A newly discovered set of footprints in Kenya provides the first evidence that two different species of ancient human relatives walked simultaneously on the same terrain 1.5 million years ago.
The researchers behind the find say the footprints belong to the species Homo erectus and Paranthropus boisei and were left within hours or days of each other – opening new mysteries about what happened when the two crossed paths.
According to a study into 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 shapes and stroke patterns of the prints, researchers discovered that the two sets were different; 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 lived side by side. 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, were probably aware that there were members of another species nearby. They would have seen each other and recognized them as different species, which raises questions about what those interactions would have been like,” said Kevin Hatala, associate professor of biology at the University of Chatham and lead author of the study. ‘Were they competitors? Were they completely okay with each other being there?
Although previously discovered fossil skeletons of Homo erectus and Paranthropus boisei showed that both species once lived in the region, the new finding provides evidence of direct overlap.
Members of the research team dig to uncover footprints.
It also shows that the two species walked on two feet in very different ways.
Although both are human relatives, Homo erectus and Paranthropus boisei exhibited very different traits, and their fates within the human evolutionary tree followed completely different routes.
Homo erectus had human-like anatomy from the neck down. The species used stone tools and may have cooked with fire. The members had a varied diet that probably included meat. The species eventually spread to Asia, Indonesia and other regions, surviving for more than 1 million years after the newly discovered footprints were made. Homo erectus last appeared in the fossil record just over 100,000 years ago.
“Many people have thought they are as good a candidate as a direct ancestor of ours,” Hatala said. “It seems to be a very successful strain.”
Paranthropus boisei, meanwhile, had smaller brains, enormous chewing muscles and large molars. William Harcourt-Smith, an associate professor of anthropology at Lehman College who was not involved in the research, said the species likely evolved to eat challenging foods such as hard nuts or grind on tough, low-quality foods such as shrubby plants.
The species did not persist on Earth nearly as long as Homo erectus.
“It’s believed they went extinct not long after, within a 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 is possible that changes in the environment are limiting their specialized diet.
An annotated aerial photo of the excavated area containing the footprint marks, with members of the research team lining the perimeter.
The footprints were first discovered in 2021 at a place called Koobi Fora, while researchers were digging for other fossils. The site is a hot spot for fossils because rising rock exposes ancient sedimentary layers at the surface, allowing researchers to access the bones of ancient humans and other animals.
The following year, researchers excavated a series of about a dozen footprints that appeared to run in a line, and then found another series of footprints that ran perpendicularly.
“We believe these footprints in the mud were made in this lakeside area. Something happened that caused sediment to come over it. It could have been a minor flood, or a rise in water levels, which carried sediments and buried the footprints shortly after they were formed,” Hatala said.
The muddy prints were not trampled by other animals and showed no signs of cracking before being buried under the sand. The researchers said this means they were left within hours or days of each other.
“The sediment protected them and prevented them from cracking, allowing them to be included in the geological record,” Hatala said.
He and his co-authors think the two species could have coexisted in the area because their diets were so different. It is also possible that they competed for resources and had an antagonistic relationship. It appears that the species has lived in the same area for hundreds of thousands of years.
Harcourt-Smith, who works as a paleontology research associate at the American Museum of Natural History, said the researchers’ analysis was sound and that the site was unique.
“I really want to emphasize how unusual it is to have a site like this, how special it is and what a great opportunity we have to find out more,” he said.
Broadly speaking, Harcourt-Smith added, it has become increasingly clear that different ancient human species have interacted with each other in different habitats over the past seven million years of evolution. The evolutionary path to modern humans is filled with offshoots and species such as Paranthropus boisei, which represent an evolutionary dead end.
“Human evolution is complicated and messy and involves a lot of experimentation. It is not a straight line,” he says.
In recent decades, scientists have developed genetic and archaeological evidence showing that humans, Denisovans and Neanderthals overlapped and sometimes interbred. The new study doesn’t discuss interbreeding, but it does show more clearly that even older species overlapped and interacted more than previously thought.
After the researchers completed their work on the Koobi Fora site, photographing and documenting the footprints in various ways, they hid them from future generations, said Craig Feibel, another author of the study.
“They have to carefully rebury it with sediment that doesn’t damage the footprints so that erosion doesn’t occur,” he said.
This article was originally published on NBCNews.com
