3.6-Million-Year-Old Footprints Suggest Early Humans Were Walking Upright Before The Genus 'Homo' Evolved

Alexandra Lozovschi

The genus Homo is thought to have emerged about 2 to 2.5 million years ago, but it seems our early hominin ancestors were already walking fully upright by then.

Fossil footprints that date back to 3.6 million years ago, and which have now become the "earliest direct evidence of hominin bipedalism," revealed that our early ancestors evolved the hallmark upright walk that we use today, with a vertical torso and extended legs, substantially earlier than previously believed, Phys.org reports.

The conclusion comes from evolutionary anthropologist David Raichlen of the University of Arizona, who presented his research at a symposium on the evolution of human locomotion, held yesterday (April 22) during the 2018 Experimental Biology Conference in San Diego.

"Upright, humanlike bipedal walking goes back 4 to 5 million years," Raichlen told the Washington Post prior to the symposium.

Hominins are thought to have begun walking on two legs around 7 million years ago, but the early walking mechanics were characterized by a crouched posture, with bent knees and hips, described by Raichlen as flexed limb mechanics. This is quite similar to the way chimpanzees walk, notes the Washington Post.

"By 3.6 million years ago, our data suggest that if you can account for differences in size, hominins were walking in a way that is very similar to living humans," Raichlen explained in a statement.

The 3.6-million-year-old footprints also belonged to members of the genus Australopithecus, which includes the famous "Lucy," a 3.2 million-year-old Australopithecus afarensis. By comparison, Homo sapiens sapiens, the species that comprises modern humans, appeared roughly 200,000 to 300,000 years ago, Phys.org points out.

Based on the fossil footprints — uncovered in 1976 and 2015, almost 40 years apart, according to Forbes — and on skeletons from the hominin fossil record, Raichlen reconstructed the walking mechanics of our early human ancestors. What he found was that the Laetoli footprints were "consistent with a fully-upright, straight-legged walking posture," states Newsweek.

At the same time, Raichlen analyzed the depth and shape of the 3.6-million-year-old footprints and compared them with two sets of experimentally generated footprints, made by eight volunteers that used both extended and flexed limb mechanics. The results showed that the Laetoli footprints were much more similar to the ones made by modern humans walking upright.

"If you were in a time machine, looking at 'Lucy' in the distance, the way they walked would have looked very humanlike," said another symposium participant according to the Washington Post.

Raichlen points out that, since the extended-limb mechanics are more efficient in terms of energy use, these findings indicate that by 3.6 million years ago our ancestors were already walking for large distances, most likely in search for food.

"Selection may have acted at this time to improve energy economy during locomotion, generating the human-like mechanics we employ today," he concludes.