Here was a prehuman – not a bear

Aiming at a Controversial Trail: A re-examination of fossilized footprints in Tanzania reveals that they were descended from a prehuman – not an erect, clumsy bear as previously suggested. However, the track features also do not match the known footprints from the region, which are attributed to Australopithecus afarensis, a relative of ‘Lucy’. Thus, it becomes clear that at least two species of hominin coexisted in East Africa about 3.7 million years ago, the researchers say.

How did the early members of our family tree develop the traits that characterize modern humans today? In addition to the properties of the brain or hands, this question focuses on another aspect that is unique to humans: walking upright. There is evidence that this mode of movement originated in our ancestors – probably more than six million years ago. However, footprints discovered in 1978 at the Laetoli site in Tanzania are believed to be the oldest clear evidence of upright walking. The traces at places G and S are dated back to approximately 3.7 million years. It is now believed to be descended from the prehuman Australopithecus afarensis – a species to which belonged the famous “Lucy”, a partial skeleton of which was found in East Africa.

A strange lead has been rediscovered

However, the famous footprints were not the only ones discovered in Laetola in the 1970s. Other prints were identified in the rock at a nearby location, marked with the letter A, but at the time they surprised researchers. There have been suggestions that they may also have come from prehumans. But the strangely wide shape also allowed it to be left behind by a young bear that walked on two legs on soft ground. Finally, to protect these ‘obscure’ marks from erosion, scientists covered them back with soil material. “With growing evidence of a variety of locomotion and species diversity in hominin fossils over the past 30 years, these remarkable footprints deserve further investigation,” said lead author of this study, Ellison McNutt of the University of Ohio.

To literally trace the origin of the footprints from site A, the team unearthed them again and cleaned them completely. They then recorded five more footprints using scanner technology. McNutt and her colleagues were then able to compare them with the traces of humans (Homo sapiens), with traces of Australopithecus afarensis and with the traces of chimpanzees (Pan troglodytes), especially black bears (Ursus americanus). To obtain traces of bears walking upright, scientists used bait to make captive cubs walk on soft ground. In addition, the video was assessed to record the natural bipedal behavior of the animals.

The bear is eliminated

They report that the findings make it very unlikely that the bear representative will leave strange traces in Laetoli. The traces of the cubs are the same size as the finds, and there are indeed similarities to those of humans – but there are also fundamental differences, the researchers explain: the heels of bears are narrowed, and their toes and front legs are fan-shaped. However, location analyzes indicate a rather wide heel compared to the front paws. The researchers also identified traces of the big toe and the next second. The difference in size between them was human, but not bearish.

Another important clue is the shape of the entire trail: when the bears walk upright, they wobble a lot, leaving a wide trail that does not match the narrow version found at the site. “They cannot walk in a way that would lead to the A footprints because their hip muscles and the shape of their knees do not allow this kind of movement and balance,” explains senior author Jeremy DeSilva of Dartmouth College, Hanover. Amazingly, the scientists point out, the mysterious creature even walked crossing one leg – a gait known as crossing. The researchers explain that bears and chimpanzees are unable to do this when walking upright. “Although people don’t usually step in step, they can use this movement to stabilize their balance,” says McNutt.

Two people walking upright

The researchers concluded that these findings and other clues about the proportions of the feet prove that the prints came from a hominin moving around on two legs. Perhaps he just walked over an uneven surface with these five steps, explains McNutt of the possible background of an unusual cross step. But who exactly could have been there? For now, it remains a mystery. Since the analyzes confirm in this context that the footprint does not come from Australopithecus afarensis: the derived foot proportions, gait parameters, and the 3D morphology of the footprints at site A differ significantly from the traces attributed to this prehuman, scientists write.

This in turn means: “Our results are therefore conclusive evidence that there were different species of hominin at about the same time that were battling in this East African landscape – but in different ways and with different rates,” says DeSilva. The prints from site A are therefore hot leads for anthropology.

Source: Dartmouth College, journal article, Nature: doi: 10.1038 / s41586-021-04187-7

Leave a Comment