Earliest Ancestor Emerges in Africa.

Our ancient kin have taken a big step back in time. An international team working in Ethiopia has found bones and teeth of the earliest known hominid, a member in good standing of humanity's evolutionary family.

The fragmentary remains come from at least five individuals--in the genus Ardipithecus--who lived between 5.2 million and 5.8 million years ago, says anthropology graduate student Yohannes Haile-Selassie of the University of California, Berkeley. He describes the finds in the July 12 NATURE.

Until now, the earliest Ardipithecus fossils came from a 4.4-million-year-old Ethiopian site. Australopithecus, the genus that includes Lucy's famous remains, lived in eastern Africa no more than about 4 million years ago.

DNA studies have suggested that a common ancestor of people and chimpanzees lived in Africa anywhere from 5 million to 7 million years ago.

"Ardipithecus was close [in time] to the common ancestor of chimpanzees and humans," Haile-Selassie says. "We don't know what that common ancestor looked like."

Much is also unknown about Ardipithecus' looks. The new finds consist of a partial jaw, a few teeth, several hand and foot bones, and pieces of an upper-arm bone and a collarbone. The bones are about the size of those from a modern common chimp. However, Ardipithecus displays dental features found in other hominids but not in any fossil or living ape.

Moreover, the new finds include a toe bone shaped like those of Lucy and her kind. This constitutes "subtle but clear evidence" that Ardipithecus, like Australopithecus, walked on two legs, says anthropologist C. Owen Lovejoy of Kent (Ohio) State University, who independently examined the toe fossil. Ongoing studies of 4.4-million-year-old Ardipithecus fossils will further illuminate this hominid's stance, Lovejoy says.

The fossils were unearthed at sites in what is now a desert. When Ardipithecus lived there, the region contained a dense forest and had a cool, wet climate, according to studies led by Giday WoldeGabriel of Los Alamos (N.M.) National Laboratory.

That work, reported in the same issue of NATURE, includes chemical analyses of fossil-bearing soil and study of the sites' other animal remains, such as extinct elephants, rats, and monkeys.

The researchers dated layers of volcanic ash above and below the fossils by measuring argon gas trapped in samples of the rock. This gas accumulates at a known rate in rocks and minerals. Age estimates based on orientation shifts in ancient Earth's magnetic field at the sites corroborated the argon-based dates.

"This is the best evidence for the earliest hominids," comments anthropologist Laura M. Maclatchy of Boston University.

The new finds raise puzzling questions about why early hominids evolved an upright stance, Maclatchy adds. Researchers have often portrayed a two-legged stride as an adaptation to trekking across hot, grassy savannas. Yet Ardipithecus lived in shady forests where a hominid would have less need to stand up to dissipate heat or walk long distances.

Earlier this year, a French team announced the discovery of fossil teeth and limb-bone fragments of a 6-million-year-old hominid in Kenya. The researchers placed this creature in a new genus, Orrorin.

Orrorin's evolutionary status is uncertain, Haile-Selassie holds. Its teeth resemble those of apes far more than those of later hominids, he says. However, Orrorin's leg bones may have supported an upright stance, Haile-Selassie acknowledges.