A STEP IN TIME: The 1.5-million-year-old footprints discovered near Ileret, Kenya, were likely made by an ancient relative, Homo ergaster, as it walked along the muddy shores of a river amidst birds and animals.
MATTHEW BENNETT/BOUMEMOUTH UNIVERSITY

Freshly discovered trails of ancient footprints, left on what was once the muddy shores of a river near Ileret, Kenya, indicate that some 1.5 million years ago human ancestors walked in a manner similar to that of people today. The international team of researchers who analyzed the prints say that those who left them had feet that looked a lot like ours.

The prints were probably left by Homo ergaster, an earlier, larger version of the widespread Homo erectus, says David Braun, a lecturer in archeology at the University of Cape Town in South Africa and co-author of the study set to be published tomorrow in Science. This discovery "lets us know that they were probably just as efficient at walking upright as we are," he says.

Previous research has shown that human ancestors were perfectly capable of getting around on their hind legs 3.5 million years ago—and perhaps even earlier. But Braun says these prints reveal, for the first time, a very modern foot with a parallel big toe—unlike an ape's opposable digit that's easily curled for grasping tree branches. Homo sapiens proper are said to have emerged about 200,000 years ago.

Footprints can tell scientists a lot about creatures that a skeleton cannot. From them, scientists can learn about the gait, weight distribution and even the approximate size of those who made them.  Braun says these prints were apparently made by pedestrians who stood just under five feet (1.5 meters) tall. A modern upright stride can indicate a lot about behavior, as well, says David Raichlen, an assistant professor of anthropology at the University of Arizona in Tucson, who cites long-distance walking and running as possible benefits of this adaptation.

"It really is a snapshot of time," Braun says. The preserved area also shows a wealth of animal prints, which gives more precise information about what creatures shared the space and time. Exhumed fossils can yield info on general environments; footprints can provide a glimpse into life over days rather than millennia. "With the footprints," Braun says, "we can almost certainly say these things lived in the same time as each other, which is unique."

It is much rarer to find footprints than bones, because conditions must be perfect for tracks to be preserved, according to Braun. In this case, the tracks were made during a rainy season near an ancient river just before that river changed course and swept a protective layer of sand over them.

The last major set of footprints, discovered in 1978 in Laetoli, Tanzania, were dated to about 3.6 million years ago. But those revealed a more ancient foot and gait, and it is still debatable whether those who made them had a stride more akin to humans or to chimpanzees, says Raichlen, who has studied the Laetoli prints.

The Ileret tracks were digitally scanned using a laser technique developed by lead study author, Matthew Bennett, a geoarchaeologist at Bournemouth University in Poole, England. Raichlen says the find gives people a rare view of those that have trod before. "It's important to think about what you're really getting: a glimpse of behavior in the fossil record that you wouldn't really get in any other way," he says. The research reveals "a moment in time when individuals are walking around the landscape. It sort of fleshes out and brings them back to life, in a way."