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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. However, a fossil lower jaw found in the Ledi-Geraru research area, Afar Regional State, Ethiopia, pushes back evidence for the human genus — Homo — to 2.8 million years ago.
According to a pair of reports published March 4 in the online version of the journal Science, the jaw predates the previously known fossils of the Homo lineage by approximately 400,000 years. It was discovered in 2013 by an international team led by Arizona State University scientists Kaye E. Reed, Christopher J. Campisano and J Ramón Arrowsmith, and Brian A. Villmoare of the University of Nevada, Las Vegas.
Researchers have found fossils that are 3 million years old and older. The most famous example of those human ancestors is the skeleton of Lucy, found in northeastern Africa in 1974 by ASU researcher Donald Johanson. Lucy and her relatives, though they walked on two feet, were smaller-brained and more apelike than later members of the human family tree.
Scientists have also found fossils that are 2.3 million years old and younger. These ancestors are in the genus Homo and are closer to modern day humans.
But very little had been found in between – that 700,000-year gap had turned up few fossils with which to determine the evolution from Lucy to the genus Homo. Because of that gap, there has been little agreement on the time of origin of the Homo lineage.
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.”
Arizona State University. “Discovery of 2.8-million-year-old jaw sheds light on early humans.” ScienceDaily. ScienceDaily, 4 March 2015. <www.sciencedaily.com/releases/2015/03/150304141454.htm>