Hominids: down-to-earth or up tree?

For more than 15 years, the way in which Lucy and other members of Australopithecus afarensis, the earliest known species in the human evolutionary family, moved about has sparked considerable debate.

One side argues that these hominids, which lived from about 4 million to 3 million years ago, preferred walking and spent only a small amount of time in the trees. New African fossil finds support this view (SN: 11/20/93, p.324).

However, evidence presented at the American Association of Physical Anthropologists' annual meeting in Denver last week depicted Lucy's species and many subsequent hominids as creatures that balanced walking with substantial tree climbing.

"Australopithecines are more similar to chimpanzees than to modern humans in their inner-ear anatomy," asserts C. Fred Spoor of the University of Liverpool in England. "This supports the view that australopithecines combined arboreal and terrestrial movement."

Inner ears like those of modern humans first emerged in Homo erectus, Spoor contends. H. erectus lived from around 1.8 million to 300,000 years ago.

A "bony labyrinth" houses the inner ear, which contains sense organs for perceiving sound, movement, and spatial orientation.

Spoor and Frans W. Zonneveld, a radiologist at the University of Utrecht, the Netherlands, took computerized tomography (CT) scans of the area around the ear in modern human, chimpanzee, gorilla, and orangutan skulls. Image enlargement made it possible to visualize each specimen's bony labyrinth.

Although humans and apes share many aspects of inner-ear anatomy, humans display markedly larger semicircular canals relative to body weight, Spoor holds. These structures support a balanced, upright stance, he says.

CT analysis of 35 hominid fossils with preserved bony labyrinths finds humanlike semicircular canal proportions only in H. erectus, according to Spoor. The fossil sample also includes specimens attributed to H. habilis and three australopithecine species - excluding A. afarensis. No fossils of Lucy's kind bearing the bony labyrinth had been found at the time of the study.

However, this structure apparently remains on an A. afarensis skull found in Ethiopia (SN: 4/2/94, p.212), says Donald C. Johanson of the Institute of Human Origins in Berkeley. Calif. CT analysis of that fossil may prove difficult, since it must remain in Ethiopia.

"Spoor has applied modern technology to hominid fossils in a new way," Johanson remarks. "There appear to be distinctions between hominid groups based on the bony labyrinth, but this needs further study."

Another study presented in Denver, directed by Randall L. Susman of the State University of New York at Stony Brook, suggests that Lucy lifted her legs rather awkwardly while walking. "It must have looked like a modern human walking at the beach while wearing a pair of flippers," Susman contends.

Susman videotaped two men and two women walking at various speeds while barefoot, wearing fitted shoes, or wearing shoes that boosted foot length by 30 percent relative to leg length. Researchers estimate that Lucy's proportional foot length was 30 percent greater than that of the average human.

Larger feet elicit greater bending at hip and knee joints, resulting in a highstepping gait, Susman asserts.

Lucy's leg appears well adapted to her foot length, Johanson responds, making it unlikely that people - whose legs are designed to go with a different foot length - can simulate her stride. With tongue half in cheek, he calls Susman's study "the clown-shoe hypothesis."