A controversial lead in sight: A renewed analysis of fossilized footprints in Tanzania shows that they come from a prehistoric man – not from an upright pounding bear, as previously suspected. The features of the trail, however, do not match the well-known footprints from the region, which Australopithecus afarensis – the relatives of “Lucy” – are assigned. The researchers say that at least two species of hominins coexisted in East Africa around 3.7 million years ago.
How did the early members of our family tree develop the characteristics that now characterize modern humans? In addition to the properties of the brain or the hands, this question focuses on another distinctive human aspect: the upright gait. There is evidence that this mode of locomotion originated early in our ancestors – possibly more than six million years ago. However, the oldest clear evidence of upright walking are footprints that were discovered in 1978 at the Laetoli site in Tanzania. The traces at the sites G and S were dated to an age of about 3.7 million years. It is now assumed that they come from the pre-human Australopithecus afarensis – the species to which the famous “Lucy” also belonged, the partial skeleton of which was found in East Africa.
A strange trace rediscovered
The footprints that have become famous, however, weren’t the only ones discovered in Laetoli in the 1970s. At a nearby point with the designation A, further imprints in the rock were identified, which, however, gave the researchers a headache at the time. There were suspicions that they could have come from pre-humans as well. But the strangely broad shape also made it seem possible that it was left behind by a young bear walking on two legs on the soft ground. In order to protect these “unclear” traces from erosion, the researchers then finally covered them again with soil material. “In view of the increasing evidence for the diversity of modes of locomotion and the biodiversity in the fossils of hominins over the past 30 years, these unusual prints now deserve another analytical look,” says the lead author of the current study Ellison McNutt of the University of Ohio.
In order to literally track down the originator of the footprints from site A, the team dug them up again and cleaned them completely. They then recorded the five successive footprints using scanner technology. McNutt and her colleagues were then able to compare them with those of humans (Homo sapiens), with traces of Australopithecus afarensis and with the footprints of chimpanzees (Pan troglodytes) and above all of black bears (Ursus americanus). In order to obtain traces of upright bears, the researchers made captive young animals run on soft ground using attractants. In addition, video material was evaluated in order to record the natural two-legged walking behavior of the animals.
Bear is eliminated
As they report, the findings make it very unlikely that a representative of the bears once left the strange tracks in Laetoli. The prints of the young animals are the same in size as the finds and indeed there are similarities to human traces – but there are also decisive differences, the scientists explain: the heels of bears are tapered and their toes and forefeet are fan-shaped. The analyzes from the site, however, show a rather broad heel compared to the front feet. The researchers also identified traces of the big toe and the following second one. The difference in size between the two was similar to that of humans, but not like that of bears.
Another important clue is the shape of the entire trail: when bears walk upright, they sway a lot, leaving a wide trail that doesn’t match the narrow version at the location where they were found. “You can’t walk in the way that left the footprints at Site A because your hip muscles and the shape of your knees don’t allow that kind of movement and balance,” explains senior author Jeremy DeSilva of Dartmouth College in Hanover. As the researchers point out, amazingly, the mysterious being even walked by placing one leg over the other – a gait known as a cross-step. Bears and chimpanzees are not able to do this when they walk upright, the researchers explain. “Although humans don’t usually cross-step either, they can use this movement to stabilize their balance,” says McNutt.
Two pre-humans on the move upright
These findings and other indications from the foot proportions now show that the prints come from a hominin who moved on two legs, the scientists sum up. Perhaps he was walking over an uneven surface with these five steps, explains McNutt about the possible background of the unusual cross step. But who exactly could have been there? This remains a mystery so far. In this context, the analyzes confirm that the trace does not come from an Australopithecus afarensis: the derived foot proportions, gait parameters and 3D morphologies of the footprints at location A differ significantly from the traces that are assigned to this pre-human, the scientists write.
This in turn means: “Our results are therefore conclusive evidence that there were different species of hominins at about the same time that moved on two legs in this East African landscape – but in different ways and with different feet,” says DeSilva. The prints from site A are therefore a hot lead for anthropology.
Source: Dartmouth College, technical article, Nature: doi: 10.1038 / s41586-021-04187-7