Ethiopian find shows human ancestors walked upright as early as 5 million years ago

By Robert Sanders, Public Affairs

Fossils from an Ethiopian site suggest that our human ancestors walked upright much earlier than previously thought. Above, a collection of fragments from Ardipithecus ramidus kadabba point to bipedalism as early as 5.2 million years ago. photo courtesy of Tim D. White\Brill Atlanta

11 July 2001 | Scouring the dry washes encircling an Ethiopian site where scientists seven years ago found fossils of 4.4 million-year-old human ancestors, Berkeley graduate student Yohannes Haile-Selassie has found even older fossils that show human ancestors walked on two legs as early as 5.2 million years ago.

The fossils are the earliest hominid known, and date from close to the time when human ancestors are believed to have split off from the chimpanzees on the first steps of their evolutionary trip to modern Homo sapiens.

The fragmentary fossils, which include teeth, a jawbone, hand, arm and collar bones, and one toe bone, appear to be from family members of the species discovered in 1994 by an international team led by Berkeley paleoanthropologist Tim White. They named that species Ardipithicus ramidus, and concluded that it was the earliest known human ancestor. Haile-Selassie, for now, has designated the new fossils as a subspecies of this earlier find: Ardipithicus ramidus kadabba.

“Its dentition is that of a hominid, its toe bone is like that of a bipedal animal,” said Haile-Selassie from Addis Ababa. “It’s definitely a hominid, and proves that the earlier 4.4 million-year-old Ardipithecus ramidus was a hominid, not an ape.”

Haile-Selassie reported his finds in the July 12 issue of Nature. A second paper in Nature by geologist Giday WoldeGabriel of Los Alamos National Laboratory, co-written by Haile-Selassie, White and others, described the paleoclimate of the area Ardipithecus roamed nearly 6 million years ago.

The publication of these two papers by Haile-Selassie and WoldeGabriel, both Ethiopians, is a major milestone in African paleoanthropology, according to White, a professor of integrative biology.

Haile-Selassie found the new fossils along the western margin of the Afar rift in the Middle Awash region of Ethiopia, about 140 miles northeast of the capital Addis Ababa and 25 kilometers from the Aramis site where White’s team found A. ramidus.

“When you don’t find anything, an hour is like a day,” Haile-Selassie said about discovering the first fossil — a mandible or jawbone with one molar — in 1997. “But when you find good stuff, you don’t even want night to come, you want to work 24 hours to find more. That’s how exciting it was.”

He subsequently found at the five sites a total of 11 fragments representing at least five individuals. Together, he said, these provide evidence that the chimpanzee-sized creature was not an ape but an early ancestor of humans.

“These canine teeth are not of humans, but no chimp has canine teeth like that either,” White said. “This argues that these fossils are not from the common ancestor of both chimps and humans, but from very early in our evolution, shortly after our ancestors parted company and before our canines fully reduced.”

Another argument that the fossils are from a hominid, not an ape, is that the toe bone shows a slanted surface at the rear joint, which is characteristic of bipedal walking. This is caused by toeing-off — pushing forward by leaving the front part of the foot on the ground and lifting the heel. This anatomy is characteristic of A. ramidus and all later hominids, but not of chimpanzees and other apes, which walk on the outside of their feet.