First articulating leg of Paranthropus robustus discovered

Fossils from Swartkrans Cave in South Africa reveal that a prehistoric relative of humans was habitually upright, but also small and vulnerable to predators

Paranthropus robustus was an early hominin species that lived in South Africa approximately two million years ago, coexisting with early Homo—a group that led directly to modern humans. Abundant skull and dental fossils have revealed much about the diet and social organization of Paranthropus robustus. However, the rarity of finding well-preserved, taxonomically diagnostic postcranial elements in the South African fossil record has complicated interpretations of its biology. Recently, an articulating leg, including hipbone, thigh bone, and shin bone from Swartkrans Member 1 (M1) has been attributed to Paranthropus robustus based on the external morphology of the proximal femur, offering, for the first time, deeper insights into the species’ body size, stature, posture, and locomotion.

Paranthropus robustus was a species of prehistoric human that lived in South Africa about two million years ago, alongside early Homo, a direct ancestor of modern humans. 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 organization 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 characterized by larger males and smaller females, indicating a mating system called polygyny, in which a single dominant male mates with multiple females.

Paranthropus robustus was an upright walker

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. A major new find from Swartkrans, the first articulating hipbone, thigh bone, and shin bone of Paranthropus robustus (designated by scientists as SWT1/HR-2), changes that.

An international research team, including a researcher from the Department of Human Origins of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany shows that this group of fossils belongs to a single, young adult Paranthropus robustus. As indicated by Marine Cazenave, “These fossils demonstrate not only that the species was, like modern humans, a habitual upright walker, but confirms it was also extremely small”. Travis Pickering, Professor at the University of Wisconsin-Madison, USA adds: “We estimate 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.”

Tooth marks and chewing damage

The small size of SWT1/HR-2 would have made it vulnerable to predators—such as sabertooth cats and giant hyenas—known to have occupied the area around Swartkrans Cave. As mentioned by Jason Heaton, from the University of Alabama at Birmingham, “This notion is confirmed by the team’s investigation of damage on the surface of the SWT1/HR-2 fossils, which includes tooth marks and other chewing damage identical to that made by leopards on the bones of their prey.”

The team’s continued investigation of the SWT1/HR-2 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 behaviors.