Two ancient hominid species with slightly different gaits crossed paths in East Africa.

Footprints preserved on what was once a muddy lakeshore indicate that the two species, each built to walk in its own way, hung out there around 1.5 million years ago.

Newly discovered foot impressions at the northern Kenyan site, and footprints previously unearthed at a nearby location, offer glimpses of coexistence and possibly direct contacts between ancient hominid species over a span of up to 200,000 years, say paleoanthropologist Kevin Hatala of Chatham University in Pittsburgh and colleagues.

Two patterns of upright walking appear in foot tracks found along an ancient lake at Koobi Fora, a set of deposits on the eastern margin of present-day Lake Turkana, the scientists report in the Nov. 29 Science. A comparable distinction applies to footprints excavated in fieldwork led by Hatala nearly 20 years ago at Ileret, another roughly 1.5-million-year-old Kenyan site, the team says (SN: 2/26/09).

Prints displaying signs of a humanlike foot anatomy and gait belonged to Homo erectus, a possible direct ancestor of H. sapiens, Hatala says. H. erectus, which lived from nearly 2 million to roughly 117,000 years ago, ate a variety of energy-rich foods to support its large brain (SN: 12/18/19).

Impressions showing fewer similarities to the feet and striding pattern of people today belonged to Paranthropus boisei, the investigators suspect. Small-brained, big-jawed P. boisei, which dates to between 2.3 million and 1.2 million years ago, had a taste for grasses and flowering plants called sedges (SN: 5/2/11).

Researchers have known for nearly 50 years that East African fossils of H. erectus and P. boisei date to about the same time in nearby locations. But those fossils accumulated slowly, and researchers could not pin down whether the two species resided simultaneously in the same place.

Preserved footprints analyzed in the new study solve that problem, says paleoanthropologist Jeremy DeSilva of Dartmouth College, who was not part of Hatala’s team. “We now know with certainty that these two kinds of [hominids] shared the same landscape and walked with slightly different gaits.”

Closely spaced footprints at the new Koobi Fora site, consisting of three H. erectus impressions and a trail of 12 impressions left by a P. boisei individual, were formed and then buried by lakeside sediments within a few days at most, the researchers say. So were footprints of large birds and animals such as antelopes and wild horses.

“Whether Homo and Paranthropus individuals passed through the area hours to a day apart, or seconds to a minute apart, they would have been aware of each other’s existence on this shared landscape,” Hatala says.

If chimpanzees and gorillas can feed peacefully in the same tree, then it’s possible that H. erectus and P. boisei “met in a 1.5-million-year-old version of a 7-Eleven store” at a lake that featured a range of desirable foods, says paleoanthropologist Bernard Wood of George Washington University in Washington, D.C. Wood did not participate in the new study.

While the footprint findings suggest that H. erectus and P. boisei interacted, “whether or when they competed, potentially due to climatic or environmental pressures, cannot be determined with the current evidence,” says paleoanthropologist Rita Sorrentino of the University of Bologna, Italy.

Whatever transpired along the ancient lakeshore, the Kenya footprints support a previous report of divergent upright stances among even older hominid species. At Tanzania’s Laetoli site, 3.6-million-year-old footprints include humanlike impressions of Lucy’s species, Australopithecus afarensis and more chimplike tracks of an unidentified hominid species (SN: 11/13/24 ; SN: 12/1/21).

In the new study, researchers compared digital 3-D models of ancient hominid footprints and trackways to those made by people today — including Kenyan herders who rarely or never wear shoes — traversing muddy soil like that along the ancient lake. Muddy tracks made by chimps provided a further comparison.

Arches formed in human footprints when walking through mud look much like those left by H. erectus at the ancient lake, Hatala says. That finding indicates that H. erectus moved its feet much as we do now, he contends.

P. boisei footprints displayed a flatter arch than those of present-day humans, showing that their foot motions and perhaps their foot anatomy differed from ours, Hatala says.

P. boisei — but not H. erectus — also possessed big toes that splayed more than those of people today, but less than observed in chimps. P. boisei’s big toes may have been more mobile than those of H. erectus or modern humans, Hatala suggests.

These foot disparities underlie two comparably effective forms of walking. “The trackway that we attribute to P. boisei reflects a fairly fast walking speed, and there is no evidence that they were off-balance or any less adept at walking on two legs than H. erectus,” Hatala says.