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Australopithecus sediba walked like a human but climbed like an ape

An international anthropology team of scientists from New York University, the University of the Witwatersrand, and 15 other institutions have announced in the open-access journal e-Life, the discovery of two-million-year-old fossil vertebrae from an extinct species of ancient human relative.

The study, “New fossils of Australopithecus sediba reveal a nearly complete lower back,” was conducted by Scott Williams, Thomas Cody Prang, Marc Meyer, Thierra Nalley, Renier Van Der Merwe, Christopher Yelverton, Daniel García-Martínez, Gabrielle Russo, Kelly Ostrofsky, Jeffrey Spear, Jennifer Eyre, Mark Grabowski, Shahed Nalla, Markus Bastir, Peter Schmid, Steven Churchill, and Lee Berger.

The recovery of new lumbar vertebrae from the lower back of a single individual of the human relative, Australopithecus sediba, and portions of other vertebrae of the same female from Malapa, South Africa, together with previously discovered vertebrae, form one of the most complete lower backs ever discovered in the early hominid record and give insight into how this ancient human relative walked and climbed.

Australopithecus sediba silhouette showing the newly found 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 newly discovered fossils, in color on the right between previously known elements in grey. Image courtesy of NYU & Wits University.

The fossils were discovered in 2015 during excavations of a mining trackway running next to the site of Malapa in the Cradle of Humankind World Heritage Site, just northwest of Johannesburg, South Africa.

Vertebrae recovered from breccia layer

In 2008 Professor Lee Berger, University of the Witwatersrand, first discovered remains of Australopithecus sediba.

The vertebrae described in the present study were recovered in a consolidated cement-like rock, known as breccia, in near articulation.

Rather than risking damaging the fossils, they were prepared virtually after scanning with a Micro-CT scanner at the University of the Witwatersrand. Once virtually prepared, the vertebrae were reunited with fossils recovered during earlier work at the site and found to articulate perfectly with the spine of the fossil skeleton, part of the original Type specimens of Australopithecus sediba first described in 2010. The skeleton’s catalog number is MH 2, but the researchers have nicknamed the female skeleton “Issa,” meaning protector in Swahili. The discovery also established that like humans, 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,” said Williams, New York University and Wits University and lead author on the paper. “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 Issa 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.

Best-preserved lower back ever found

“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,” said Berger, leader of the Malapa project. 

Berger added that this combination of completeness and preservation gave the team an unprecedented look at the anatomy of the lower back of the species.

Previous studies of the incomplete lower spine by authors not involved in the present study hypothesized that sediba would have had a relatively straight spine, without the curvature, or lordosis, typically seen in modern humans. They further hypothesized Issa’s spine was more like that of Neandertals and other more primitive species of ancient hominins older than two million years.

Australopithecus sediba silhouette showing the newly-found vertebrae (colored) along with other skeletal remains from the species. Image courtesy of NYU & Wits University.

Lordosis is the inward curve of the lumbar spine and typically is used to demonstrate strong adaptations to bipedalism. However, with the more complete spine, and excellent preservation of the fossils, the present study found the lordosis of sediba 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 Russo, Stony Brook University.

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

According to Nalla, University of Johannesburg and Wits, who is an expert on ribs and a researcher on the present study, when combined with other parts of torso anatomy, this indicates that sediba retained clear adaptations to climbing.

Previous studies of this ancient species have highlighted the mixed adaptations across the skeleton in sediba that have indicated its transitional nature between walking like a human and climbing adaptations. These include features studied in the upper limbs, pelvis, and lower limbs.

“The spine ties this all together,” said Prang, Texas A&M, who studies how ancient hominins walked and climbed. “In what manner these combinations of traits persisted in our ancient ancestors, including potential adaptations to both walking on the ground on two legs and climbing trees effectively, is perhaps one of the major outstanding questions in human origins.”

The study concluded that sediba is a transitional form of ancient human relative and its spine is clearly intermediate in shape between those of modern humans (and Neandertals) and great apes.

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

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