Hominid bones show strides toward walking.

Animal fossils stored in a box since their excavation 15 years ago at an underground cave in South Africa have yielded an unexpected discovery--four foot bones that fit together to form the left instep of a hominid that lived about 3.5 million years ago. The bones provide the first clear fossil evidence that members of the human evolutionary family evolved in stages from climbing in trees and moving about on all fours to walking upright.

"This partial foot provides a locomotor missing link in the hominid fossil record," asserts Phillip V. Tobias, an anthropologist at the University of the Witwatersrand Medical School in Johannesburg. "It combines a weight-bearing heel used for two-legged walking with a chimplike big toe capable of grasping."

Tobias and Witwatersrand colleague Ronald J. Clarke unveil the foot fossils in the July 28 Science.

In conjunction with an excavation last year of the oldest fossil-bearing sediment in South Africa's Sterkfontein cave, Clarke examined material that had been recovered from the same soil layer in prior fieldwork and stored in boxes. One such container, thought to hold only the bones of nonhominids, disclosed the new foot specimens.

Until now, the oldest hominid remains at Sterkfontein dated to between 2.6 million and 2.8 million years ago, based on analyses of animal fossils that lay near them and magnetic reversals in the caves' sediment layers. These hominid finds have been assigned to the species Australopithecus africanus.

The new fossils come from a deeper soil layer that probably dates to about 3.5 million years ago, according to Tobias and Clarke. The bones most likely belong to a species of Australopithecus, perhaps the earliest known A. africanus, the scientists contend.

The heel bone, fully capable of supporting body weight during two-legged walking, closely resembles that of modern humans, Tobias says. The remaining three bones become progressively more apelike as they move away from the heel. The bone that forms the base of the big toe angles away from the other toes and shows evidence of considerable mobility, as in chimpanzees. And a large muscle that helped the hominid grasp objects was once attached to a deep groove in that bone, Tobias holds.

Anthropologists have long debated whether early hominids favored two-legged walking or balanced their walking with considerable tree climbing. Bones from a 2.5-million-year-old Sterkfontein hominid (SN: 4/22/95, p.253), as well as an analysis of hominid inner-ear bones (SN: 4/9/94, p.231), have recently indicated that an upright gait emerged gradually.

Some investigators assert that the approximately 3.6-million-year-old hominid footprints preserved in volcanic ash at the Laetoli site in East Africa, which were discovered in 1978, indicate a strikingly human foot anatomy and gait. A reconstruction of the entire Sterkfontein foot, derived partly from other early hominid foot specimens found in East Africa, closely matches the Laetoli footprints, Tobias contends.

"This is an exciting find because it helps to confirm the belief that hominids came down from the trees at some point and that there was an intermediate stage on the way to modern human feet," notes Michael Day, an anthropologist at the British Museum in London. Day examined the Sterkfontein foot bones last week on a visit to South Africa.

Day argues, however, that Clarke and Tobias' foot reconstruction is probably not accurate. East African specimens cannot provide a reliable picture of toe length or other features of the South African hominid, in his opinion.

Feet like those of the Sterkfontein hominid could not have made the Laetoli prints, which look like the footprints of modern humans who habitually walk barefoot, Day maintains.

"Our foot reconstruction may change, but the important point is the transitional nature of the bones we have now," Tobias remarks.