It suggests that some 3.6 million years ago, more than one hominin species walked on two legs.

In the 1970s, researchers in Laetoli, located in northern Tanzania, discovered millions of years old footprints. These footprints are said to have been left by Australopithecus afarensis – the species of hominid to which the famed ‘Lucy’ belongs. However, the researchers also found other mysterious footprints in the area. And that led to more discussion. Some thought they had been left by a bear that walked on its hind legs, while others argued that they belonged to a human species. A new study provides the answer.

Laetoli footprints

The Laetoli footprints are very special, not least because they testify that hominins in present-day Tanzania moved upright about 3.6 million years ago. These tracks therefore represent the oldest, direct evidence of ‘bipedia’ in humanoids. While most scientists agree that the first pair of footprints were left by relatives of Lucy, the footprints found at a nearby location look very different.

Left: the controversial footprints. Right: Footprints probably left by a relative of Lucy. While the length is similar, the left print is much wider than the right. Image: Jeremy DeSilva and on right by Eli Burakian/Dartmouth.

The unusually shaped footprints sparked a heated debate. For example, they were dismissed by some as possibly being left behind by a young bear, which had stood upright on its hind legs. Others were convinced that it must have been a humanoid after all.

Study

To get to the bottom of things, researchers returned to the crime scene in Tanzania in a new study. Here they subjected the tracks to a thorough inspection. The footprints were measured, photographed and 3D scanned. They then compared the tracks to the footprints of contemporary American black bears, chimpanzees and humans.

Bear

To determine whether the prints were immortalized in the sand by a bear, the researchers collected four semi-feral, young black bears with legs about the same size as the prints at Laetoli. Using maple syrup or applesauce, each bear was enticed to stand on its hind legs and walk along a mud-filled path. The researchers also looked at 50 hours of footage of bears.

Experiment with a bear, studying its footprint (right). Image: Jeremy DeSilva and Ellison McNutt

The study leads to some interesting conclusions. For example, the team notes that the controversial Laetoli prints resemble those of humans more than those of bears or chimpanzees. The latter, for example, have relatively narrow heels compared to their forefoot, while the enigmatic millions of years old footprints actually show broad heels compared to their forefoot. In addition, there are features of a big toe and a smaller second toe from the footprints. The size difference between these two toes is similar to that of humans and chimpanzees, but doesn’t really resemble the toes of black bears. In addition, the video analysis shows that wild American black bears almost never walk on their hind legs. And, while thousands of animal remains have been found in Laetoli, none belonged to a bear.

Still a humanoid

According to the researchers, this can only mean one thing. And that is that the 3.6 million-year-old footprints were left by a humanoid that walked on two legs. However, this does not alter the fact that this humanoid had a special walk. Therefore, the researchers hypothesize that the tracks were left by an as-yet-unknown hominin that moved in an unusual, intersecting manner.

It means the mystery has been solved: The footprints belong to early humans, not a bear. And that has interesting implications. For example, it provides more insight into the origin of human bipedalism. In addition, it suggests that several species of hominids probably coexisted in the area. “Thanks to this research, we now have compelling evidence that several hominins walked across the landscape on two legs, but in different ways and with different feet,” said study researcher Jeremy DeSilva. “We’ve actually had this evidence since the 1970s. But thanks to the rediscovery and more detailed analysis of these beautiful footprints, we are only now sure.”