1.5 million-year-old fossil traces show that two ancient human species coexisted

About 1.5 million years ago, individuals of two different species of human evolutionary ancestry ran across a muddy lakeshore in northern Kenya, leaving behind crisscrossing tracks along with the tracks of antelopes, horses, wild boars, giant storks and other animals.

These tracks turned into fossils that scientists have now discovered at a site called Koobi Fora, providing the first evidence that these two species – Paranthropus boisei and Homo erectus – shared the same landscape, literally crossing paths. The discovery raises interesting questions about the relationship between the two species and any competition for resources.

Paranthropus boisei, the more distantly related to modern humans of the two, lived from about 2.3 to 1.2 million years ago, standing about 4 feet 6 inches (137 cm) tall. They had a skull adapted for large masticatory muscles, including a cranial crest like that of male gorillas, as well as huge molars. Their legs bore ape-like features, including the big foot.

Homo erectus, an early member of our evolutionary line with body proportions similar to those of Homo sapiens, lived from about 1.89 million to 110,000 years ago, with a height of about 4 feet 9 inches to 6 feet 1 inch (145-185 cm ). They had large brow ridges and a larger brain than Paranthropus boisei, although smaller than our species.

Researchers found the tracks in 2021 in the Lake Turkana region. They found a long trail of 12 tracks, each about 10.25 inches (26 cm) long, attributed to an adult Paranthropus boisei based on its shape and how it moved.

Three isolated tracks, ranging from 8-9.25 inches (20.5-23.5 cm) and resembling those of modern humans, were almost perpendicular to the main corridor. Two were complete enough to be attributed to Homo erectus, possibly a juvenile. The third was more difficult to assign with certainty.

The researchers said the tracks appear to have been left within a few hours or perhaps a few days – the mud had never dried and cracked – and the individuals may have seen each other. There was no evidence of interaction.

“The fossils provide us with a clear picture of this moment, 1.5 million years ago. The various human ancestors may well have passed by each other, walking in shallow water, possibly hunting and gathering,” said paleoanthropologist Louise Leakey. , director. of the Koobi Fora research program and co-author of the study published Thursday in the journal Science, opens a new tab.

The researchers reanalyzed tracks previously discovered nearby, determining that both species were present together on fossilized mudflats over a period spanning about 200,000 years.

“It’s possible they were in direct competition, but it’s also possible they weren’t in direct competition, and both had access to the resources they needed in this shared landscape,” said paleoanthropologist and lead study author Kevin Hatala of Chatham University in Pittsburgh.

Nutritional differences may have moderated competition.

“Paranthropus boisei consumed low-quality forage that likely required repeated chewing. Homo erectus was likely omnivorous, using tools to butcher carcasses and also had meat in its diet,” Leakey said.

The human and chimpanzee evolutionary lineages split about 7 million years ago in Africa. Species of the human lineage are called hominids.

Tracks provide information about anatomy, movement, behavior and environments that skeletal fossils or stone tools cannot. The legs of these two species were anatomically distinct and had a different gait.

“The footprints we attribute to Homo erectus have a very modern, human-like shape with a tall arch of midfoot sediment that suggests a stiff foot and a gait that involves toe thrusting,” said the Harvard evolutionist. biologist and study co-author Neil Roach.

“The tracks of Paranthropus lack this high arch and indicate a more flat-footed gait. In addition, the tracks of Paranthropus show a big toe that is slightly more divergent than the other toes and more agile. Both of these aspects are more like the way a chimpanzee walks, although clearly distinct and intermediate between chimpanzee and human tracks,” Roach said.

Paranthropus boisei disappeared a few hundred thousand years after these traces, while Homo erectus flourished. Possibly a direct ancestor of Homo sapiens, Homo erectus was the first human species to spread beyond Africa.

The fossil site was a resource-rich lakeshore near the mouth of a river.

“The fact that we consistently see two species of hominids in these landscapes, despite the presence of dangerous animals such as hippos and crocodiles, suggests that these environments were important enough to our ancestors to be worth the risk of visiting them,” said Roach. .