About 1.5 million years ago, individuals from two different species in the human evolutionary lineage walked on the shore of a muddy lake in northern Kenya, leaving behind criss-crossing tracks alongside the footprints of antelopes, horses, warthogs, giant storks and other animals.
Those tracks turned into fossils that scientists have now discovered at a site called Kobi Vora, providing the first evidence that these two species — Paranthropus boisei and Homo erectus — shared the same landscape, and literally intersected. This discovery raises interesting questions about the relationship between the two species and any competition for resources.
Paranthropus boisei, the closer to modern humans of the two, lived about 2.3 to 1.2 million years ago, and was about 4 feet 6 inches (137 cm) long. They had a skull adapted to accommodate large chewing muscles, including a crest of the skull like that of male gorillas, as well as massive molars. Their feet bear ape-like features including a big toe.
Homo erectus, one of the first members of our evolutionary line with body dimensions like Homo sapiens, lived from about 1.89 million to 110,000 years ago, and ranged in length from about 4 feet 9 inches to 6 feet 1 inch (145-185 cm). They had large brow ridges and larger brains than Paranthropus boisei, although they are smaller than our own species.
Researchers found the footprints in 2021 in the vicinity of Lake Turkana. They identified a long track of 12 tracks, each about 10.25 inches (26 cm) long, attributed to an adult individual of the species Paranthropus boisei based on their shape and manner of movement.
Three isolated footprints, 8 to 9.25 inches (20.5 to 23.5 cm) long and similar to those of modern people, were roughly perpendicular to the main path. Two of them were complete enough to be attributed to Homo erectus, and they were probably juveniles. The third was more difficult to set with confidence.
The tracks appeared to have been left within hours or perhaps a few days — the clay never dried or cracked — and the individuals may have seen each other, the researchers said. There was no evidence of interaction.
“Fossil footprints provide us with a clear picture of that moment in time, 1.5 million years ago,” said paleontologist Louise Leakey, director of the Department of Paleoanthropology. “Different human ancestors may have passed each other, waded in shallow waters, and may have been hunter-gatherers.” From the Koobi Fora research project and co-author of the study published Thursday in the journal Science Opens a new tab.
The researchers reanalyzed tracks previously discovered nearby, and determined that both species existed together on fossilized muddy surfaces over a period spanning nearly 200,000 years.
“It is likely that they competed directly, but it is also possible that they were not in direct competition and both had access to the resources they needed in this common landscape,” said paleontologist and lead author of the study Kevin Hatala of Chatham University in Pittsburgh.
Dietary differences may have mitigated competition.
“Paranthropus boisei consumed low-quality forage that likely required frequent chewing. Homo erectus was likely omnivorous, used carcass tools, and included meat in its diet,” Leakey said.
The evolutionary lineages of humans and chimpanzees split about 7 million years ago in Africa. Species in the human lineage are called hominins.
Footprints provide information about anatomy, locomotion, behavior, and environments that skeletal fossils or stone tools cannot provide. The feet of these two species were anatomically distinct, and they had a different gait.
“The tracks we attribute to Homo erectus have a very modern, human-like shape with a long arch of deposits in the middle of the track suggesting a calloused foot and a gait that involves propulsion from the toes,” Harvard Evolutionary said. Biologist and study co-author Neil Roach.
“Paranthropus tracks lack this long arch and indicate flat-footed walking. Additionally, Paranthropus tracks show a big toe that is slightly more deviated than the other toes and is more mobile. Both sides are more similar to the way a chimpanzee walks,” Roach said. “Although it is clearly distinct and intermediate between chimpanzee and human footprints.”
Paranthropus boisei disappeared a few hundred thousand years after these footprints appeared, while Homo erectus flourished. Homo erectus was likely the direct ancestor of Homo sapiens, and was the first human species to spread outside of Africa.
The excavation site was a resource-rich lake shore near the mouth of the river.
“The fact that we consistently see two species of hominins in these landscapes, despite the presence of dangerous animals like hippos and crocodiles, suggests that these environments were important enough to our ancestors to be worth the risk of visiting,” Roach said. .