Even if the early pre-humans still looked quite similar to the great apes on the outside, they showed clear differences in one point: in their way of moving. Because their legs, hips and feet were already adapted to walking upright on two legs, at least part of the time. Now, paleoanthropologists are presenting new evidence that the earliest hominins walked upright. Analysis of leg and arm bones from the pre-human Sahelanthropus tchadensis, which is seven million years old, provides evidence for this. They suggest that although he still spent a lot of time climbing and dangling on trees, he also walked on two legs more often.
Walking upright is one of the defining characteristics of humans and their direct ancestors. Walking on two legs still distinguishes us from our closest relatives, the chimpanzees, who mainly walk on all fours. But so far it is unclear when the upright gait developed in our early ancestors. Australopithecus fossils show that the feet of these hominins, who lived a good three million years ago, were already clearly adapted to bipedal walking - although these prehumans also spent some of their time in trees. Leg bones from even earlier prehuman forms, such as Ardipithecus and Orrorin, which lived around six million years ago, suggest that they, too, at least occasionally walked the landscape bipedally. This brings the development of upright walking relatively close to the splitting of the human and chimpanzee lineages, which took place seven to eight million years ago.
How did Sahelanthropus move?
New evidence that one of the earliest known representatives of hominins, Sahelanthropus tchadensis, was able to walk upright comes even closer. The first fossils of this pre-human, who lived around seven million years ago, were discovered in Chad in 2001. Scientists then recovered an almost complete skull, jaw and several individual teeth from this species. Even then, the position of the occipital foramen on the skull provided the first indications that this pre-human did not walk on four legs, but mostly assumed an upright posture. The opening that connects the head to the spine was relatively far forward and centered below the skull in this fossil, while it is typically further back in quadrupedal mammals. In the absence of leg or foot fossils, however, it remains a matter of debate whether the Sahelanthropus really walked upright or whether it only climbed and shimmyed a lot in an upright position.
Now other fossils, most likely from the Sahelanthropus, may have provided the missing piece of the puzzle. It is a femur and two cubits (ulna) from the forearm of Sahelanthropus, which were found at the same site as the skull, but could not initially be assigned. Comparative analyzes only showed in 2009 that they probably also come from Sahelanthropus tchadensis. Guillaume Daver from the University of Poitiers in France and his colleagues have now specifically examined these three bones for characteristics that can provide information about the movement of this pre-human.
Leg bones show signs of walking upright
Examination of the thigh bone revealed that its upper part is slightly flattened and oriented forward - this is considered typical of upright creatures. The attachment points of the large buttock muscle, gluteus maximus, which is important for upright walking, are also relatively robust and are similar to ours, as the researchers report. In their analyzes of the bone density of virtual cross sections of the femur, there were also slight thickenings in the areas that are exposed to greater lateral forces when walking upright. "The hominin-like density distribution of the femur supports the assumption that the individual movement pattern of Sahelanthropus tchadensis also included bipedal gait," write Daver and his colleagues. "Based on several pieces of evidence, we conclude that the leg bone discovered in Chad shows features for habitual bipedalism." In other words: Sahelanthropus walked upright for at least part of its time.
However, the arm bones of this pre-human paint a somewhat different picture. The curved shape of the ulna and the morphology of the elbow suggest that Sahelanthropus was often climbing and swinging. "The ulnae display a number of morphological features that suggest significant arboreal behavior," the researchers report. Sahelanthropus must therefore still have spent a large part of its time in trees. It is fitting that this pre-human lived in an environment in which wooded areas, savannas and wet meadows alternated with bodies of water. "This enabled the hominins from Chad to forage both in the trees and on the ground and to seek out water resources," explain Daver and his colleagues. This may have enabled the Sahelanthropus and the pre-humans that came after it to occupy their own ecological niche and thus successfully continue to evolve. In the course of time, the pre-humans stayed less and less in trees and adapted better and better to the two-legged gait.
Source: Guillaume Daver (CNRS, Université de Poitiers) et al., Nature, doi: 10.1038/s41586-022-04901-z