JAWBONE FILLS A GAP IN EARLY HUMAN EVOLUTION

On the morning of Jan. 29, 2013, Chalachew Seyoum was climbing a remote hill in the Afar region of his native Ethiopia, his head bent, eyes focused on the loose sediment. The site, known as Ledi-Geraru, was rich in fossils. Soon enough, he spotted a telltale shape on the surface — a premolar, as it turned out. It was attached to a piece of a mandible, or lower jawbone. He collected other pieces of a left mandible, and five teeth in all. Mr. Seyoum, a graduate student in paleoanthropology at Arizona State University, had made a discovery that vaulted evolutionary science over a barren stretch of fossil record between two million and three million years ago. This was a time when the human genus, Homo, was getting underway. The 2.8-million-year-old jawbone of a Homo habilis predates by at least 400,000 years any previously known Homo fossils. William H. Kimbel, director of the Institute of Human Origins at Arizona State, said the Ledi-Geraru jaw “helps narrow the evolutionary gap between Australopithecus and early Homo,” adding that it was an excellent “transitional fossil in a critical time period in human evolution.”

Dr. Spoor said in an email that he agreed with the hypothesis that the new Ledi-Geraru mandible “derives from Australopithecus afarensis, and at 2.8 million years shows morphology that is ancestral to all early Homo.” Dr.
Spoor’s predictions were drawn from a digital reconstruction of the disturbed remains of the jaws of the original 1.8-million-year-old Homo habilis specimen found 50 years ago by the legendary fossil hunters Louis and Mary Leakey at Olduvai Gorge in Tanzania.

The reconstruction, suggesting a plausible evolutionary link between A. afarensis and H. habilis, yielded a remarkably primitive picture of a deep-rooted diversity of a species that emerged much earlier than the 2.3 million years ago suggested by some specimens. The teeth and jaws appeared to be more similar to A. afarensis than to subsequent Homo erectus or Homo sapiens, modern humans that emerged about 200,000 years ago.

Dr. Spoor’s analysis also seemed to put a new face on H. habilis. He said that individual species of early Homo were more easily recognizable by jaw structure and facial features than by differences in brain size, which tend to be highly variable. Dr. Villmoare and colleagues made similar observations in their article. Both the predictions and the mandible findings called attention to smaller teeth with the emergence of H. habilis and evidence suggesting that the species probably split in different evolutionary lines, only one of which might have been ancestral to later H. erectus and H. sapiens.

In an email, Dr. Spoor explained that the split occurred sometime before 2.3 million years ago. The lineage leading to H. habilis must have kept the primitive jaw morphology. The Ledi-Geraru specimen kept the primitive, sloping chin that links it to a Lucy-like ancestor. Other lineages must account for the fact that H. erectus and H. habilis existed together for a period more than a million years ago.

In a second report for the journal Science, Erin N. DiMaggio of Penn State and other geologists examined soil, vegetation and fossils at Ledi-Geraru. They determined that when the H. habilis left its jaw there, the habitat was dominated by mammals that lived in a more open landscape — grasslands and low shrubs — than the more wooded land often favored by A. afarensis.

But after about 2.8 million years ago, increased African aridity has been cited as a possible result of widespread climate change affecting species changes and extinctions. Kaye E. Reed, co-leader of the Arizona State team, noted that the “aridity signal” had been observed at the Ethiopian fossil site. However, she said, “it’s still too soon to say this means climate change is responsible for the origin of Homo.”

For that, Dr. Reed said, “we need a larger sample of hominin fossils, and that’s why we continued to come to the Ledi-Geraru area to search.” That, and to learn more about the evolution of our genus, Homo.