Homo erectus—the direct ancestor of modern man and longest-lived hominid– walked on Earth for about two million years. But he didn’t walk alone. The most striking example of this fact is the Koobi Fora footprints found in northwestern Kenya. First discovered in 2021 and dating back 1.5 million years in the Calabrian Subdivision of the Pleistocene Epoch, these tracks span 26 feet long and contain a set of 12 footprints made by an individual and three footprints made by others (along with the noticeably different tracks of a long-extinct giant marabou stork, Leptoptilos falconeri).

Of course, footprints of ancient human species have already been found in East Africa. In 1974, for example, footprints of Australopithecus afarensis– which wandered through wet volcanic ash millions of years ago – were discovered in Tanzania, providing one of the first evidence of the existence of human bipedalism. Despite this watershed moment in anthropology, the Koobi Fora footprints show something completely different: they actually contain evidence of two distinct species of humans.

In a study published last week in the newspaper Scienceresearchers say they have discovered that the Koobi Fora footprints, likely located along an ancient lake shore 1.5 million years ago, actually contain a set of footprints belonging to both H. erectus (which had a gait similar to that of modern humans) and Paranthropus bouilei (a broader-faced Australopithecus with specialized adaptations for intensive chewing). Using 3D X-ray imaging, scientists analyzed the footprints and I discerned that the continuous track – the one containing a dozen steps – belongs to P. bouilei, while the others belonged to H. erectus.

Scientists were able to discern these differences mainly thanks to the position of P. Boilei a flatter foot and big toe, which had a greater range of motion (up to 19 degrees) than that seen in modern humans (about 10 degrees). It may not seem like a big difference, but this subtle adaptation is what allows humans to to walk and run without accumulating physical abnormalities of the foot over time.

“These fossilized footprints are as close as possible to a time machine to take us back to the edge of an East African lake 1.5 million years ago,” Bernard Wood, study co-author and paleoanthropologist at George Washington University, said Nature. “We can only assume that they were aware of each other’s presence. How exactly they interacted, or if they learned from each other or what, is still a mystery.

This depicts a remarkable scene of two ancient human species being able to interact and coexist with each other in East Africa. Of course, evidence of hominid fraternization exists in our DNA, as many people of European and Asian descent carry at least one to two percent of their blood. Neanderthal DNA in their genome. But fossils are rarely able to capture such a unique moment.

Darren lives in Portland, has a cat, and writes/edits about science fiction and how our world works. You can find his previous work on Gizmodo and Paste if you look hard enough.