Fossils unearthed in Ethiopia offer a glimpse of the time when humans and chimps were first going their separate evolutionary ways--and may represent the earliest known human ancestor. The remains--a jawbone with teeth as well as arm, hand, and foot bones--have been dated at between 5.2 million and 5.8 million years old. From the shape of one nearly complete foot bone, the discoverers conclude that their specimen walked upright, a hallmark of all hominids.
The find comes hot on the heels of the report of 6-million-year-old bones found in Kenya's Tugen Hills, also hailed by their discoverers as belonging to the earliest known hominid (ScienceNOW, 22 February). The two creatures share a crucial feature: Both appear to have lived in relatively wet woodlands. If either is indeed a hominid, that could overturn a long-held theory that bipedalism evolved when forest-dwelling apes moved out into open savannas, possibly as a result of climate change.
The Ethiopian fossils were found between 1997 and early this year by an Ethiopian-American team led by Yohannes Haile-Selassie and Tim White of the University of California, Berkeley. The dates--determined by argon-argon dating and confirmed by paleomagnetic measurements and analysis of animal bones found in the same sediments--place the teeth and bones around the time when most geneticists believe that humans and chimpanzees split from a common ancestor, between 6 million and 9 million years ago. Describing their find in two papers in the 12 July issue of Nature, the team has named its specimen Ardipithecus ramidus kaddaba. The researchers argue for its hominid status partly because of its lower canine teeth, which in cross section are diamond-shaped like those of later hominids rather than V-shaped like those of apes. They also note that the foot bone has features--such as a joint's orientation--similar to those of later hominids.
This orientation "suggests bipedality," says Juan Luis Arsuaga of the University of Madrid, although "the evidence is still weak" because so far it is based primarily on a single bone. Fred Spoor of University College London agrees that the jury is still out on hominid status: "Neither this nor the Orrorin paper make a really watertight case," he says.