New fossil discovery of an early human ancestor reveals that it walked upright, just like humans

Paranthropus robustus was a species of prehistoric human that lived in South Africa about two million years ago, alongside Homo ergaster, a direct ancestor of modern people. Fossils of Paranthropus robustus are found in abundance at Swartkrans Cave, situated about halfway between Johannesburg and Pretoria. Much has been revealed about the diet and social organisation of this extinct species based on studies of its many skulls and hundreds of teeth, which have been recovered from Swartkrans since scientific excavations began there in 1948. 

For instance, the extremely heavy jaws and thickly enameled teeth of Paranthropus robustus suggest that, when times were lean, it was capable of subsisting on low quality foods that were difficult to chew. Moreover, some of the skulls and teeth of Paranthropus robustus are exceptionally large, while others are robust but not as large as those in the first group. This suggests that Paranthropus robustus was characterised by larger males and smaller females, indicating a mating system called polygyny, in which a single dominant male mates with multiple females.

Unfortunately, Swartkrans has over the years yielded many fewer bones from the rest of the Paranthropus robustus skeleton, limiting our understanding of its stature, posture, and locomotion, essential characteristics related to finding food and mates. A major new find from Swartkrans, the first articulating hipbone, thigh bone, and shin bone of Paranthropus robustus, changes that. 

A team of international researchers affiliated to the Evolutionary Studies Institute at the University of the Witwatersrand (Wits University), in South Africa including Travis Pickering, Matthew Caruana, Marine Cazenave, Ron Clarke, Jason Heaton, A.J. Heile, Kathleen Kuman, and Dominic Stratford, indicates in new research that this group of fossils belong to a single, young adult Paranthropus robustus. The fossil not only demonstrates that the species was, like modern humans, a habitual upright walker, but also confirms it was also extremely small. The research was published in the Journal of Human Evolution.

“It is estimated that this individual, probably a female, was only about a meter tall and 27 kg when it died, making it even smaller than adults from other diminutive early human species, including those represented by the famous ‘Lucy’ (Australopithecus afarensis, about 3.2 million years old) and ‘Hobbit’ (Homo floresiensis, about 90,000 years old) skeletons, from Ethiopia and Indonesia, respectively,” says Professor Pickering from the Univesity of Wisconsin-Madison, who led the research. 

The small size of the new Paranthropus robustus individual would have made it vulnerable to predators — such as sabertooth cats and giant hyenas — known to have occupied the area around Swartkrans Cave. This notion is confirmed by the team’s investigation of damage on the surface of the fossils, which includes tooth marks and other chewing damage identical to that made by leopards on the bones of their prey. 

“Although it seems that this particular Paranthropus robustus individual was the unfortunate victim of predation, that conclusion does not mean that the entire species was inept. We know that Paranthropus robustus survived in South Africa for over a million years and is found invariably, and at various sites, in spatial association with stone and bone tools,” says Pickering. 

Those implements were used for a variety of purposes, including butchering animals for their meat and digging for edible roots and underground insects. It is a matter of current research whether Paranthropus robustus, contemporaneous Homo ergaster, or both, was the maker and user of those important tools—but the Swartkrans team believes that Paranthropus robustus very likely possessed the cognitive and physical capabilities to do both.

The team’s continued investigation of the fossils includes CT-scan analyses of internal bone structures, which will provide additional information on the growth and developmental patterns of Paranthropus robustus, as well as adding details to our growing appreciation of its locomotor behaviours.

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