Ancient jaw may hold clues to origins of human genus

Researchers have discovered what they regard as the oldest known fossil from the human genus, Homo. But questions about the evolutionary status of the approximately 2.8-million-year-old lower jaw have already emerged.

Found in 2013 resting atop eroding soil in Ethiopia’s Ledi-Geraru research area, the fossil jaw contains several signature Homo features, including small and symmetrically shaped teeth, say paleoanthropologist Brian Villmoare of the University of Nevada, Las Vegas and his colleagues. The new find also contains more apelike traits found in Australopithecus afarensis, a hominid species that died out about 3 million years ago. A. afarensis fossils, unearthed at a nearby Ethiopian site called Hadar, include Lucy’s famous partial skeleton. The similarities suggest Lucy’s species may have been an evolutionary precursor of the human genus, Villmoare’s team proposes online March 4 in Science.

Until now, the oldest fossil attributed to the Homo genus was a 2.3-million-year-old upper jaw from Hadar. The Ledi-Geraru find shows that substantial tooth and jaw changes had already occurred in early members of Homo within 200,000 years of the demise of A. afarensis.

More fossil evidence is needed to assign a species to the ancient jaw, the researchers say.

The discoverers of the Ledi-Geraru fossil assign it to Homo based solely on jaw and tooth traits. But it’s not clear whether additional evidence for a large brain, long legs, regular toolmaking and meat-eating, or some combination of these traits is needed to define the human genus, remarks paleoanthropologist Chris Stringer of the Natural History Museum in London. A partial jaw with a handful of teeth “may be too incomplete to recognize a fossil as Homo,” Stringer says.

Without a more complete skeleton, there’s no way to know whether the Ledi-Geraru jaw is the earliest representative of Homo “or just another australopithecine,” adds paleoanthropologist Christoph Zollikofer of the University of Zurich.

Study coauthor William Kimbel responds that the jaw probably comes from an early Homo species that split into two evolutionary lines.“The new jaw from Ledi-Geraru likely represents a transitional population between a Lucy-like Australopithecus ancestor and later Homo populations,” says Kimbel, director of Arizona State University’s Institute of Human Origins in Tempe.

One lineage, he says, included the 2.3-million-year-old Hadar jaw, which displays a wide, deep palate and small teeth characteristic of the Homo genus. The second line included Homo habilis, a species known from 1.8-million-year-old jaw, braincase and hand fossils found in Tanzania’s Olduvai Gorge in 1960.

A new digital reconstruction of that H. habilis jaw shows that it had a long, narrow shape similar to that of jaws from Lucy’s much older species. In contrast, H. habilis’ reconstructed partial braincase is larger than previously thought and comparable to that of Homo erectus specimens from around the same time. Evolutionary anatomist Fred Spoor of University College London and colleagues present these results in the March 5 Nature.

Spoor agrees with Kimbel that the Ledi-Geraru jaw came from a transitional Homo population. In Spoor’s view, differences in jaw shape indicate that three Homo species existed between 2.1 million and 1.6 million years ago — H. habilis, H. erectus and Homo rudolfensis.

But Zollikofer says shape differences between those proposed species are statistically weak. He and his colleagues suspect the Homo genus initially evolved as one species (SN: 11/16/13, p. 6).

Fossil evidence that Homo emerged 2.8 million years ago in East Africa shows that Australopithecus sediba — a 1.9-million-year-old hominid from South Africa with some humanlike traits — cannot have been an evolutionary bridge to Homo, Kimbel contends. Discoverers of two A. sediba skeletons, on the other hand, view that species as a possible Homo ancestor (SN: 8/10/13, p. 26).

Stringer proposes that East African and South African Homo species evolved from different Australopithecus ancestors. If so, that would put A. sediba in the thick of Homo evolution.