Footprints show two species of ancient human relatives shared the same ground at the same time

A newly discovered set of footprints in Kenya provides the first evidence that two different species of ancient human relatives walked the same ground simultaneously 1.5 million years ago.

The researchers behind the finding say the footprints belong to the species Homo erectus and Paranthropus boisei and were left within hours or days of one another — which opens up new mysteries about what happened when the two crossed paths.

According to a study about the discovery, published Thursday in the journal Science, the footprints were embedded in dried mud near a lake in northern Kenya and buried in sediment at a prominent fossil site. By analyzing the prints’ shapes and strike patterns, researchers found that the two sets were distinct; the best explanation, they concluded, is that two different species left the tracks.

The study adds to a growing understanding among anthropologists and paleontologists that ancient human relatives likely interacted and co-existed. And, by extension, it raises questions about what kind of relationship the species had.

“We think that these individuals, these two species that were there, probably would have been aware there were members of another species nearby. They would have seen each other and recognized each other as different species, which raises questions about what those interactions would have been like,” said Kevin Hatala, an associate professor of biology at Chatham University and a lead author of the research. “Were they competitors? Were they totally OK with each other being there?”

Although previously discovered fossil skeletons of Homo erectus and Paranthropus boisei showed that both species lived in the region at some point in time, the new finding provides evidence of direct overlap.

It also shows that the two species walked on two feet in very different ways.

While both are human relatives, Homo erectus and Paranthropus boisei featured very different traits, and their fates within the human evolutionary tree took starkly different routes.

Homo erectus had a humanlike anatomy from the neck down. The species used stone tools and might have cooked with fire. Its members had a varied diet that likely included meat. The species eventually spread to Asia, Indonesia and other regions, and survived for more than 1 million years after the newly discovered footprints were made. Homo erectus last appeared in the fossil record a little over 100,000 years ago.

“Lots of people have thought they are as good of a candidate as there is to be a direct ancestor of ours,” Hatala said. “They seem to be a very successful species.”

Paranthropus boisei, meanwhile, featured smaller brains, massive chewing muscles and large molar teeth. William Harcourt-Smith, an associate professor of anthropology at Lehman College who was not involved in the study, said the species likely evolved to eat challenging foods like hard nuts or to grind away at tough, low-quality foods like shrubby plants.

The species did not last on Earth for nearly as long as Homo erectus.

“They’re thought to have gone extinct not too long after this, within the next few hundred thousand years,” Hatala said, referring to the time of the footprints. He added that no one knows for sure what happened, but it’s possible changes in the environment restricted their specialized diets.

The footprints were initially discovered in 2021 in a place called Koobi Fora, while researchers were digging for other fossils. The site is a fossil hotspot because uplifting rock exposes old sedimentary layers at the surface, allowing researchers to access the bones of ancient humans and other animals.

The following year, researchers excavated a set of about 12 footprints that appeared to walk in a line, then found another set of footprints running perpendicular.

“We think these footprints were made in the mud in this lake shore environment. Something happened to bring sediment in over top of it. It could have been a small flood or a rise in the water level that carried in sediments and buried the footprints soon after they were formed,” Hatala said.

The muddy prints weren’t trampled over by other animals and they showed no sign of cracking before they were buried by sand. The researchers said that means they were left within hours or days of one another.

“The sediment protected them and prevented them from cracking and allowed them to be entered into the geologic record,” Hatala said.

He and his co-authors think the two species might have been able to co-exist in the area because their diets were so different. It’s also possible they competed for resources and had an antagonistic relationship. The species seem to have lived in the same general area for hundreds of thousands of years.

Harcourt-Smith, who works as a research associate in paleontology at the American Museum of Natural History, said the researchers’ analysis was sound and that the site was unique.

“I really want to stress how unusual it is to have a site like this, how special it is and what a great chance we have to find out more,” he said.

Broadly speaking, Harcourt-Smith added, it has become increasingly clear that various ancient human species interacted across different habitats over the past 7 million years of evolution. The evolutionary pathway to modern humans is filled with side branches and species like Paranthropus boisei, which represent an evolutionary dead end.

“Human evolution is complicated and messy and there’s lots of experimentation. It’s not a straight line,” he said.

Over the past few decades, scientists have developed genetic and archaeological evidence showing that humans, Denisovans and Neanderthals overlapped and sometimes interbred. The new study doesn’t address interbreeding, but it does show more clearly that even older species overlapped and had more interaction than previously understood.

After the researchers completed their work at the Koobi Fora site, photographing and documenting the footprints in several ways, they covered them up for future generations, said Craig Feibel, another author of the study.

“They have to carefully rebury it with sediment that won’t damage the footprints so erosion won’t occur,” he said.

This article was originally published on NBCNews.com