Footprints Reveal Two Early Human Species Walked the Same Lakeshore in Kenya 1.5 Million Years Ago

Roughly 1.5 million years ago, members of two different species of early humans walked barefoot across a muddy lakeshore in what is now Kenya.

Their footprints, made within hours or days of each other, provide the first direct, physical evidence that multiple early human species coexisted in the same habitat, according to a new study published last week in the journal Science.

“It really is a snapshot in time,” says Tracy Kivell, a paleoanthropologist at the Max Planck Institute for Evolutionary Anthropology who was not involved with the study, to Nature News’ Miryam Naddaf.

The “mind-blowing” discovery also raises new questions about how the two species—thought to be Homo erectus and Paranthropus boisei—interacted, as Briana Pobiner, a paleoanthropologist at the Smithsonian’s National Museum of Natural History who was not involved with the research, tells CNN’s Katie Hunt.

“Maybe they actively competed for the same food; maybe they just eyed each other warily from across a grassy patch,” Pobiner adds. “Maybe they ignored each other entirely… It’s impossible to go back in time to actually observe these species 1.5 million years ago—but having both of their footprints on the same surface? That’s the next best thing.”

Researchers first discovered the fossilized footprints in July 2021 while excavating the eastern shore of Lake Turkana in Kenya at a site called Koobi Fora. They initially found one hominin footprint, as well as tracks that were probably made by large birds, reports CNN. The team covered the footprints with fine sand until they could take a closer look.

In 2022, researchers returned and excavated a 249-square-foot area. Those efforts unveiled 12 total hominin tracks in a straight line, which were probably made by the same individual. Nearby, they found three additional isolated hominin footprints that appear to have been made by three different individuals.

They also uncovered dozens of tracks made by wildlife, including three from horse-like creatures and 30 made by cow-like animals. They found 61 tracks left by birds, including some that were probably made by an extinct species of giant stork called Leptoptilos falconeri. The largest bird track was more than ten inches wide.

Researchers suggest the footprints were made at least 1.52 million years ago, then covered up almost immediately with sand and sediment. The footprints do not show any signs of cracking, which suggests they weren’t exposed to the sun and wind for very long—and that they were made very close to the same time.

At first, scientists couldn’t tell whether the footprints were left by different species. And, since few fossilized remains have been discovered, they couldn’t “do the Cinderella thing of fitting the foot skeleton into the footprint,” study co-author Kevin Hatala, a paleobiologist at Chatham University, tells the New York Times’ Katrina Miller.

But after creating detailed 3D images and comparing them to other specimens, the team found differences in gait and foot anatomy—such as toe angles and the depth of the arch—suggesting at least two species had walked along the lakeshore.

The linear, 12-print path was likely made by P. bosei, a hominin that had large molars and big chewing muscles that it likely used to chow down on tough plants and hard nuts. The small-brained, flat-footed species went extinct within a few hundred thousand years after the footprints were made, but researchers don’t know exactly why.

The discovery of P. bosei tracks is significant in its own right, because scientists had not previously known whether the hominins walked on two legs or four. The footprints are “unequivocal evidence of walking on two legs,” says Ashleigh Wiseman, a biological anthropologist at University College London who was not involved with the research, to New Scientist’s Michael Marshall.

The three scattered footprints, meanwhile, were probably made by H. erectus, which had bigger brains and human-like anatomy from the neck down. The bipedal species ate a diverse diet, used stone tools and might have cooked with fire. They survived for at least another one million years after the footprints were made and spread to new parts of the world.

Skeletal remains of H. erectus and P. boisei are the only early hominin remains to have ever been found in the area, which supports the scientists’ idea about who made the footprints.

Fossil evidence suggests the two species may have shared the site for as long as 100,000 years. This long timeframe suggests H. erectus and P. boisei must have achieved some sort of harmonious coexistence, Hatala tells the New York Times—or, at the very least, they likely didn’t view each other as direct competitors.

One possible explanation for this seemingly peaceful co-habitation is the two species’ differing diets, reports Science News’ Bruce Bower. H. erectus was an omnivore that ate a wide variety of foods, including meat, to fuel its large brain. P. boisei, meanwhile, was mostly eating grass-like plants known as sedges.

If the species weren’t battling for the same resources, they could treat the lake like a “1.5-million-year-old version of a 7-Eleven store,” says Bernard Wood, a paleoanthropologist at George Washington University who was not involved with the study, to Science News. Still, the footprints alone can’t reveal the nature of the species’ interactions, and it remains possible that they competed for resources.

It’s also possible the two species occasionally mated with each other—just like Neanderthals, Homo sapiens and Denisovans did—but researchers haven’t found any evidence of interbreeding.

“Human evolution is complicated and messy, and there’s lots of experimentation,” says William Harcourt-Smith, a paleoanthropologist at the American Museum of Natural History and the City University of New York who was not involved with the study, to NBC News’ Evan Bush. “It’s not a straight line.”

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Sarah Kuta is a writer and editor based in Longmont, Colorado. She covers history, science, travel, food and beverage, sustainability, economics and other topics.