To be bipedal is to be human, we like to think. The truth is to be bipedal is also to be a bird. Moreover, certain lizards can scuttle bipedally and even cockroaches rear up and run on two legs when sufficiently startled. Then there are the kangaroo, the jird and the gerbil (rodents) and T-Rex – all famed for rapid bipedal progression.
So, bipedalism is clearly not the hallmark of beings human, but there are differences between our mode of locomotion and that of our primate cousins that stem, among other things, from the unique evolution of our hip joints. The great apes can rise up on their legs and walk – but they run quadrupedally. In humans, from early toddlerhood, we move about exclusively on two legs. The question is when this distinction arose.
Now, researchers report in American journal Proceedings of the National Academy of Sciences, on the analysis of fossilized remains of two hominins found 60 years ago in the Sterkfontein Caves in South Africa – that don’t help to shed light on the mystery.
The project was led by Dr. Leoni Georgiou, Dr. Matthew Skinner and Prof. Tracy Kivell at the University of Kent’s School of Anthropology and Conservation, and included an international team of biomechanical engineers and paleontologists.
One of the Sterkfontein specimens dates to about 2.8 to 2 million years ago and has been identified as Australopithecus africanus.
The other is younger, dating to about 2 to 1.2 million years ago; the team thinks it may have been a Paranthropus robustus or a very early human on a different branch of the evolutionary tree than our ancestor.
The rub is that scanning the bones shows the geologically younger hominin (the robustus) retained an archaic locomotive form while the older one (the australopithecine) was regularly walking bipedally, which is counterintuitive.
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Externally, in both the Sterkfontein hominins, the hip is more like ours than that of apes, researchers say, which is indicative of bipedalism.
But to deduce how these two beings actually moved, the team compared the internal structure of their femurs (the trabecular porous tissue inside the ends of long bones) with that of modern humans and great apes. The trabecular bone changes during one’s lifetime based on how one uses one’s limbs, the scientists explain.
The spherical head of the Australopithecus specimen resembles ours and he likely had a predominantly bipedal, human-like gait. But the corresponding internal leg structure in the robustus (if that’s what it was) suggests that its hip joints could be highly flexed like in the great apes, a posture associated with climbing trees. That suggests the robustus regularly both climbed trees and walked.
Now, if human evolution was a simple linear progression, we would expect a linear progression of the evolution of bidepalism. The discovery in Sterkfontein teaches that not only did multiple hominins coexist – but multiple forms of locomotion among these hominins coexisted too.
As little as a million or two years ago, at least one member of the Homo family hadn’t fully abandoned the trees.
“It has been challenging to resolve debates regarding the degree to which climbing remained an important behavior in our past,” stated Dr. Matthew M. Skinner of Johannesburg’s University of Witwatersrand.
“Evidence has been sparse, controversial and not widely accepted, and as we have shown in this study the external shape of bones can be misleading. Further analysis of the internal structure of other bones of the skeleton may reveal exciting findings about the evolution of other key human behaviors such as stone tool making and tool use,” Skinner added.
Previous research based on the fossil foot of a toddler Australopithecus indicated that while the adults walked upright on two legs and apparently lived on the ground, their kids may have clung to the trees for protection.
The coexistence of different human species throughout millions of years, in fact until as recently as about 30,000 years ago, has been amply demonstrated, including through the revelation that present-day people have some genes from Neanderthals, Denisovans and a host of “ghost” species – which means that our ancestors mixed with Neanderthals, Denisovans and species we have yet to identify. Also, Neanderthals and Denisovans mixed too.
Robustus has so far only been found in South Africa and was only about a meter tall on average. They had very large molars, the origin of the “robust” in their name, and a large sagittal crest (a ridge of bone running lengthwise along the skull). Their position in the Homo family tree is a mystery. So is ours. The only thing we know for sure is that we are the only hominins left.