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Paleoanthropologists have discovered and examined the fossil lumbar vertebrae of Australopithecus sediba, a small hominin that lived about 2 million years ago. Their results suggest that Australopithecus sediba would have had an upright posture and comfortably walked on two legs, and the curvature of their lower back was similar to modern humans; however, other aspects of the bones’ shape suggest that as well as walking, this hominin probably spent a significant amount of time climbing in 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 that lived 2 million years ago in what is now South Africa.

In 2008, fossils from an adult female Australopithecus sediba were discovered at a cave site called Malapa.

However, the fossils of the lower back region were incomplete, so it was unclear whether the female — referred to as 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 uncovered new fossils — mainly bones from the lower back — at the Malapa site.

They fit together with the previously discovered MH2 fossils, providing a nearly complete lower spine.

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

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

“Associated series of lumbar vertebrae are extraordinarily rare in the hominin fossil record, with really only three comparable lower spines being known from the whole of the early African record.”

The discovery of the new specimens means that MH2 (also known as ‘Issa,’ meaning protector in Swahili) now becomes one of only two early hominin skeletons to preserve both a relatively complete lower spine and dentition from the same individual, allowing certainty as to what species the spine belongs to.

“While Issa was already one of the most complete skeletons of an ancient hominin ever discovered, these vertebrae practically complete the lower back and make Issa’s lumbar region a contender for not only the best-preserved hominin lower back ever discovered, but also probably the best preserved,” Professor Berger said.

Australopithecus sediba silhouette showing the newfound vertebrae along with other skeletal remains from the species; the enlarged detail (a photograph of the fossils in articulation on the left; micro-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 MH2’s spine was more like that of the extinct species Neanderthals and other more primitive species of ancient hominins older than 2 million years.

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

However, with the more complete spine, and excellent preservation of the fossils, Professor Berger and colleagues found the lordosis of MH2 was in fact more extreme than any other australopithecines yet discovered, and the amount of curvature of the spine observed was only exceeded by that 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 large and upward oriented transverse processes, that suggest powerful trunk musculature, perhaps for arboreal behaviors,” said Professor Gabrielle Russo, a researcher at Stony Brook University.

Strong upward oriented transverse spines are typically indicative of powerful trunk muscles, as observed in apes.

“When combined with other parts of torso anatomy, this indicates that Australopithecus sediba retained 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 ancient human relative and its spine is clearly intermediate in shape between those of modern humans and Neanderthals and great apes.

“Issa walked somewhat like a human, but could climb like an ape,” Professor Berger said.

The findings were published in the journal eLife.

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

 



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