Muddy footprints suggest that 1.5 million years ago, two species of ancient humans coexisted in Kenya.

WASHINGTON (AP) — Muddy footprints left on the shore of a Kenyan lake suggest two of our earliest human ancestors lived next door to each other about 1.5 million years ago.

The tracks were left in the mud by two different species “over a period of hours or at most days,” said paleontologist Louise Leakey, co-author of the study published Thursday in the journal Science.

Scientists previously knew from fossil evidence that these two extinct branches of the human evolutionary tree, called Homo erectus and Paranthropus boisei, lived around the same time in the Turkana Basin.

But the dating of the fossils is not precise. “It’s a few thousand years, give or take,” said paleontologist William Harcourt-Smith of Lehman College and the American Museum of Natural History in New York, who was not involved in the study.

Yet despite the fossilized traces, “a real point in time has been preserved,” he said. “This is an amazing discovery.”

Trace fossils were discovered in 2021 at the site of what is now Koobi Fora in Kenya, said Leakey, who is based at Stony Brook University in New York.

Regardless of whether the two people walked along the eastern shore of Lake Turkana at the same time – or a day or two apart – they were likely aware of each other’s existence, said study co-author Kevin Hatala, a paleoanthropologist at Chatham University in Pittsburgh.

“They probably saw each other, probably knew each other was there, and probably influenced each other in some way,” he said.

Scientists were able to differentiate the two species thanks to the shape of the footprints, which provides clues to the anatomy of the foot and how it is used.

H. erectus appears to have walked in a similar way to how modern humans walk: first striking the ground with its heel, then rolling onto the balls of its feet and toes, and pushing off again.

The other species, which also walked upright, moved “in a way unlike anything we’ve seen anywhere else before,” said co-author Erin Marie Williams-Hatala, a human evolutionary anatomist at Chatham.

Among other details, the footprints suggest greater mobility of the big toe compared to H. erectus or modern humans, Hatala said.

Our common primate ancestors likely had hands and feet adapted for grasping branches, but over time, the feet of human ancestors evolved to walk upright, the researchers say.

The new study adds to a growing body of research that shows the transition to bipedalism—walking on two legs—did not happen at one point or in one particular way.

Most likely, ancient people learned to walk, run, stumble, and slide along prehistoric mud slopes in a variety of ways.

“It turns out there are different gait mechanics—different ways of being on two legs,” Harcourt-Smith said.

___

The Associated Press Health and Science Department receives support from the Howard Hughes Medical Institute’s Science and Education Media Group. AP is solely responsible for all content.