Our ancestors may have spent most of their time in the trees, but their feet were made for walking 2 million years earlier than thought. Footprints made in Tanzania, East Africa, by our hominin ancestors some 3.5 million years ago suggest they walked with an upright gait that is distinctly human.
The Laetoli trail, as these footprints are known, was made by hominins who walked through powdery volcanic ash. The prints were cemented by a soft rain and preserved long enough to be unearthed in 1976.
"A few people argue that the Laetoli prints were created in an ape-like way," with a forward-bend posture, study researcher Robin Crompton of the University of Liverpool said. "Our findings are very different. They support the opposite interpretation that they are very modern footprints in many respects."
The researchers took very precise digital scans of the footprints and found small changes in their depth to indicate where more force was placed – either at the front or the back of the foot. They compared these prints to footprints from modern humans (who typically wear shoes), some examples of barefoot walkers from Kenya and India (who normally don't), and footprints from chimpanzees and bonobos walking upright.
"These [Laetoli] footprints are quite definitely well within the modern human range, I'm sure of that," Crompton said. "This is a very early date for human-like walking." Specifically, the impressions showed that the hominins pushed off from the surface using their big toes; today's great apes, on the other hand, push off with their mid-foot.
The only human-like primate known to be around when these prints were laid down was Australopithecus afarensis, typified by the Lucy specimen discovered in 1974 and known to be 3.2 million years old.
Lucy in the trees
While the current study indicates our upright gait has been similar for millions of years, Crompton believes A. afarensis still spent most of his time in the trees. These primates probably couldn't walk that far, he said — maybe about six miles (10 kilometers) but not 120 miles (200 km).
The changing landscape of the time, which featured drying and separation of the forest canopy, probably made traveling through the trees progressively more difficult and prompted hominins to take to the ground.
"Humans managed to maintain and increase their range by becoming able to use both the ground and, when they needed, go to the trees for escape from predators," Crompton said. "This is a clear indication that walking on two feet evolved not on the ground but in the trees."
"These hominins used human-like walking on the ground but were still capable of using resources in the trees," David Raichlen, a researcher at the University of Arizona who wasn't involved in the study, told LiveScience in an email. "However, this study does support the idea that some of the adaptations for climbing we see in the feet of other apes were not present in Laetoli hominins."