A fossilised left leg unearthed in South Africa belongs to one of the smallest adult hominins ever discovered – smaller even than the so-called “hobbit”, Homo floresiensis.
The diminutive hominin was a member of the species Paranthropus robustus. This was one of several species of Paranthropus, a group of ape-like hominins that shared the African landscape with the earliest representatives of our human genus, Homo, between about 2.7 and 1.2 million years ago. Paranthropus had heavily built skulls that housed small brains and large teeth – which like a cow.
Little is known about Paranthropus anatomy because most fossils found to date are teeth or parts of the skull. This makes the discovery of a relatively complete leg of P. robustus an important find. “We knew immediately that we had an astonishing fossil on our hands,” says at the University of Wisconsin-Madison.
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One thing that was instantly clear was that the fossil – a thighbone, shinbone and part of the hip – belonged to an unusually small hominin. “It’s impressive how small it is compared with the shortest of the short we’ve known about so far,” says at the Smithsonian Institution in Washington DC, who wasn’t involved in the analysis.
Pickering’s team estimates that the hominin, which was probably a young adult female, stood just 103 centimetres tall. For comparison, the best-preserved H. floresiensis individual – – was 109 cm tall.
Such a small hominin may well have been an easy target for predators, says Pickering, and so it might have sought shelter in trees. However, there are no clear indications in the leg bones that P. robustus had special adaptations for climbing. That is a surprise: a fragmentary skeleton of a related species – Paranthropus boisei – was discovered about a decade ago, and it .
The two species “may have engaged in different behaviours”, says at the University of Alcalá, Spain, who led the analysis of the P. boisei skeleton. This doesn’t necessarily mean that P. robustus couldn’t climb, says Pickering, although it is unclear why it lacked the climbing adaptations seen in P. boisei.
Another puzzle is how P. robustus fed. Researchers have previously suggested that the species . This would imply that it spent long periods of time sitting, kneeling or squatting to dig in the dirt – but there are no clear signs in the knee joint of the fossilised leg that P. robustus engaged in such behaviour. “This was a surprising finding to me,” says Pickering.
It is certainly an interesting discovery, says at the City University of New York, who wasn’t involved in the analysis – but he adds that it is too early to say for sure that P. robustus really didn’t sit or squat often. “It’s a hypothesis that needs further testing,” he says.
The fossil has, however, resolved at least one point of debate about P. robustus: exactly how it moved around on the ground. Although it is certain that the species could walk on two legs, it has long been unclear whether it did so most of the time or just occasionally. Thanks to the leg fossil, Pickering says we can now confirm P. robustus did walk on two legs most of the time. This, he says, is “the real revelation of our research”.
Journal of Human Evolution
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