(Inside Science) -- A nearly complete 3.8 million-year-old skull found in Ethiopia offers the first detailed look at the face of the earliest known member of the extinct group at the base of the human family tree. The discovery, described in two papers published today in the journal Nature, sheds new light on how these ancient relatives of humans evolved, and brings into question which species may have given rise to the human lineage.
Among the earliest known relatives of humanity that evolved to walk upright was Australopithecus afarensis, the species that includes the famed 3.2 million-year-old "Lucy." Australopithecines such as A. afarensis are among the leading candidates for the direct ancestors of Homo, the human lineage.
Until now researchers had found mostly only partial jaw bones and isolated teeth of the oldest known australopithecine, Australopithecus anamensis, dating between 4.2 million and 3.9 million years old. In contrast, they have found multiple skulls from younger australopithecine species.
Now scientists have revealed a mostly complete skull they identified as A. anamensis based on its teeth and jaw. Unearthed from the Woranso-Mille site in Ethiopia in 2016 and about 55 kilometers north of where Lucy was discovered, the fossil likely came from an adult male with a brain roughly one-third the size of the average modern human's, said paleoanthropologist Yohannes Haile-Selassie, curator and head of physical anthropology at the Cleveland Museum of Natural History.
The specimen was more apelike than A. afarensis was, with a more protruding face to support chewing of tough food, as well as larger canine teeth. It was found in the sandy deposits of a delta where a river entered a lake. The species likely lived along forested river and lake shores amid dry shrubland and steep hillsides that were occasionally blanketed with ash and lava, said sedimentologist Beverly Saylor at Case Western Reserve University in Cleveland. This diverse, active landscape may have at times isolated groups of australopithecines, spurring evolutionary variation, she added.
The new findings suggest A. anamensis and A. afarensis overlapped in time for at least 100,000 years, contradicting long-held assumptions that the former evolved into the latter in a linear manner, Haile-Selassie said. The fact that two or more australopithecine species may have coexisted could mean that one of them gave rise to the genus Homo.