Fossilized Footprints Revealed Coexistence Between Human Species

More than 1.5 million years ago, there were two different species of ancient human who crossed paths on a lakeshore, perhaps locking eyes with each other. These early forerunners of Homo sapiens wandered in a landscape teeming with wildlife, including giant maribou storks that stood 2 meters (6.5 feet) tall.

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

"It’s surprising that you would have two kinds of similarly sized, large-bodied hominin species on the same landscape," said Kevin Hatala, first author of a study on the footprints that published in the journal Science on 28 November.

"We see them in the very same lake margin environment, passing within this area within hours to a couple of days of one another. They probably would have been aware of each other’s existence. They saw each other and they might have interacted," added Hatala, who is an 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, where the skeletal remains of several ancient human relatives have been found. That excavation revealed one hominin footprint, alongside several other tracks made by large birds. The team decided to rebury the tracks with fine sand until a detailed excavation was possible.

The dig took place in 2022, when Hatala and his colleagues exposed 23 square meters (248 square feet) of sediment, revealing 11 more hominin tracks similar to the first in a line that suggested they were made by the same individual, plus three isolated footprints that were oriented in a perpendicular direction.

The researchers also found 94 nonhuman tracks belonging to birds and cow- and horse-like animals. The largest bird track was 27 centimeters (10.6 inches) across and likely belong to a kind of giant stork known as Leptoptilos.

"There’s one long trackway with 12 (hominin) footprints in it. It was made at a decent walking pace … especially since they were walking through mud. There’s not a clear destination at the end," Hatala said.

"It’s hard to say what exactly they were doing, but they walked in that perfect zone of mud,” he said. “If you think about the shoreline of a lake or a modern beach, you’ve got kind of a narrow zone where the mud is perfect for making footprints. If you move too far this way, it’s too dry, if you move too far the other way, it’s too wet. And they’re walking almost like in a straight line through the perfect area for their footprints to be made, which is very lucky for us," he said.

The three other footprints perpendicular to the trackway were scattered across the site. Hatala thinks they were made by three separate individuals, their other tracks perhaps wiped out by other animals walking around the surface at the same time.

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

However, it wasn’t immediately obvious that the footprints were made by two different species. Hatala, who is an expert in foot anatomy, determined they reflected different patterns of gait, stance and motion only after detailed 3D imaging and analysis.

Through experiments in the field and in the lab, he compared the footprints with those made by living humans, including 59 of the Daasanach people in Ethiopia, who typically don’t wear shoes, as well as other fossilized hominin prints and tracks made by chimpanzees.

Hatala found that the 12-print trackway was made by an individual whose footprints didn’t fall within the range of variation seen in Homo sapiens, unlike the three scattered footprints, which plotted more closely to those made by living humans.