On a hot January morning 2 years ago, Chalachew Seyoum was searching for fossils at a desolate site in Ethiopia called Ledi-Geraru, where no human ancestor had turned up in a decade of searching. But Seyoum, an Ethiopian graduate student at Arizona State University (ASU), Tempe, was upbeat after a week off. “I had a lot of energy and fresh eyes,” he says. “I was running here and there. I went up a little plateau and over the top when I spotted this specimen popping right out.”
He sat down and closed his eyes. When he opened them, he could more clearly see the gray fossil poking out of the bleached sand and mudstone, and he realized that he had found the jawbone of a hominin—a member of the human family. He called out for the ASU expedition leader: “Kaye Reeeed!” Reed scrambled up the steep slope on her hands and knees, saw the fossil, and yelled “Woo-hoo!”
Their excitement was justified. In two papers online this week in Science, the ASU team and co-authors introduce the partial lower jaw as the oldest known member of the genus Homo. Radiometrically dated to almost 2.8 million years ago, the jaw is a window on the mysterious time when our genus emerged. With both primitive and more modern traits, it is a bridge between our genus and its ancestors and points to when and where that evolutionary transition took place. As a transitional form “it fits the bill perfectly,” says paleontologist Fred Spoor of University College London.
Together with a reassessment of known fossils, published in Nature this week by Spoor and colleagues, the find is stimulating new efforts to sort out the mixed bag of early Homo remains and to and to work out which forms emerged first. “This causes us to rethink early Homo,” says paleoanthropologist Bernard Wood of George Washington University in Washington, D.C.
Researchers agree that small-brained hominins in the genus Australopithecus evolved into early Homo between 3 million and 2.5 million years ago, but the Homo fossil trail disappears at the crucial time. Until now, the oldest known Homo fossil was a 2.3-million-year-old upper jaw from Hadar, Ethiopia, that has not been classified into a species. It and other early Homo fossils paint a confusing picture. Some have big skulls, others small; some consist of a bit of skull, others only a jaw, resulting in a grab bag of mismatched parts. As a result, researchers have argued about whether there was a single species of early Homo or three. The type specimen of H. habilis, for example, includes a 1.8-million-year-old lower jaw called OH 7 from Olduvai Gorge in Tanzania. But the type specimen of another species, H. rudolfensis, is a 2.1-million-year-old skull without teeth or a lower jaw.
This week’s papers advance the work on two fronts. Spoor and colleagues created a virtual reconstruction of the OH 7 specimen, which was found 55 years ago, to correct for postmortem distortion. They used computed tomography and 3D imaging to digitize and reassemble pieces of the jaw in the computer. Then they compared OH 7 with other specimens and found that it has more primitive features, such as a long narrow palate, than do the older Hadar jawbone and members of H. rudolfensis. Although OH 7 itself is relatively recent, their analysis suggested that H. habilis arose earlier than the other two species.
Meanwhile, the ASU team spent years doing targeted searches for an older ancestor. They hunted in sediments that were the right age—2.58 million to 2.84 million years old—and in an epicenter of early human evolution. Ledi-Geraru is only 30 kilometers from Hadar, home of the 2.3-million-year-old Homo jaw, as well as to more than 100 individuals of Australopithecus afarensis, the species of the famous skeleton called Lucy. The oldest known stone tools, dated to 2.6 million years ago, are only 40 km away at Gona.
The new Ledi-Geraru discovery fits best in Homo, says ASU paleoanthropologist William Kimbel. Its molars are slimmer than those of Australopithecus, the third molar is smaller, and the jawbone is shaped differently. The ASU team hasn’t assigned the jaw to a species yet because they hope to find more parts, but say that it most closely resembles H. habilis.
In fact, the new jaw looks a lot like what Spoor imagined for the ancestor of OH 7. That suggests that although the two specimens are separated by almost 1 million years, they belong to the same lineage, and that the oldest Homo looked most like H. habilis, just as Spoor and others have predicted.
But the Ledi-Geraru specimen is not likely to be a member of H. habilis itself, Spoor says. The jaw also has traits that link it with A. afarensis, such as a rounded chin region. The similarities strengthen the proposal that Lucy’s species, which lived from 2.95 million to 3.8 million years ago, was the direct ancestor of Homo. But other types of australopiths also lived during that time, making the genealogy exercise premature.
The ASU team does rule out A. sediba from South Africa as the Homo ancestor, because at 1.9 million years old it is too recent. But its discoverer, paleoanthropologist Lee Berger of the University of the Witwatersrand in Johannesburg, South Africa, says that the known A. sediba skeletons might simply be late examples of the species.
The new data may help solve a puzzle: Why did so many kinds of hominins roam East Africa between 2 million and 3 million years ago? To understand this burst of evolution, the ASU team analyzed the bones of other species living at that time. As they report in the second Science paper, fully one-third of the Ledi-Geraru mammals were new species, not seen in older sediments at nearby Hadar.
Three million years ago, Hadar was home to monkeys, giraffes, and elephants that favored a patchwork of woods and grasslands. Ledi-Geraru hosted a different fauna just 200,000 years later, with grazers such as gazelles, zebras, wild pigs, and a baboon at home in open grasslands like the Serengeti. Climate change and the shift to more open terrain may have spurred the emergence of many species, including members of Homo and Australopithecus, Reed says. “This is a snapshot of a hominin in a landscape that’s really open,” agrees paleoclimatologist Peter deMenocal of Columbia University’s Lamont-Doherty Earth Observatory in Palisades, New York, who has argued that climate change sparked intense periods of speciation.
Researchers often reconstruct ancient climate from clues in sediment cores. To better correlate climate and human evolution, in 2013 researchers cored lakebeds close to key fossil sites. “Stay tuned,” deMenocal says. “We’ll be answering this [climate question] within a year.”