Footprints suggest H. erectus walked the modern human walk: 1.5-million-year-old fossils excavated in eastern Africa.

Human ancestors created some remarkably lasting impressions on the eastern African landscape around 1.5 million years ago. Walking across a muddy patch of terrain near what's now Ileret, Kenya, these ancient individuals left footprints that hardened and have now been excavated by a team of scientists.

On close inspection, the preserved footprints provide the oldest evidence for a virtually modern-human foot and walking style in a human ancestor, report Matthew Bennett of Bournemouth University in Poole, England, and his colleagues in the Feb. 27 Science. The findings provide new clues to the evolution of upright stance and walking in modern humans.

Bennett's team identifies the ancestor as an early Africa-based Homo erectus, or Homo ergaster as some scientists call it.

Measures of the size, spacing and depth of the Ileret impressions allowed the researchers to estimate individuals' heights, weights and stride lengths, all of which fell within the ranges of modern humans. Digitized images of the prints show a big toe in line with the other toes, an arrangement that contrasts with the angled, grasping big toes of apes. Other humanlike features include a pronounced arch and short toes.

Ancient foot impressions at Ileret complement fossil leg and pelvis finds in Africa indicating that H. erectus, which appeared about 2 million years ago, displayed much the same body size and proportions as modern humans, the researchers say.

"The Ileret footprints add to evidence that early Homo erectus had a body adapted to traveling long distances, at a time when food sources were patchily distributed across the landscape," says excavation director John Harris of Rutgers University in New Brunswick, N.J.

Harvard University anthropologist Daniel Lieberman says the African footprints support his 2004 proposal that around 2 million years ago the Homo genus evolved bodies capable of running long distances. Springlike arches and short toes observed on the Ileret footprints would have enabled endurance running. "How could H. erectus have hunted more than a million years before the invention of tipped spears, as we know it did, without the ability to run well?" Lieberman says.

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That's a plausible hypothesis, comments Susan Anton of New York University, but she says the footprints might instead come from either of two other species, Homo habilis or Paranthropus boisei. Scientists know little about the size variation in H. erectus (SN: 12/6/08, p. 14) or the other two species.

Other footprints from members of the human evolutionary family, dating to 3.5 million years ago, were discovered by Mary Leakey between 1977 and 1979 at Laetoli, Tanzania. Researchers disagree about whether these finds--often attributed to Australopithecus afarensis, a fossil species that includes the partial skeleton of Lucy--reflect an apelike or humanlike foot anatomy.

Bennett's team argues that the Laetoli prints, which are smaller than those at Ileret, show signs of an upright gait, shallower arch and more angled big toe.

Bernard Wood of George Washington University in Washington, D.C., agrees. The Ileret footprints underscore skeletal evidence for a radical shift in anatomy sometime between the demise of A. afarensis around 3 million years ago and the appearance of H. erectus nearly 1 million years later, Wood holds.