ARI SHAPIRO, HOST:
Near a lake in Kenya, researchers have found ancient footprints of large birds, horse and antelope relatives, and they've also found evidence of two distinct human ancestors. As NPR's Jessica Yung reports, the evidence points to a complex evolutionary history of bipedalism.
JESSICA YUNG, BYLINE: As you walk around, you might not think about how you walk or the footprints you leave behind, but Kevin Hatala does - down to the arch, which he says can tell you a lot about how a foot moves, especially on soft ground, like the muddy surroundings of this Kenyan lake.
KEVIN HATALA: You make an arched footprint because of the way that your foot hits the ground with your heel, rotates over, and then you push off with your forefoot.
YUNG: Hatala is an associate professor of biology at Chatham University. And in 2022, he and a research team excavated the footprints fossilized in mud, which are over 1.5 million years old. The researchers made 3D models of the footprints, analyzed them and then determined that these footprints were likely made by two very different-looking species of human ancestors. There was Homo erectus, which looked a lot like modern humans, and Paranthropus boisei, which had smaller brains and a large jaw. Hatala also says these two species likely had two different ways of walking on two legs, something they determined with X-rays.
HATALA: To actually look through muds and understand how people's feet moved within the mud to see how the actual shape of a footprint develops.
YUNG: The researchers think the 12 footprints, made in one path, belonged to a flatter-footed Paranthropus boisei individual, while the other cluster of footprints were likely made by three different Homo erectus individuals with human-like arches. This is not the first time human ancestor species with different walking patterns have been found, but previous evidence relied on skeletal remains, oftentimes found thousands of miles apart and not definitively datable to the same time period.
HATALA: With these footprints, we know that these sites form and are preserved on the scale of, like, hours to days. They were there at basically at the same time. You know, they might not have been walking together or literally crossing paths, but they were there within a window of time where they very likely would have known that each other was there, living in the same area.
YUNG: Hatala says he and his team think that the two species coexisted with little competition since there's been no evidence of high aggression found so far. It seems like they were both able to access the resources they needed, and there was likely something special about areas around lakes like this one that drew both species to spend so much time there.
Jessica Yung, NPR News.
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