If we want to work out today how our early ancestors moved, the first thing we do is look at the shape of their bones. This provides important information about the posture adopted by early hominids millions of years ago.
But external appearance is not everything: using modern computed tomography techniques, it is possible to analyse bone structure, compare fine geometric details and measure variations in bone density. This provides information about how the bone has been stressed. In this way, it has now been possible to determine: The species Paranthropus robustus, which was found in South Africa, was capable of walking upright, but apparently spent a great deal of time engaging in movements consistent with tree-climbing. The older Australopithecus, which lived in the same region, is thought to have relied far less on climbing, however.
Daily wear and tear alters the bone
During the early history of humankind, several different hominin species lived in South Africa. Paranthropus robustus belonged to a side branch that later became extinct, whereas Australopithecus is thought to have been at least closely related to the hominids from which modern humans subsequently evolved. For a long time, it was assumed that Paranthropus robustus and Australopithecus lived in much the same way. The shape of their bones suggested that they mainly walked on two legs.
As part of a research project led by the Max Planck Institute for Evolutionary Anthropology in Leipzig, fossilised bones have now been examined in greater detail at TU Wien: using high-resolution computed tomography, it is possible to study not only the external shape but also the internal structure of the bones. Bone is not a homogeneous block; it contains fine bone trabeculae inside, similar to a three-dimensional lattice structure. “This structure is not fixed,” explains Sebastian Bachmann from the Institute of Lightweight Structures and Structural Biomechanics at TU Wien. “It can change over the course of a person’s life. Depending on how the bone is stressed, and which forces act on it and in which direction, it can alter its shape or density in certain areas.”
A 3D map enables precise comparisons
A team from TU Wien – Dieter Pahr, Alexander Synek and Sebastian Bachmann – has been researching prehistoric bones for years. They used a large number of bones to create a reference model on the computer – a sort of ‘average bone’ against which individual finds can be compared. Only then does it become clear which part of one bone needs to be compared with which part of the bone from the other species.
This revealed unexpected differences between Paranthropus and Australopithecus: although both walked on two legs, Paranthropus is thought to have spent a great deal of time climbing – unlike Australopithecus, who, although physically capable of climbing, apparently tended to stay on the ground.
This means that the evolution of bipedalism did not follow a linear timeline. Although Paranthropus robustus is younger than the Australopithecus specimens from the Sterkfontein karst caves in South Africa, the Sterkfontein Australopithecus is thought to have been capable of walking on two legs more frequently. In Paranthropus, the hips, knees and ankles were likely to have been bent very often, as is necessary when climbing trees.
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Dr. Sebastian Bachmann