Humanity's missing link: 2-million-year-old bones shed light on human spine

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The Australopithecus sediba is estimated to have lived nearly two million years ago and is thought to be a predecessor to the Homo genus, to which modern humans – Homo sapiens – belong.

489077Malapa Hominin 1 (MH1) left, Lucy (AL 288-1 (Centre), and Malapa Hominin 2 (MH2) right. Image compiled by Peter Schmid courtesy of Lee R. Berger, University of the Witwatersrand.

Modern humans have been evolving and developing the ability to walk upright for over two million years, new research into a fossilized spine of an early human ancestor has revealed.

A peer-reviewed study published by eLife and led by New York University paleoanthropologist Scott Williams looks to shed new light on the development of bipedalism – walking on two legs – through examining the near-complete fossilized lower back of a female Australopithecus sediba. 

A. sediba is estimated to have lived nearly two million years ago and is thought to be a predecessor to the Homo genus, to which modern humans, Homo sapiens, belong – but this is contested by some who suggest it may be a sister species. 

The first A. sediba fossil was discovered in 2008 at the Malapa Fossil Site near Johannesburg, South Africa, but the fragments were too incomplete to determine whether or not the species had been bipedal or quadrupedal. However, a more recently discovered fossil specimen from the same location, labeled MH2, has provided a more complete picture of this early human ancestor.

Human bipedalism is achieved through the forward curve of the lower section of the spine, enabling an upright posture. This differs from other ape species – chimpanzee, orangutan and gorilla – all of whom lack this curvature and walk on four limbs when not up in the trees. 

489079The cranium of Malapa hominid 1 (MH1) from South Africa, named ''Karabo''. The combined fossil remains of this juvenile male is designated as the holotype for Australopithecus sediba. (credit: Wikimedia Commons)

The structure of the fossilized A. MH2 vertebrae suggests that the A. sediba would have walked comfortably on two legs, but other aspects of the species' bone structure indicate that it would probably still have spent a significant amount of time climbing trees.

The knee and ankle bones of the A. sediba "demonstrate clear evidence for bipedal locomotion," the study states, explaining that they possess human-like traits such as the ankle joint and the angle of the femur bone. Most importantly, the well-preserved MH2 spine indicates a clear curvature, more so than any other early human fossil, backing up the hypothesis that the species was well adapted to walking on two legs.

That is not to say that the species' spine had fully evolved into the spine of a modern human, however. The study notes that although it is "somewhat human-like in overall shape, its costal processes [a section of the cervical vertebrae] are long and cranially oriented, unlike modern humans," and that it is somewhere between the spine of the modern Homo sapien or extinct Homo neanderthalensis and the great apes. 

he upper body of the A. sediba was still well-equipped for climbing, with powerful trunk musculature that would have been suited for climbing and traveling via treetops rather than on the ground. Its shoulder blade is most similar to that of an orangutan, and it has an elongated humerus bone much like non-human ape species.

What is notable, however, is that, as mentioned in a 2011 study, the hand features a relatively long thumb and short fingers, similar to modern humans, indicating the ability to create complex stone tools.

"The findings provide new insights into our evolutionary history – and ultimately our place in the natural world around us," Williams said.

"Our lower back is prone to injury and pain associated with posture, pregnancy and exercise (or lack thereof)," he said. "Therefore, understanding how the lower back evolved may help us to learn how to prevent injuries and maintain a healthy back."