More than 1.5 million years ago, two different types of ancient humans crossed paths on a lake shore. These early precursors of Homo sapiens roamed a landscape teeming with wildlife, including the 2-meter-tall giant maribu storks.

A stunning discovery of fossilized footprints pressed into the mud preserved the unexpected and extraordinary moment, suggesting that the two distinct hominin species were able to live as neighbors sharing a habitat, rather than as competitors standing on their own territory.

"It's surprising that you would have two similar-sized, large-bodied hominin species in the same landscape," said Kevin Hatala, first author of a study on the tracks that was published in the journal Science on Thursday.

"We see them in the same lake-margin environment, passing within this area within hours to days of each other. They would probably have been aware of each other's existence. They saw each other and could have interacted," added Hatala, associate professor of biology at Chatham University in Pittsburgh.

The first part of the find occurred in July 2021 during an excavation at Koobi Fora, on the eastern shore of Lake Turkana in Kenya.

The researchers found 12 hominin footprints in a row, suggesting they were made by the same individual. The excavation took place in 2022, when Hatala and his colleagues discovered 11 more hominin footprints similar to the first in a row that suggested they were made by the same individual, plus three isolated footprints that were oriented perpendicular direction.

The researchers also found 94 footprints belonging to birds and cow- and horse-like animals. The largest bird track was 27 centimeters wide and likely belonged to the giant stork Leptoptilos.

The other three tracks perpendicular to the track were scattered around the country. Hatala thinks they were made by three separate individuals.

Hatala and his colleagues could not directly date the footprints. But Hatala said the fossils were found below — and therefore "slightly older" than — a layer of volcanic ash at the same site known as the Elomaling'a Tuff, which is dated to 1.52 million years ago according to the study. .

However, the researchers said they are confident the footprints were pressed within hours, as there are no cracks in the surface of the footprints, which would occur if they were exposed to air and dried in the sun for a longer period. long.

The scientists said all the tracks were preserved intact under the accumulated layers of sediment thanks to the fine sand.

The term hominin refers to the species in the human family tree that appeared after the split from the ancestors of the great apes 6-7 million years ago. This group includes extinct species such as Neanderthals 40 thousand years ago and Australopithecus afarensis, represented by the Lucy skeleton in Ethiopia - 3.2 million years old.

Homo sapiens, our species, is the only living hominin species, making the idea of ​​meeting another species from the same lineage particularly appealing to imagine. Researchers found some clues about which groups of ancient people crossed paths during this meeting.

The team concluded that smaller-brained hominins belonging to the species Homo erectus and Paranthropus boisei made the tracks. P. bosei made the long track, while Homo erectus made the other three tracks, the study suggested. Skeletal remains of both species have been found at the site.

However, it was not immediately clear that the tracks were made by two different species. Hatala, who is an expert in foot anatomy, determined that they reflected different patterns of walking, standing and movement only after detailed 3D imaging and analysis.

Through field and laboratory experiments, he compared the footprints to those from living humans, including 59 from the Daasanach people in Ethiopia, who usually don't wear shoes, as well as other fossilized hominin footprints and footprints from chimpanzees.

Hatala found that the 12 tracks were made by an individual whose tracks did not fall within the range of variation seen in Homo sapiens, unlike the three scattered tracks.

"Homo erectus, from the neck down, look very similar to modern humans, and they are the best candidate to be our direct ancestors. We assume that those more human-like footprints are more likely Homo Erectus just because the rest of their anatomy is so human," Hatala said, adding that Paranthropus boisei look quite different.

"They have very large jaws, very large teeth and large connections for the muscles of mastication. It looks like they adapted to eat a very different kind of diet than Homo erectus."

Shown here is a fossilized footprint hypothesized to have been made by Homo erectus, a species of ancient man. Hatala and his colleagues examined the old fossil record from the site and found evidence that the two species overlapped at the site for a considerable period of time, perhaps over the course of 100.000 years.

The footprints are the first physical evidence that different hominin species overlapped at the same time and space, avoiding predators and foraging in the ancient landscape, according to the study.

Homo erectus continued to flourish for another 1 million years. Paranthropus boisei became extinct within a few hundred thousand years for still unknown reasons.