Swartkrans Paranthropus and Sterkfontein Australopithecus  had different locomotor repertoires

Paranthropus robustus from southern Africa combined upright walking and more frequent climbing than the older Australopithecus in the same region

To the Point

• Locomotor differences: The internal bone structure of the extremities of the lower limb of Paranthropus robustus more closely resembles that of African great apes. This suggests more frequent flexion of the ankle, knee, and hip joints, as required for climbing, than in Australopithecus from Sterkfontein. P. robustus may have used a more diverse repertoire of locomotion, whereas Australopithecus from Sterkfontein appears to have engaged more consistently in terrestrial bipedalism, with a greater terrestrial-based home range.
• Distinct ecological niche: P. robustus combined bipedal walking on the ground with frequent climbing—in contrast to Australopithecus, which relied more heavily on bipedal locomotion. The two taxa, therefore, exploited different ecological niches. This findings raise the question of whether Paranthropus boisei in East Africa frequently adopted highly flexed postures of the hip, knee, and ankle— questioning whether similar locomotor behaviours may have characterized the genus more broadly.
• Non-linear evolution:  The evidence for frequent climbing in P. robustus, which postdates the Australopithecus from Sterkfontein, points to a non-linear evolution of bipedalism in the hominin lineages.

Paranthropus robustus was an early hominin species that lived in southern Africa approximately two million years ago, coexisting with early Homo, a group that led directly to modern humans, and following earlier hominins such as Australopithecus from Sterkfontein Member 4 (ca. 3.5–3.4 Ma). The rarity of well-preserved, species-diagnostic postcranial elements in the southern African fossil record has long complicated interpretations of the P. robustus biology.

Fossils of P. robustus are found in abundance at Swartkrans Cave, a hominin fossil site situated not far from Johannesburg and Pretoria in South Africa. Much has been revealed about its diet and potential social organization based on remains of the many skulls and teeth that are attributed to this species, which have been recovered from Swartkrans since research began there in 1948. For instance, the extremely robust jaws and thickly enameled teeth of P. robustus suggest that it was capable of subsisting on low-quality foods that were difficult to chew. In contrast, many Australopithecus specimens from Sterkfontein (ca 1 km from Swartkrans Cave) have less specialised jaws and teeth, suggesting that they likely relied less on such mechanically challenging foods.

However, although a few studies have suggested some degree of morphological—and thus locomotor—diversity, it has long been assumed that both taxa were broadly similar, with their lower limb morphology indicating bipedalism as the primary mode of terrestrial locomotion, while aspects of their upper limb morphology suggest some degree of arboreal activity.

Recently, an articulating lower limb, comprising part of the pelvis, a nearly complete femur and complete tibia, from Swartkrans Member 1 (M1) has been attributed to P. robustus, offering, for the first time, deeper insights into the ecology of this species. Although its external morphology reveals clear adaptations to bipedalism, Cazenave et al. (2026) investigated the cortical and trabecular bone, plastic tissues that adapt to the mechanical loading or stress experienced by the bone or joint during life, to gain further functional insight into how P. robustus moved within its environment. The results show that the internal bone structure of the P. robustus lower limb is more similar to that of African apes, indicating more frequent ankle, knee, and hip flexion than in Sterkfontein Australopithecus.

What do we learn from the new findings?

Most of the fossil record for P. robustus is made up of skulls and teeth, with much less known about the appearance of the rest of its skeleton. Thus, the first articulating hip bone, thigh bone, and shin bone of Paranthropus robustus (designated as SWT1/HR-2) provides much-needed information about the appearance of the P. robustus lower limb and how it moved within its environment.

An international research team, including several researchers from the Department of Human Origins of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, shows that the internal structure of the bone of the hip, knee and ankle of the individual is like that of living African apes. As indicated by Marine Cazenave, “These results indicate that, although P. robustus was certainly a biped, it was also using more flexed hip, knee, and ankle postures during life, which are necessary for climbing, far more so than the older Australopithecus species from the same region.” Travis Rayne Pickering, Professor at the University of Wisconsin–Madison and Principal Investigator at Swartkrans, adds that “along with the differences observed in the skulls and teeth of the two taxa, the present results suggest that P. robustus from Swartkrans Member 1 exploited an ecological niche distinct from that of Sterkfontein Member 4 Australopithecus.”