Around 1.5 million years ago, four walkers traversed the muddy shore of a lake, leaving footprints. If they did not cross paths, they would have missed each other narrowly — probably by hours if not minutes.
What makes the discovery of these footprints so remarkable is the identity of those who left them.
Using innovative technology, researchers have analyzed the prints in detail, concluding that the footprints were made by two distinct species of hominins — our prehistoric relatives — at around the same time.
While one set of tracks belonged to an ancient primate, Paranthropus boisei, the other footprints were left behind by three Homo erectus individuals, an archaic human species, researchers believe. Their findings were published in the journal Science on Thursday.
Scientists say that the footprints, unearthed in present-day Kenya, are thefirst clear evidence that these two species lived alongside each other in a shared habitat — a finding that raises questions about how they interacted. The answer, not yet known, could further our understanding of how our ancestors evolved.
“This is the first time that we know they were living alongside each other,” said Kevin Hatala, a paleoanthropologist at Chatham University who helped excavate the footprints and analyze them using 3D documentation, in a phone interview. “They probably would have been aware of one another’s existence, living in such close proximity to one another. That raises some interesting questions about competition and coexistence.”
A crew of paleontologists and excavators unearthed the footprints in 2021 near Lake Turkana, in northern Kenya, where they were buried beneath layers of sand and volcanic rock deposited around 1.5 million years ago.
By analyzing the shape of the feet that left the imprints, the study’s authors found that the sets of tracks belonged to individuals from different hominin species. “They appear to be walking in slightly different fashions. They’re both bipeds. They’re both moving adeptly. But there are still subtle differences in how they walk,” Hatala said.
The analysis suggested that the bipeds — meaning they walked upright on two legs — were probably adults. One of them, the ancient primate Paranthropus boisei, was walking at around 4 mph, a modestly fast pace, according to an analysis of their stride length.
Within a few hundred thousand years of the encounter, the Paranthropus boisei would be extinct for reasons that are still unknown. By contrast, the Homo erectus species persisted for another million years — most likely becoming a predecessor of the modern human being. Both species are hominins, members of the same human lineage that split from apes’ ancestors more than 6 million years ago.
Craig Feibel, a geologist at Rutgers University who was a co-author on the study and helped date the findings, said the walkers left sharply defined footprints on a moist surface that was gently buried by a layer of sand soon after, before the surface had time to dry out and crack. That immediate burial allowed scientists to estimate that the tracks were left within hours of each other.
Unlike bones, fossil footprints cannot be transported by water, predators or scavengers — meaning scientists can confidently pinpoint them to a precise location.
There was already evidence that the two species overlapped in time, but no scientist has ever presented such direct proof that they also shared the same habitat, the scientists behind the study say. Reexamining similar footprints found in the same region using the same techniques revealed other sites at which different species of hominin also crossed paths, the authors said.
Thursday’s findings also raises a question: How did these two distinct species of prehistoric human relatives interact?
The answer, which scientists do not know, could one day shed light on the evolutionary path taken by humanity’s ancestors.
The study authors hypothesize that, for much of the time that the species coexisted, they consumed different diets. While the primate was well adapted to chewing vegetables, the Homo erectus was more of a hunter-gatherer. That offers a clue — but not much more — that they may not have been in fierce competition for resources at the time.
Feibel said the lake shore traversed by these two individuals would have been hot and seasonally rainy, similar in climate to the modern Serengeti in Africa. It was a plentiful environment, full of fauna and flowing with water, which the walkers would have shared with a diverse plethora of prehistoric creatures, including pigs, antelopes, saber-tooth cats and giant giraffes. Other tracks found on the same surface were made by bird, bovid or equid creatures, the study said. The bird tracks were “unusually large,” with the largest exceeding 10 inches wide.
It’s possible that subsequent changes in the climate pitted the Paranthropus boisei and the Homo erectus species into stronger competition as depleting resources forced our ancestors to adapt toward riskier, higher-reward food acquisition strategies, the study authors said, referencing existing research into the evolution of throwing, endurance running and changing diets in our prehistoric relatives.
Hatala said it was “definitely possible” that interactions between the two species “would have had some influence on the evolution of Homo erectus.”
Fred Spoor, a professor of paleontology at London’s Natural History Museum who was not involved in the research, said the study presented a good working hypothesis that two species coexisted at this lakeside.
“That is not to say that they were there to congregate and shake hands,” he said in a phone interview. Observing how chimpanzees and gorillas interact in Central Africa might be a good point of reference for imagining an encounter between these two hominin species, he added.
Ultimately, the scope and nature of any interaction between Paranthropus boisei and Homo erectus is unknown at this stage. However, Thursday’s findings align with our understanding that early human evolution was a messy and uneven process — with several species of hominins overlapping chronologically to form a family tree with multiple offshoots rather than a single evolutionary chain.
“It was a much more complicated, bushy affair, with multiple branches,” Spoor said. For most of the last 6 million years, different species of hominin overlapped, he added, with the last Neanderthals dying out just 38,000 years ago — a blink in prehistoric time. “The situation that we as humans are in, that we are alone at the moment, is rather unusual.”
