Scientists say they're filling in some of the most frustrating gaps in the story of human evolution — thanks to the discovery of the oldest known fossil from the genus to which we belong, plus the computer-aided reinterpretation of another key fossil.
The revelations, published Wednesday by the journals Science and Nature, focus in on a key time frame for those who study human origins: the transition from early human ancestors such as Australopithecus afarensis, represented by the famous Lucy skeleton, to the species within the genus Homo. We're the only remaining members of that genus, as members of the species called Homo sapiens.
The now-extinct Homo species range from Neanderthals to Homo habilis ("Handy Man"), which has been considered the oldest member of the genus.
The newly announced discovery pushes the dawn of Homo back to 2.8 million years, which is hundreds of thousands of years earlier than previously thought. This ancient fossil of a partial jawbone and teeth was found in Ethiopia's Afar region, roughly the same area where Lucy was discovered more than 40 years ago.
"It's the first fossil we have on the branch that leads toward us," Brian Villmoare, an anthropologist at the University of Nevada at Las Vegas who's the principal author of one of the studies in Science, told NBC News on Thursday during a teleconference.
The fossil blends characteristics traditionally associated with Homo, such as the slim and symmetrical shape of the jaw, with the sloping chin of Lucy and her Australopithecus kin. It's known as the Ledi-Geraru fossil, or LD 350-1, after the place in Afar where it was found two years ago.
Meanwhile, a separate group of researchers took a fresh look at a 1.8 million-year-old set of Homo habilis fossils from Tanzania's Olduvai Gorge, known as OH 7 or "Jonny's Child." These bones were discovered back in 1960, but they were preserved in a fractured and distorted condition. That made the fossils hard to analyze, because researchers have to rely on precise measurements of the bones — particularly the jaw and teeth — to classify and compare specimens.
The team behind the Nature paper, led by Fred Spoor of the Max Planck Institute for Evolutionary Anthropology, used CT scanning and 3-D imaging software to create a virtual reconstruction of the lower jaw and braincase.
They determined that Jonny's Child was actually more like Lucy than previously thought, due to the long and narrow shape of the reconstructed lower jaw. The brain size, however, was larger than previous estimates suggested.
This posed a big problem: Anthropologists had linked Jonny's Child with an earlier specimen known as AL 666-1, which was found in Ethiopia and dated to 2.3 million years ago. But AL 666-1 had a more modern-looking jaw, suggesting that the two fossils represented separate branches of Homo that diverged. All of a sudden, the researchers didn't know where Jonny's Child fit.
"By digitally exploring what Homo habilis really looked like we could infer the nature of its ancestor, but no such fossils were known," Spoor said in a news release.
Solving the mysteries
Spoor said the Ledi-Geraru fossil turned up "as if 'on request'" to solve the problem. Scientists saw a progression in the evolution of the jawbone, from Lucy to Ledi-Geraru to Jonny's Child. They concluded that AL 666-1 and similar Homo fossils belonged to a different lineage that must have split off well before 2.3 million years ago. Such a lineage probably included a species known as Homo rudolfensis, they said.
Spoor and his colleagues also concluded that brain size was highly variable among human ancestors, and that the brain metric alone couldn't be used as a reliable guide to determine which species evolved from which.
What caused the split between these species? In another paper published by Science, Penn State University's Erin DiMaggio and her colleagues say the geological evidence from the spot where the Ledi-Geraru fossil was found suggests that the environment was dominated by open grasslands and shrub lands, with trees lining rivers or wetlands. That fits a scenario in which Ethiopia's Afar region became less forested and more arid around 2.8 million years ago — right about the time when the Ledi-Geraru human ancestor lived.
"We can see the 2.8 million-year-old aridity signal in the Ledi-Geraru faunal community," Arizona State University's Kaye Reed, a co-author of the study, said in a news release. "But it's still too soon to say that this means climate change is responsible for the origin of Homo."
The anthropologists are still looking for the fossils of human ancestors in the Ledi-Geraru area, in hopes of fleshing out the story further.
Arizona State University's Don Johanson, who discovered and analyzed the Lucy fossils back in 1974, said the newly reported research is "closing the gap" between Lucy and our more advanced Homo ancestors. He's reluctant to use the term "missing link," but he says the Ledi-Geraru fossil adds another link to the chain of human evolution. Or should that be "chains"?
"This implies that there were probably two or three lineages of Homo," Johanson told NBC News.
In addition to Villmoare, DiMaggio and Reed, the authors of "Early Homo at 2.8 Ma From Ledi-Geraru, Afar, Ethiopia" include William Kimbel, Chalachew Seyoum, Christopher Campisano, John Rowan, David Braun and J. Ramon Arrowsmith.
In addition to DiMaggio, Reed, Campisano, Rowan and Arrowsmith, the authors of "Late Pliocene Fossiliferous Sedimentary Record and the Environmental Context of Early Homo From Afar, Ethiopia" include Guillaume Dupont-Nivet, Alan Deino, Faysal Bibi, Margaret Lewis, Antoine Souron and Lars Werdelin.
In addition to Spoor, the authors of "Reconstructed Homo Habilis Type OH 7 Suggests Deep-Rooted Species Diversity in Early Homo" include Philipp Gunz, Simon Neubauer, Stefanie Stelzer, Nadia Scott, Amandus Kwekason and M. Christopher Dean.