Earliest hominid: Not a hominid at all?

The earliest known hominid fossil, which dates to about 7 million years ago, is not a hominid at all, but actually some kind of ape, according to an international team of researchers led by U-M.

The finding, they say, suggests scientists should consider that we didn’t actually descend from apes resembling chimpanzees, which are considered our closest relatives.

Cranial bases of the reconstructed TM 266 vault (center), compared with STS 5, the australopithecine (left), and a female gorilla from the Senckenberg collection (right). The surprise is that TM 266 should resemble any gorilla (or other ape) given the expectation of adaptations to bipedalism if it is a hominid. (Photo courtesy Department Of Anthropology)

U-M anthropologist Milford Wolpoff and colleagues examined images and the original paper published on the discovery of the Toumaï cranium (TM 266) or Sahelanthropus tchadensis, as well as a computer reconstruction of the skull, originally thought to be of the primate family, Hominidae, of which Homo sapiens (humans) is the only living species. Two other colleagues actually were able to examine the skull, Wolpoff said, in addition to the images and the computer reconstruction.

The research team concluded that the cranium did not sit atop the spine but in front of it, indicating the creature walked on all fours like an ape. Hominids, he said, are distinguished from all other primates by walking upright. Hominids are everything on the line leading to humans after divergence with chimpanzees, which genetic data suggests happened 4 to 6 million years ago. Upright bipedalism is the single best way of identifying which fossils are hominids.

The Toumaï cranium is what until now has been believed to be the first hominid, or relative of the earliest hominid, but it’s not a hominid at all, Wolpoff said, adding, this “is very important in trying to understand where humans come from.”

Nor does the skull resemble a living chimpanzee—no fossil records of chimpanzees exist so it’s impossible to compare to earlier descendents, Wolpoff said. Because it’s thought that chimps and humans diverged from 4 to 6 million years ago, it’s difficult to prove that a 7-million-year-old fossil is a hominid without overwhelming evidence, he said.

“The big message it sends us is our ancestors never looked like a chimpanzee,” Wolpoff said. “This thing is clearly saying that chimpanzees are just as different from this ancestor as we are. They are just different in a different way.”


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