Remarkable Fossil Footprints Show Two Hominin Species Coexisting 1.5 Million Years Ago

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Approximately 1.5 million years ago, two human relatives belonging to two distinct species made their way along the shore of an ancient lake. Researchers know this because the hominins’ footprints fossilized in the mud, alongside the prints of giant birds that occupied the paleoenvironment.

The footprints were made by Homo erectus and Paranthropus boisei, long-extinct species that shared eastern Africa in the ancient past. Together, the footprints are a remarkable window into the lives of our nearest relatives and ancestors. The prints show how hominins overlapped as they eked out existence in ancient Africa; according to the research team, if the hominins who made the prints didn’t overlap at the site, they crossed it within hours of one another. The team’s research was published today in Science.

“In biological anthropology, we’re always interested in finding new ways to extract behavior from the fossil record, and this is a great example,” said Rebecca Ferrell, a program director at the National Science Foundation, in a Rutgers University release. “The team used cutting-edge 3D imaging technologies to create an entirely new way to look at footprints, which helps us understand human evolution and the roles of cooperation and competition in shaping our evolutionary journey.”

Homo erectus made its way out of Africa as far as eastern Asia; a different paper published earlier this year posited that the “hobbits” of Flores (Homo floresiensis) descended from Homo erectus in Java. Homo erectus only went extinct about 110,000 years ago, though P. boisei died out around 1.2 million years ago—shortly after it made tracks on the ancient Kenyan lakeshore.

The prints were found in 2021 by a team organized by the paleontologist Louise Leakey. A field team excavated the prints the following summer.

“These are actually the only two hominins currently known from the Turkana Basin at this particular time period,” said Kevin Hatala, an evolutionary biologist at Chatham University and lead author of the paper, in an email to Gizmodo. “Homo habilis is known from sediments just slightly older than these, and it is very possible that they lived at the same time and their fossils just haven’t yet been found. But for now, we believe it is most parsimonious to hypothesize that the footprints were made by H. erectus and P. boisei.”

Though general consensus is that Homo erectus is an ancestor of Homo sapiens, Paranthropus has a different story. Paranthropus is an extinct human relative with a broad face and the largest teeth of any primate. Last year, a team of researchers found a set of three-million-year-old tools alongside Paranthropus remains, indicating our hominin relatives may have been butchering critters long before anatomically modern humans came along.

Both species were upright, bipedal—they walked on two legs—and agile. And of course, both made use of the ancient lakefront near the modern Lake Turkana.

“This proves beyond any question that not only one, but two different hominins were walking on the same surface, literally within hours of each other,” said Craig Feibel, an anthropologist at Rutgers University and co-author of the paper, in the same release. “The idea that they lived contemporaneously may not be a surprise. But this is the first time demonstrating it.”

As noted, the fact that these two species lived at the same time is not a huge surprise. Our own species, Homo sapiens, emerged roughly 300,000 years ago and lived contemporaneously with several other hominins, including the Neanderthals, the Denisovans, and the aforementioned H. erectus. Additionally, there was H. naledi, H. floresiensis, H. luzonensis, possibly H. heidelbergensis, and the yet-to-be-named “hobbit” species described earlier this year. Evidence of any of these hominins residing in the same environment at the exact same time is slim to none, save for the fact that H. sapiens interbred with both Neanderthals and Denisovans, as evidenced in our DNA.

Evidence of Homo erectus and P. boisei coexisting has turned up before; in 2020, a skull cap belonging to a H. erectus toddler was found alongside cranial remains of P boisei in South Africa’s Drimolen site, part of the group of sites known as the Cradle of Humankind.

“The study of Hatala et al. also offers a tantalizing glimpse into the behavioral ecology of contemporary hominin species,” said William Harcourt-Smith, a paleoanthropologist at the American Museum of Natural History, in an accompanying Perspectives article. “Based on the proximity of the different footprints, the authors argue that H. erectus and P. boisei lived in the same geographic area and likely had low levels of competition between each other,” he wrote. The distinct contrasts in diet and life history between the two species make this a truly “fascinating suggestion,” Harcourt-Smith added.

More fossil evidence could help explain the interactions of ancient hominins that clearly overlapped not just in time, but in geography. Plenty of questions remain about how the hominins made use of their environment, as well as how they may have engaged with one another. But the recently studied fossil site is an amazing window into the history of our family tree.

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