Different human ancestors walked together, footprints suggest

In 2021, a team led by paleontologist Louise Leakey discovered fossilized footprints on the shores of Lake Turkana in Kenya, revealing that two different human species, Homo erectus and Paranthropus boisei, lived at the same time and in the same area. The footprints, dating back 1.5 million years, provide direct evidence of their coexistence in a lakeside environment. The findings, published in the journal Science, shed new light on early hominin coexistence.

"This is the first direct snapshot of the two species together on the same immediate landscape," said Dr. Kevin Hatala, the lead author of the study and a paleoanthropologist at Chatham University. The footprints were left in the mud by Homo erectus and Paranthropus boisei, who walked along the same lakeshore at the same time. Evidence suggests they were present together within a few hours or days, as indicated by the absence of cracks on the surface.

The New York Times, The Guardian, Le Figaro, The Washington Post, and Scientific American reported on the finding, among others. 

"It's possible they passed each other by. They might have looked up in the distance and seen another member of a closely related species living in the same landscape," added Dr. Hatala.

The footprints were discovered in an area where an expedition led by Louise Leakey also found fossil bones. The tracks were gouged out of what was once soft, squishy mud and were analyzed using advanced 3D scanning technology. This allowed the researchers to reconstruct the tread that created the impressions and distinguish the prints based on anatomy and movement patterns.

"The idea that they lived contemporaneously may not be a surprise. But this is the first time demonstrating it. I think that's really huge," said Professor Craig Feibel from Rutgers University, one of the authors of the study. By analyzing the sediments around the footprints, Professor Feibel was able to narrow down the timing of the two early humans' presence to a very close time window.

"With these kinds of data, we can see how living individuals, millions of years ago, were moving around their environments and potentially interacting with each other, or even with other animals. That's something that we can't really get from bones or stone tools," noted Dr. Hatala.

The study found that Homo erectus and Paranthropus boisei walked upright on two legs but exhibited different styles of bipedalism. The footprints of Homo erectus reflect walking patterns and posture closer to modern humans, with a well-defined arch and a walking style that involves pushing off with the toes. In contrast, Paranthropus boisei had a more flat-footed walk, similar to chimpanzees, and their footprints suggest more mobility in the big toe compared to Homo erectus or modern humans.

Paranthropus boisei, known for its adaptations for powerful chewing muscles and large molars, stood about 1.37 meters tall and had a skull adapted to house large muscles responsible for chewing, including a cranial crest like that of male gorillas. It had a diet primarily composed of tough plant materials. Homo erectus, on the other hand, had a humanlike anatomy from the neck down and body proportions similar to those of modern humans. They had larger brains than Paranthropus boisei, although smaller than those of modern humans, and are considered direct ancestors of Homo sapiens.

"Given their different dietary adaptations, it's possible the two species did not directly compete for resources," suggested Dr. Hatala. The habitat around Lake Turkana was rich in resources but also posed dangers, as there were hippos, crocodiles, and other dangerous animals in the area. "It would have been a dangerous area; there would have been hippos, crocodiles, and other dangerous animals that also lived in those areas," added Dr. Hatala.

This discovery provides the first evidence that Homo erectus and Paranthropus boisei coexisted in the same area at the same time, highlighting the complexity and diversity of human evolution. The findings not only provide insight into the coexistence of these species but also raise questions about their interactions and how their coexistence could have influenced human evolution.

"It's wonderful that these early human relatives can now be placed directly in a lakeside landscape, walking and wading on wet surfaces, and probably feeding on the plant and animal resources there," said Professor Chris Stringer, head of human origins at the Natural History Museum in London.

"In biological anthropology, we’re always interested in finding new ways to extract behavior from the fossil record, and this is a great example," said Rebecca Ferrell, a program director at the National Science Foundation.

"Short of a time machine, fossil footprints are the next best way to capture a snapshot in time of our ancestors' daily lives," remarked paleoanthropologist Briana Pobiner.

This article was written in collaboration with generative AI company Alchemiq