The excavation of the 2-million-year-old Homo naledi, our most recently discovered human ancestor, shocked the anthropological world when it was revealed that it appeared to bury its dead – a trait previously thought to be exclusive to humans.

Now, it looks like another ancient ancestor has just let slip a secret that will change our understanding of our own evolution yet again. As revealed by fossilized footprints, Homo erectus walked just like modern humans, which suggests our walking style evolved as long as 1.9 million years ago, around the time this species evolved.

H. erectus once lived across much of Africa and Asia, and was still around 143,000 years ago, about 57,000 years after H. sapiens, our own species, first appeared. It is generally recognized as being the oldest human ancestor to have modern human-like body proportions, elongated legs, and the ability to walk upright to a degree and even trek long distances – as opposed to climbing trees, as its close ancestors and cousins did. Despite these similarities, scientists still tend to consider H. erectus still quite different from ourselves.

This new study in the journal Scientific Reports confirms that H. erectus was more like us than previously thought. Back in 2006, a series of 1.5-million-year-old footprints were found in Kenya, and they were identified as clearly belonging to H. erectus. Since then, a team of researchers have been reconstructing how these footprints were made, and they are confident that not only did their owners walk upright, but that they were fully bipedal.

Although evidence of walking on two feet dates back to 6-7 million years ago, a sparse fossil record means that ascertaining just how efficiently bipedal and human-like our ancestors’ gaits is often very difficult. These well-preserved 97 tracks, created by at least 20 individual members of H. erectus, allowed researchers to create digital models of them, before comparing them to habitually barefoot members of local tribespeople.

Without a doubt, these ancient footprints are indistinguishable from the modern equivalent. “Our analyses of these footprints provide some of the only direct evidence to support the common assumption that at least one of our fossil relatives 1.5 million years ago walked in much the same way as we do today,” Kevin Hatala, a researcher at the Max Planck Institute for Evolutionary Anthropology and The George Washington University and lead author of the study, said in a statement.

^{Image in text: Reconstruction of an adult female. H. erectus. John Gurche/Tim Evanson/Smithsonian Museum of Natural History/Wikimedia Commons; CC BY-SA 2.0}

The authors of this paper note that the footprint sizes indicate that multiple males were walking together in the group, which implies a sense of cooperation and a move away from the sole male dominated hierarchies seen in other primate species. So not only did they have similar walking styles, but similar social styles to contemporary humans.

The big question, though, is what happened to allow such a swift transition from tree-climbing adaptations to efficient walking mechanisms? Some have suggested that the need to use hands for food gathering or tool-wielding prompted the change, whereas others think that it simply requires less energy compared to scampering around on all fours. Perhaps it simply made co-operation easier somehow – either way, it’ll always be difficult to really know what triggered it.

Along with a recent, dramatic re-dating of the revelatory H. naledi fossils, this definitive discovery regarding H. erectus makes it very clear that each new discovery brings with it more questions than answers. Just when we think we know exactly how our species arose from our past, another new piece of information highlights just how much more we’ve got left to understand.

^{Snap! Credit: Kevin Hatala}