Fossil skull points to single root for human evolution

A remarkably complete, roughly 1.8-million-year-old fossil skull with a surprising set of features adds key evidence to the controversial idea that early members of the Homo genus evolved as one species living in both western Asia and Africa, scientists say.

The new find, and the remains of four other skulls previously unearthed at a site called Dmanisi, in the nation of Georgia, belonged to Homo erectus despite some big differences in shape and size, say paleoanthropologist David Lordkipanidze of the Georgian National Museum in Tbilisi and his colleagues. The magnitude of skull diversity at Dmanisi indicates that African Homo fossils dating to shortly before and after 1.8 million years ago can be folded into a single, intercontinental H. erectus population, the researchers conclude in the Oct. 18 Science.

Some researchers challenge that assertion. Scientists usually sort the African fossils into four species: H. habilis, H. rudolfensis, H. ergaster and H. erectus. 

“After finding these five fossil skulls from a snapshot of time at Dmanisi, we’re reconsidering what we know about Homo erectus,” Lordkipanidze says. African sites have yielded individual Homo fossils from around 2 million years ago, he adds, but those finds reveal nothing about physical differences in populations at that time.

Working at Dmanisi in 2000, Lordkipanidze’s team found a lower jaw from a member of the Homo genus, the set of related species that includes people today. Five years later, the researchers uncovered the rest of the skull from the same ancient individual. Unlike any previously discovered Homo skull, the new find combines a relatively small brain case with a long face, projecting jaws and large teeth. The five skulls found at Dmanisi come from a regional branch of H. erectus that featured small-faced, straight-jawed youngsters and adult females along with large-faced, jut-jawed adult males, the investigators say.

Shape and size differences among the Dmanisi skulls don’t exceed those found in modern populations of people and chimpanzees, they add.

The five Dmanisi individuals offer a good yardstick for gauging the extent to which skeletal features varied among individuals in ancient Homo species, Lordkipanidze says. Early Homo fossils from East Africa — including a 2.3-million-year-old upper jaw, a nearly 2-million-year-old partial skull classified by some researchers as H. rudolfensis (SN: 9/8/12, p. 8) and a 1.8-million-year-old upper jaw often assigned to H. habilis — display enough similarities to the Dmanisi crowd to qualify as H. erectus, his group concludes.

“Whatever you call them, the Dmanisi hominids provide an amazing window into what our ancestors looked like when they left Africa for the first time,” remarks paleoanthropologist Tim White of the University of California, Berkeley.

White finds it plausible that a single early Homo species spread from Africa to Georgia. More fossil discoveries are needed to establish whether additional African Homo species existed alongside H. erectus, he says.

But some paleoanthropologists don’t see the Dmanisi finds as reason to put all early Homo fossils in one species.

“I’m prepared to think that we’re witnessing H. erectus in the making at Dmanisi,” says Fred Spoor of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. H. rudolfensis, however, evolved distinct facial features in East Africa that don’t appear on any Dmanisi skulls, Spoor holds.

Bernard Wood of George Washington University in Washington, D.C. agrees, saying that the Dmanisi fossils are probably examples of a hominid species that evolved in western Asia. “A raft of evidence” from the brain case, teeth, limbs, hands and feet suggests that H. erectus and H. habilis were different species, he says.

Another research team has proposed that a roughly 2-million-year-old southern African hominid, Australopithecus sediba, evolved into the first Homo species (SN: 8/10/13, p. 26). But if H. erectus originated in eastern Africa and western Asia that relegates A. sediba to a dead-end branch of hominid evolution, Lordkipanidze says.

Lee Berger of the University of the Witwatersrand in Johannesburg, who leads ongoing excavations of A. sediba fossils, disagrees. Several skull traits considered critical for H. erectus in the new study appear on the skull of a 9-year-old A. sediba boy, Berger contends. Those traits include the size of the face relative to the body and the extent of jaw projection. A. sediba’s entire skeleton, not just the skull, may show enough similarities to the Dmanisi fossils to support the argument that the southern African species was an evolutionary precursor of the Homo line, Berger suggests.

Lordkipanidze’s team did not compare the overall shape of H. erectus skulls at Dmanisi to either of two A. sediba skulls.