The recent discovery of a 2.8 million year-old fossilized jawbone in Ethiopia is helping scientists clear up a clouded period in the evolution of modern humans.

The African continent is rich with fossils documenting the earliest phases of human evolution, yet fossilized remains dating from 2 to 3 million years ago have been hard to find. UNLV anthropologist Brian Villmoare is co-director of an international research team that found the jawbone and presented its findings in the March 4 online issue of the journal Science.

The unearthed jawbone is the earliest evidence of the genus Homo - which shared both human and ape-like features - and helps fill in a particularly elusive gap in the fossil record.

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

Scientists found the fossil in 2013 in the Ledi-Geraru research area in the Afar Regional State in Ethiopia. The jaw predates the previously identified fossils of the Homo lineage by approximately 400,000 years.

The Ledi-Geraru jawbone provides insight to developmental changes in the jaw and teeth in Homo only 200,000 years after the last known occurrence of Australopithecus afarensis - made famous by the 1974 discovery of "Lucy" in the nearby Ethiopian site of Hadar.

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 with ASU's Institute of Human Origins, revealed advanced features. For example, the fossil has slim molars, symmetrical premolars, and an evenly proportioned jaw.

These features distinguish the early species on the Homo lineage from the more apelike early Australopithecus, yet the fossil's primitive, sloping chin links it to a Lucy-like ancestor.

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

The Ledi-Geraru Research Project is based in the Institute of Human Origins at Arizona State University.