New fossils discovered in the Afar desert of eastern Ethiopia are a missing link between our ape-man ancestors some 3.5 million years ago and more primitive hominids a million years older, according to an international team led by the University of California, Berkeley, and Los Alamos National Laboratory in New Mexico.
The fossils are from the most primitive species of Australopithecus, known as Au. anamensis, and date from about 4.1 million years ago, said Tim White, a UC Berkeley professor of integrative biology and one of the team's leaders. The hominid Australopithecus has often been called an ape-man because, though short-statured, small-brained and big-toothed, it walked on two legs unlike the great apes.
More primitive hominids in the genus Ardipithecus date from between 4.4 million and 7 million years ago and were much more ape-like, though they, too, walked on two legs.
"This new discovery closes the gap between the fully blown Australopithecines and earlier forms we call Ardipithecus," White said. "We now know where Australopithecus came from before 4 million years ago."
The fossil finds and an analysis of the hominid's habitat and evolutionary position are reported by White and co-authors from Ethiopia, Japan, France and the United States in the April 13 issue of Nature.
Since the first Australopithecus skull, the famous Taung child, was discovered in South Africa 82 years ago by Raymond Dart, fossils of this hominid have been found all over eastern Africa spanning a 3-million-year time period. Seven separate species have been named, including the most primitive, Au. anamensis, which dates from 4.2 million years ago, and Au. africanus, Dart's find. The most specialized species, Au. boisei, died out about 1.2 million years ago, long after the genus Homo had spread throughout the Old World.
The most famous of the Australopithecine fossils was "Lucy," a 3.5-foot adult skeleton discovered in the Afar depression in 1974. Her analytical team included White. Subsequently named Au. afarensis, this hominid, which lived between 3.6 and 3 million years ago, was also discovered in the Middle Awash study area, where the new Au. anamensis fossils were found.
Ardipithecus, on the other hand, was discovered by White and his team in 1992, based on fossils from Aramis, a village in the Awash Valley of Ethiopia's Afar rift. White and his team named the 4. 4 million-year-old fossils Ardipithecus ramidus.
The relationship between Australopithecus and Ardipithecus remained unclear, however, because of a million-year gap between these two genera. The new fossil finds, jawbones and teeth from each of two localities, bridge that gap. With Ardipithecus in older rocks and Au. afarensis in overlying rocks, the newly announced fossils are intermediate in time and anatomy.
The teeth tell a story about the organism's diet, White said. Australopithecus's large cheek teeth - anthropologists refer to the hominid as a megadont, meaning large-toothed - allowed it to subsist on a broader diet of tough, fibrous plants. The teeth of Ardipithecus were smaller, restricting it to a diet of softer, less abrasive food, White said.