FOOTPRINTS laid down near Lake Turkana in north-west Kenya 1.5 million years ago were made by human ancestors with essentially modern foot anatomy and gait.
They are the second oldest hominin footprints known, after the 3.7-million-year-old Laetoli prints found by anthropologist Mary Leakey in Tanzania in 1978, which are more ape-like and are suspected to be from a species of Australopithecus (see diagram).
More importantly, they are the oldest made by human ancestors; probably early Homo erectus, who was thought to share our stature, foot anatomy and springy, efficient stride. Because fossilised foot bones are fragile, and thus scarce, researchers have not previously been able to verify their inferences about the foot anatomy or gait of H. erectus.
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鈥淏etween 1.8 and 1.5 million years ago, H. erectus evolved rapidly, [becoming] very different to anything that came before,鈥� says Matthew Bennett of Bournemouth University, UK, who was part of the team. Those differences included shorter arms, longer legs and 鈥� the footprints show 鈥� a modern foot and an energy-saving bounce from the arch and big toe. Like modern apes, earlier human ancestors had short legs and strides, and a flat-footed, less efficient gait.
The evolved stride matters, says Bennett, because the increasing mobility of H. erectus opened up a wider range of habitats to occupy and exploit.
Bennett and his colleagues unearthed two layers of footprints at Koobi Fora, an archaeological site near the town of Ileret in Kenya. The lower layer includes two trails, each with two footsteps, and an isolated footprint; the upper layer had three trails, one with seven steps. The prints are sandwiched between layered deposits of volcanic ash whose age is well known and indicates that both sets of prints were made between 1.53 and 1.51 million years ago.
鈥淲e see characteristic human prints, with a clear heel, an imprint on the outside of the foot, and then a really deep impression underneath the ball of the foot,鈥� says co-author Brian Richmond, a palaeoanthropologist at George Washington University in Washington DC. 鈥淲hereas the Laetoli prints have debatable evidence of these modern characteristics, the prints from Kenya are compelling.鈥�
On several key features, including length, a pronounced arch, and the position of the big toe in relation to the others, the Ileret prints are much more modern than those from Laetoli (Science, ). Crucially, they reveal a modern stride in which weight shifts from the heel to the ball of the foot and then to the big toe, providing a platform to push off from.
For team member Jack Harris of Rutgers University in New Brunswick, New Jersey, the footprints confirm what researchers have been piecing together about how H. erectus evolved. 鈥淭hey had the anatomy to range over longer distances and move into more diverse habitats,鈥� he says.
鈥淭he footprints show Homo erectus could range over longer distances and move into more diverse habitats鈥�
Rather than mobility in itself, he suggests, it was the consequent adaptation to a wider range of environments that enabled H. erectus to become the first hominin to leave Africa. 鈥淭hey just happened to adapt to environments that eventually connected them with Europe and Asia,鈥� he says.
Palaeoanthropologist William Jungers at Stony Brook University in New York state, who was not a member of the team, is intrigued by the footprints. They are 鈥渁 precious window into precisely how these ancestors walked鈥�, he says. 鈥淭he emergence of our own genus, Homo, is linked to the adaptive shift revealed by these fossilised footprints.鈥�