These two ancient human relatives crossed paths 1.5 million years ago

Some 1.5 million years ago, two ancient species crossed paths on a lake shore in Kenya. Their footprints in the mud were frozen in time and lay undiscovered until 2021.
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Now, analysis of the impressions reveals that they belonged to Homo erectus, a forebear of modern humans, and the more distant relative Paranthropus boisei. The two individuals walked through the lake area within hours or days of each other — leaving the first direct record of different archaic hominin species coexisting in the same place.
“This is the first snapshot we have of those two species living on the same immediate landscape, potentially interacting with one another,” says study co-author Kevin Hatala, a palaeoanthropologist at Chatham University in Pittsburgh, Pennsylvania. The study was published in Science on 28 November1.
The prints preserved details about the individuals, including the height of their foot arches, the shape of their toes and their walking patterns.
“It really is a snapshot in time,” says Tracy Kivell,a palaeoanthropologist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany.
“These fossilized footprints are as close as we are going to get to having a time machine to take us back to an eastern African lakeshore 1.5 million years ago,” says Bernard Wood, a palaeoanthropologist at George Washington University in Washington DC.
Previous studies, based mostly on the fossil record, have suggested that different hominin species lived alongside each other. But fossils are often scattered over large areas and their estimated dates span thousands of years. “You don’t know if they’re actually bumping into each other or not,” says Kivell.
In July 2021, researchers discovered multiple sets of ancient footprints at the Koobi Fora site in the East Turkana Area of Kenya, including a continuous path of impressions left by one hominin individual and isolated prints left by at least three others. The surface dates to 1.52 million years ago, and impressions of rippled sand, reed beds and fish nests suggest that the area was a lake shore with shallow water.
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The path comprises 13 footprints. Hatala and his team estimated that the hominin that created it walked at 1.81 metres per second, similar to a modern human walking briskly.
Using 3D X-ray-based imaging techniques, the researchers studied how the motion of a foot shapes the tracks it leaves. They compared arch depth and toe angles in the hominin footprints with those of humans. The analysis suggests that the isolated footprints belong to individuals of H. erectus, thought to be the first human species to walk and run upright like modern humans.
The researchers attributed the continuous trail to an individual from the species Paranthropus boisei, which also seemed to walk upright. That species had a flatter foot and the position of its big toe changed from step to step. The big toe had a greater range of motion — able to angle outwards by up to 19 degrees in the right foot and nearly 16 degrees in the left foot — compared with human big toes, which extend outwards by up to about 10 degrees. “There’s a bit of mobility in the big toe that goes beyond what we see in modern people,” says Hatala.
The footprints of H. erectus and P. boisei are within metres of each other. “We can only assume they were aware of each other’s presence. Exactly how they interacted, or whether they learned from each other or what, that’s still a mystery,” says Wood.
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Alongside the hominin footprints, the site contained preserved tracks from 30 relatives of cattle, three horse-like animals and 61 birds, including a giant extinct stork, Leptoptilos falconeri.
Hatala hopes to combine data from fossil footprints and bone fossils to “give a really high-resolution picture of what’s happening in this area during this phase of human evolution”.
Wood says that future studies could focus on the animals and birds. “That just brings the whole thing alive in a way that, with regular fossil evidence, it’s difficult to do.”