Australopithecus sediba walked comfortably on two legs, but could climb like a monkey

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Paleoanthropologists discovered and examined the fossil lumbar vertebrae of Australopithecus sediba, a small hominid that lived about 2 million years ago. Their results suggest that Australopithecus sediba would have had an upright posture and would have walked comfortably on two legs, and the curvature of the lower back was similar to that of modern humans; however, other aspects of bone shape suggest that in addition to walking, this hominid likely spent a lot of time climbing trees.

Life reconstruction of Australopithecus sediba commissioned by the University of Michigan Museum of Natural History. Image credit: Elisabeth Daynes / S. Entressangle.

Australopithecus sediba is a close relative of modern humans who lived 2 million years ago in what is now South Africa.

In 2008, fossils of an adult female Australopithecus sediba were discovered in a cave called Malapa.

However, the fossils in the lower back region were incomplete, so it was not clear whether the female – called Malapa Hominin 2 (MH2) – had a forward curving spine and other adaptations needed to walk. on two legs.

In 2015, Professor Scott Williams, a paleoanthropologist at New York University and the University of the Witwatersrand, and his colleagues discovered new fossils – mostly lower back bones – at the Malapa site.

They interlock with the previously discovered MH2 fossils, providing an almost complete lower spine.

The discovery also shows that, like humans, Australopithecus sediba had only five lumbar vertebrae.

“The lumbar region is essential to understanding the nature of bipedalism in our early ancestors and to understanding how well they were adapted to walking on two legs,” said Professor Williams.

“Associated series of lumbar vertebrae are extraordinarily rare in the hominid fossil record, with in reality only three comparable lower spines known to all of the earliest African records.”

The discovery of the new specimens means that MH2 (also known as ‘Issa’, which means protector in Swahili) is now one of the first two hominid skeletons to preserve both a lower spine and relatively dentition. complete data from the same individual, which allows for certainty as to which species the spine belongs to.

“While Issa was already one of the most complete skeletons of an ancient hominid ever discovered, these vertebrae virtually complement the lower back and make Issa’s lumbar region a candidate for not just the hominin lower back. the best preserved ever discovered, but also possibly the best preserved, ”said Professor Berger.

Silhouette of Australopithecus sediba showing new vertebrae as well as other skeletal remains of the species;  the enlarged detail (a photograph of the articulated fossils on the left; computed tomography models on the right) shows the fossils, in color on the right between previously known elements in gray.  Image credit: Williams et al., Doi: 10.7554 / eLife.70447.

Australopithecus sediba silhouette showing the vertebrae found as well as other skeletal remains of the species; the enlarged detail (a photograph of the articulated fossils on the left; computed tomography models on the right) shows the fossils, in color on the right between previously known elements in gray. Image credit: Williams et al., doi: 10.7554 / eLife.70447.

Previous studies of the incomplete lower spine hypothesized that MH2 would have had a relatively straight spine, without the curvature, or lordosis, typically seen in modern humans.

They further hypothesized that MH2’s spine looked more like that of extinct Neanderthals and other more primitive species of ancient hominids over 2 million years old.

Lordosis is the inward curve of the lumbar spine and is typically used to demonstrate strong adaptations to bipedalism.

However, with the fuller spine and the excellent preservation of the fossils, Professor Berger and his colleagues found that MH2 lordosis was in fact more extreme than any other Australopithecus yet discovered, and the amount of curvature of the spine observed was only exceeded by this. seen in the spine of the 1.6 million year old Turkana boy (homo erectus) from Kenya and some modern humans.

“While the presence of lordosis and other features of the spine represent clear adaptations to walking on two legs, there are other features, such as the wide and upward-facing transverse processes, which suggest a powerful trunk musculature, possibly for arboreal behaviors, “said Professor Gabrielle Russo, a researcher at Stony Brook University.

Strong transverse spines pointing upwards usually indicate strong trunk muscles, as seen in monkeys.

“When combined with other parts of the torso anatomy, it indicates that Australopithecus sediba kept clear adaptations to climbing, ”said Professor Shahed Nalla, a researcher at the University of Johannesburg and the University of the Witwatersrand.

The authors concluded that Australopithecus sediba is a transitional form of an ancient human parent, and its spine is clearly intermediate in shape between that of modern humans and Neanderthals and great apes.

“Issa walked a lot like a human, but could climb like a monkey,” said Professor Berger.

The results were published in the journal eLife.

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Scott A. Williams et al. 2021. New fossils from Australopithecus sediba reveal an almost complete lower back. eLife 10: e70447; doi: 10.7554 / eLife.70447


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