Discovery of 2.8-million-year-old jaw sheds light on early humans

For decades, scientists have been searching for African fossils documenting the earliest phases of the Homo lineage, but specimens recovered from the critical time interval between 3 and 2.5 million years ago have been frustratingly few and often poorly preserved. As a result, there has been little agreement on the time of origin of the lineage that ultimately gave rise to modern humans. At 2.8 million years, the new Ledi-Geraru fossil provides clues to changes in the jaw and teeth in Homo only 200,000 years after the last known occurrence of Australopithecus afarensis ("Lucy") from the nearby Ethiopian site of Hadar.

Found by team member and ASU graduate student Chalachew Seyoum, the Ledi-Geraru fossil preserves the left side of the lower jaw, or mandible, along with five teeth. The fossil analysis, led by Villmoare and William H. Kimbel, director of ASU's Institute of Human Origins, revealed advanced features, for example, slim molars, symmetrical premolars and an evenly proportioned jaw, that distinguish early species on the Homo lineage, such as Homo habilis at 2 million years ago, from the more apelike early Australopithecus. But the primitive, sloping chin links the Ledi-Geraru jaw to a Lucy-like ancestor.

"In spite of a lot of searching, fossils on the Homo lineage older than 2 million years ago are very rare," says Villmoare. "To have a glimpse of the very earliest phase of our lineage's evolution is particularly exciting."

In a report in the journal Nature, Fred Spoor and colleagues present a new reconstruction of the deformed mandible belonging to the 1.8 million-year-old iconic type-specimen of Homo habilis ("Handy Man") from Olduvai Gorge, Tanzania. The reconstruction presents an unexpectedly primitive portrait of the H. habilis jaw and makes a good link back to the Ledi fossil.

"The Ledi jaw helps narrow the evolutionary gap between Australopithecus and early Homo," says Kimbel. "It's an excellent case of a transitional fossil in a critical time period in human evolution."

Global climate change that led to increased African aridity after about 2.8 million years ago is often hypothesized to have stimulated species appearances and extinctions, including the origin of Homo. In the companion paper on the geological and environmental contexts of the Ledi-Geraru jaw, Erin N. DiMaggio, of Pennsylvania State University, and colleagues found the fossil mammal assemblage contemporary with this jaw to be dominated by species that lived in more open habitats--grasslands and low shrubs--than those common at older Australopithecus-bearing sites, such as Hadar, where Lucy's species is found.

"We can see the 2.8 million year aridity signal in the Ledi-Geraru faunal community," says research team co-leader Kaye Reed, "but it's still too soon to say that this means climate change is responsible for the origin of Homo. We need a larger sample of hominin fossils, and that's why we continue to come to the Ledi-Geraru area to search."

The research team, which began conducting field work at Ledi-Geraru in 2002, includes:

• Erin N. DiMaggio (Pennsylvania State University), Christopher J. Campisano (ASU Institute of Human Origins and School of Human Evolution and Social Change), J. Ramón Arrowsmith (ASU School of Earth and Space Exploration), Guillaume Dupont-Nivet (CNRS Géosciences Rennes), and Alan L. Deino (Berkeley Geochronology Center), who conducted the geological research
• Faysal Bibi (Museum für Naturkunde, Leibniz Institute for Evolution and Biodiversity Science), Margaret E. Lewis (Stockton University), John Rowan (ASU Institute of Human Origins and School of Human Evolution and Social Change), Antoine Souron (Human Evolution Research Center, University of California, Berkeley), and Lars Werdelin (Swedish Museum of Natural History), who identified the fossil mammals
• Kaye E. Reed (ASU Institute of Human Origins and School of Human Evolution and Social Change), who reconstructed the past habitats based on the faunal communities
• David R. Braun (George Washington University), who conducted archaeological research
• Brian A. Villmoare (University of Nevada Las Vegas), William H. Kimbel (ASU Institute of Human Origins and School of Human Evolution and Social Change), and Chalachew Seyoum (ASU Institute of Human Origins and School of Human Evolution and Social Change, and Authority for Research and Conservation of Cultural Heritage, Addis Ababa), who analyzed the hominin fossil