human fossil evidence is likely to turn up in West and Central Africa now that scientists know it's a good place to look. "This [discovery] makes us realize how limited a view we have of human evolution because until now we've concentrated on East Africa," he said.
The Rift Valley has long been associated with early human evolution because many scientists believe the opening of the rift millions of years ago—which left jungle on the west and savanna or grasslands to the east—was an important factor that helped shape adaptation.
One widely held theory suggests that early human ancestors became bipedal—walking upright—when they moved out of forests and trees and into savannas. Presumed traits of the new hominid are somewhat at odds with that view.
Brunet believes Sahelanthropus was bipedal at least part of the time. Yet his colleague Patrick Vignaud and other members of the research team have published a paper in the same issue of Nature indicating that the environment where the ancient skull was found consisted of lake, forest, river, and wooded savanna during the Upper Miocene. This suggests bipedalism may have developed apart from migration to savannas, the researchers say.
The region where Sahelanthropus dwelled was teeming with other animals, according to Brunet and his colleagues. Since 1994, they have recovered tens of thousands of vertebrate fossils from the excavation site in northern Chad. The fossil remains represent 42 species that include elephants, giraffes, antelopes, hippopotamus, crocodiles, lizards, monkeys, fish, and wild boar.
Exactly where Sahelanthropus belongs on the family tree is not possible to determine at this time.
Despite the detailed analysis and published claims, the question of identity remains open-ended. Is it actually a new hominid, or a variation of some other previously identified species, or perhaps even an ape?
Some observers have suggested, for example, that because of its small canine teeth, Sahelanthropus may be a female chimp. Brunet was in the field in Chad and unavailable for comment. In an interview with a newspaper reporter in Chad, however, he said: "This brow ridge is thicker than that of a male gorilla so the probability that it's a female is very low."
"It is a hominid," he declared.
Stringer said such questions in the world of paleontology are always complex because evidence is usually incomplete and there is little agreement about what key features characterize a distinct human ancestor. "Everyone has a favorite model of or take on what would identify early members of the human line—it's a matter of interpretation," he said.
"This creature could be our missing ancestor, it could be on the human line of evolution. But I don't think we can really say yet that it's a human relative, or even whether it's male or female," he said. "We simply don't have the signposts to know what the ancestors of gorillas and chimps and humans looked like."
Brunet and his colleagues argue that age and primitive anatomical features of Sahelanthropus suggest it may be closely linked to the last common ancestor of humans and chimpanzees, making it "a likely ancestor of all later hominids."
Scientists believe that the two branches of primates—apes and humans—diverged five to eight million years ago and evolved along separate paths. Molecular and DNA analysis in the past decade has indicated that humans are most closely related to chimpanzees, sharing as much as 98 percent of their genetic material. This means they shared a common ancestor somewhere along the way.
According to Wood, "molecular clock" studies indicate that the hypothetical common ancestor of modern humans and chimpanzees probably lived between about five and seven million years ago.
Brunet said of his new fossil discovery: "Here we are not far from the divergence between chimp and human. The next skull we have is four million years later, so we don't know what happened in between. But with this new guy and species, we have the beginnings of new knowledge. This is just the beginning of our knowledge of the human lineage."
In Wood's view, the chief significance of the Sahelanthropus find is not the issue of whether it's a human ancestor, but the clues it offers into the unsuspected diversity of ancient fossil hominids. "One of the real surprises," he said, "is what an amazing mix of anatomy this creature has. It says, 'Hey, things are much more diverse than we'd thought.'"
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