Fossil footprints discovered in Tanzania reveal that two human species once lived in the same place at the same time, NPR reports.
Australopithecus afarensis has long been assumed to be the only human species living 3.7 million years ago, and scientists have considered it an ancestor of modern humans. But recent discoveries of other remains, such as jaws, skulls, and foot bones, suggest that a diversity of hominins may have existed during this time.
At Laetoli in northern Tanzania, all the footprints come from the same layer of mud. Individuals from these two early human species must have moved within hours or days of one another, noted Ellison McNutt, a biological anthropologist at the Ohio University Heritage College of Osteopathic Medicine.
Walking on two legs is a unique and distinctive human trait.
"It's a very strange way of kind of moving through the world, and it's very different from other animals," McNutt said.
In the 1970s, paleontologists uncovered tracks from animals including ostriches, giraffes, hyenas, and Australopithecus afarensis.
Another set of tracks revealed bipedal walking, but those didn't look so human. Scientists determined they must have been left by a young bear, as bears were prevalent in Africa. A bear's hind feet resemble human feet, bears can walk bipedally, and they don't always leave distinctive claw-prints, scientists noted.
Footprints, unlike bones, capture an actual behavior of early humans, and let scientists imagine these species at Laetoli.
By examining the different footprints, the research team determined that the fossils' features looked more human-like. The walking pattern preserved in the tracks shows a cross-step, with the foot crossing in front of the mid-line of the body. People can cross-step while walking over uneven surfaces or regaining their balance.
"That actually is one more piece of evidence that it's not a bear," says McNutt, "because bears don't have the anatomy at their hip and their knee to allow them to stand up and maintain their balance while cross-stepping."
The researchers also observed videos taken of wild black bears and found that they hardly ever walk on their hind legs.
"When a bear stands up, he's usually holding on to a tree," says McNutt. "Or maybe he'll take a step or two, but he doesn't usually take four or more, which is what you'd really need to get to the five we have preserved at this site."
"If it's the case that this is a second species, this discovery shows that Australopithecus afarensis and something else were really in the same time, at the same place," said Stephanie Melillo, a paleoanthropologist at the Max Planck Institute for Evolutionary Anthropology in Germany who was not part of the research team.
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