Modern feet step back 1.5 million years

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 geologist Matthew Bennett of Bournemouth University in Poole, England, and his colleagues in the Feb. 27 Science. Finding what amounts to the fossilized behavior of these creatures provides 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 range of modern humans. Digitized images of the newly discovered footprints 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 of the prints include a pronounced arch and short toes.

Ancient foot impressions at Ileret complement earlier fossil leg and pelvis finds in Africa indicating that, by about 2 million years ago, early H. erectus 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 anthropologist and 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, Lieberman remarks.

“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?” he asks.

That’s a plausible hypothesis, comments anthropologist Susan Antón of New York University, but she says the Ileret footprints might instead come from either of two other species in the human evolutionary family, Homo habilis or Paranthropus boisei. Scientists know little about the size variation in H. erectus (SN: 12/6/08, p. 14) and the other two species. Some individuals in any of these species may have had feet as big as those that made the Ileret impressions, Antón says.

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 combined with a shallower arch and more angled big toe than seen at Ileret.

Anthropologist Bernard Wood of George Washington University in Washington, D.C., agrees. The Ileret footprints underscore skeletal evidence for a radical shift in anatomy that occurred 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.

The Ileret finds are “a treasure” that strongly supports a modern-humanlike stance by 1.5 million years ago, says anthropologist Russell Tuttle of the University of Chicago. But the nature of the Laetoli individuals’ feet and gait, as well as their evolutionary identity, remains unresolved, in Tuttle’s view.

Digitized analyses of the Laetoli footprints have not been conducted, he notes. Weathering of the Tanzanian impressions after excavation, followed by reburial of the finds because of ground movement and vegetation growth, will obstruct further study, adds Tuttle, who studied the Laetoli footprints shortly after their discovery.

Scientific prospects appear brighter at Ileret. From 2005 to 2008, excavations there revealed footprints in two sediment layers, one about five meters above the other. Earlier research had dated both layers to about 1.5 million years ago, separated by 10,000 to 20,000 years.

Three footprint trails appear on the upper sediment layer — two trails of two prints each and one of seven prints. Another six isolated prints dot the surface. On the lower layer, the researchers found one trail of two prints and a single, relatively small print that was probably made by a young individual. A variety of four-legged animals also left footprints on this layer.

“I have no doubt that we’ll find more footprints at this site,” Harris says.