Paranthropus robustus Walked Upright: What the New Swartkrans Fossils Reveal

Northwest of Johannesburg, in the limestone region nicknamed the Cradle of HumankindCradle of HumankindA karstic area northwest of Johannesburg, inscribed by UNESCO, among the richest deposits of human fossils in the world.→, the SwartkransSwartkransA fossil-bearing cave in the Cradle of Humankind, South AfricaAfricaThe cradle of humankind: the continent where the first hominins appeared, then Homo sapiens around 300,000 years ago, before the expansion to the rest of the world.→, rich in remains of Paranthropus robustus and early tools.→ cave has just yielded a set of fossils that is rare among all others: a partial hip bone, a nearly complete femur and a complete tibia belonging to one and the same individual. These bones, described in the Journal of Human Evolution by a team that includes Travis Pickering and Jason Heaton, are the first post-cranial remains of a ParanthropusParanthropusA genus of fossil hominins known as robust australopithecines, with a powerful chewing apparatus, that lived in Africa about two to one million years ago.→ robustus ever found still articulated¹. Their message is plain: this robust homininHomininMember of the subtribe Hominina, comprising the human lineage (Homo, Australopithecus, Paranthropus…) but excluding orangutans and gibbons. The term progressively replaces "hominid" in its narrow sense.→, long known from its jaws and massive teeth alone, moved about upright, with a bipedalismBipedalismA mode of locomotion on two hind limbs, the defining trait of the human lineage, appearing over 7 million years ago. Visible in the anatomy of the pelvis, femur and foramen magnum.→BipedalismA mode of locomotion involving standing and moving upright on the two lower limbs.→ of decidedly human character.

The discovery cuts against the common image of a hominin still half at home in the trees. It brings weighty support to a long-running debate about how the robust australopithecines moved, while raising fresh questions about their small stature, their place in a landscape teeming with carnivores and their relationship to the earliest tools. Above all, it reminds us that upright walking was not the preserve of our direct ancestors, but a shared trait, refined in various ways, within a human tree far bushier than once supposed.

A robust cousin of the human lineage

Paranthropus robustus belongs to the group of so-called robust australopithecinesAustralopithecineA genus of bipedal hominins from Africa (c. 4.2–1.9 Ma) with a brain still close to that of great apesgreat apesThe family of great apes (Hominidae) comprising orangutans, gorillas, chimpanzees, bonobos and humans.→ (400–550 cm³) but walking upright. Lucy (<em>Au. afarensis</em>) is the most famous specimen.→, a branch of hominins that lived in southern Africa roughly two to one million years ago. It was first described in 1938 by the South African paleontologist Robert Broom, from remains found at the site of Kromdraai, not far from Swartkrans³. In the years that followed, Broom and his collaborators multiplied their finds, especially at Swartkrans, opened to excavation from 1948, which became one of the richest deposits for the species.

What strikes one at once in Paranthropus robustus is its chewing apparatus. The molars and premolars are enormous, sometimes almost twice the size of our own, and capped with very thick enamel. The face is broad and flat, fitted with powerful arches to anchor considerable chewing muscles; in males, a bony crest often ran along the top of the skull to attach those muscles. Specialists readily speak of a mortar-and-pestle dentition, built to grind and crush tough foods. This equipment let the animal exploit low-quality resources, roots, tubers, seeds and fibrous plants, during lean times when softer food ran short. The brain, by contrast, stayed modest, on the order of five hundred cubic centimeters, barely larger than that of a living great ape. For decades, these skulls and teeth, the most durable parts of the skeleton, made up almost everything we knew of the species, leaving its body in the shadows.

Unprecedented bones freed from the rock

It is precisely this gap that the new study fills. The fossils come from Member 1 of the Swartkrans Formation, more specifically from the unit called the Hanging Remnant, a remnant of breccia left clinging beneath the roof of the cave. The caves of the Cradle of Humankind were never dwellings: they are ancient natural traps, shafts and fissures where bones and debris collected, cemented over time into a hard rock, the breccia².

In March 2019, a technician at the University of the Witwatersrand, Lukas Sekowe, spotted what looked like the upper end of a fossilized femur, exposed at the tip of a block of breccia prised from a fossil-rich layer. Painstaking work with small compressed-air drills gradually revealed a remarkably well-preserved hip, femur and shin bone, still in anatomical connection. The team, affiliated with the University of the Witwatersrand and led by Travis Pickering of the University of Wisconsin-Madison, dated the deposit to around two million years, within a range of roughly 2.3 to 1.7 million years¹. For a species of which only rare isolated fragments below the pelvis had been known, having a hip bone and a lower limb that certainly belong to the same individual changes everything: one can at last reason about a complete chain of joints, from hip to knee to ankle, rather than about scattered bones whose attribution remained uncertain.

A decidedly bipedal gait

Comparing these bones with those of other fossil hominins and present-day human populations, the team concluded that Paranthropus robustus was an accomplished upright walker. The shape of the hip, the knee and the ankle resembles that of modern humans far more than that of a tree-climbing great ape¹. The lower end of the tibia, at the ankle joint, further suggests the presence of an arched foot, another signature of efficient upright walking.

This verdict is no small matter, for the locomotion of the robust australopithecines has drawn much ink. Certain analyses of the internal structure of limb bones assigned to the species had suggested that it frequently flexed its hips, knees and ankles, a motion linked to climbing, and that it therefore retained more arboreality than the older Australopithecus of SterkfonteinSterkfonteinA fossil-bearing cave in the Cradle of Humankind, South Africa, rich in australopithecines and central to the debate over their dating.→. The new fossils do not entirely close this discussion, but they shift its center of gravity firmly toward a habitual, committed bipedalism. In other words, even if these hominins could still climb on occasion, their way of getting about on the ground was that of a biped, standing on two legs, in the manner in which we walk today. One sizable detail nonetheless tempers the picture: the Swartkrans individual was tiny.

Tiny, mobile and vulnerable

The bones point to a young adult whose height the team estimates at about one meter and whose weight it puts at some 27 kilograms, among the smallest known members of the human lineage¹. She was a little shorter and lighter than the famous Lucy, the Australopithecus afarensis of Ethiopia, or than the human of Flores, the celebrated hobbit of Liang Bua cave. Since some specimens of the species weigh almost twice as much, the researchers read in this gap a strong sexual dimorphism, with males markedly larger than females; it is on this basis, and cautiously, that they propose to identify the Swartkrans individual as a female.

Among living primates, such a contrast in size between the sexes often goes with a social arrangement in which one dominant male mates with several females. Small size had its advantages: a light body slips easily through dense vegetation, feeds at lower energy cost and copes better with fragmented habitats. But it had a dire downside. Across the savanna and thickets of the South African PleistocenePleistoceneThe geological epoch of the great ice ages (c. 2.6 Ma–11,700 BP), spanning most of human prehistory.→ prowled leopards, giant hyenas and saber-toothed cats. A hominin one meter tall made choice prey. The bones themselves bear the mark of it: the team recorded on the hip and femur bite marks and tooth impressions matching those left by modern leopards, and uncovered a complete leopard skull in the very same block of breccia as the limb bones¹. This finding extends an idea put forward as early as the 1970s by the South African paleoanthropologist C. K. Brain, the so-called Swartkrans leopard hypothesis: big cats hauled their carcasses into trees overhanging the fissures, and the leftovers eventually dropped into the caves, explaining the abundance of tooth marks on the Cradle's fossils⁴. The famous child's skull SK 54, pierced by two holes whose spacing exactly matches a leopard's canines, remains the most striking illustration. Such a reading belongs to taphonomyTaphonomyThe study of the processes affecting organic remains between death and fossilisation, including the marks left by predators.→, the science of what happens to bones between death and fossilization.

Does this mean the species was doomed to failure? Not at all, insists Travis Pickering. That one individual ended up devoured does not mean the whole species was ill-adapted. Paranthropus robustus thrived in southern Africa for more than a million years, proof of a survival strategy different from that of our ancestors but remarkably effective over the long run.

The puzzle of the tools and the lesson on bipedalism

One question still divides specialists: did Paranthropus robustus make tools? The sites that yield its remains, Swartkrans foremost among them, have also produced flaked stone tools and, remarkably, bone tools polished by use. Analyses of the micro-wear traces have shown that some of these bones served to dig, probably to extract termites from their mounds, a foraging behavior that fits well with the plant diet suggested by the dentition⁵.

But assigning these tools to a particular species is tricky, for Paranthropus robustus did not reign alone at Swartkrans. It shared the site with a member of the genus Homo, often referred to Homo ergasterHomo ergasterThe early African form of Homo erectusHomo erectusAncient human species, the first hominin to leave Africa and the maker of the earliest Acheulean handaxes.→, contemporary with Paranthropus robustus and recognized for making stone tools.→, the early African form of Homo erectus, which is known in any case to have worked stone. So which of the two produced and used the cave's tools? Nothing in the rock allows a certain answer: the objects carry no maker's signature. Some researchers argue for a tool-making Paranthropus, given hands whose anatomy allowed a precision grip; others find it more parsimonious to credit Homo, a proven artisan. The case remains open, and the new fossils, which concern the lower limb and not the hand, do not close it.

What the young adult from Swartkrans does illuminate forcefully, by contrast, is the history of bipedalism itself. Standing upright is no late invention peculiar to our genus. Signs of upright walking appear very early in the human tree, in forms such as Sahelanthropus, Orrorin and Ardipithecus, six to seven million years ago, and then assert themselves among the australopithecines, whose footprints at Laetoli, in Tanzania, preserve the fossil trace from about 3.6 million years ago. By showing that a robust paranthropine walked upright as a matter of habit two million years ago, the study confirms that bipedalism was a foundation shared by several contemporary branches, each perfecting it in its own way. Our direct ancestors of the genus Homo were not the only ones to stand tall: they inherited and extended an adaptation already ancient, which cousins such as Paranthropus robustus practiced in parallel. That is the whole meaning of these three bones freed from a block of breccia: they remind us that human evolution was less a ladder than a bush, in which several ways of being bipedal coexisted before one of them, ours, finally prevailed.