Ancient hominin walked like a human but climbed like an ape

Early hominins used their upper limbs to climb like apes and their lower limbs to walk like humans, according to new fossil evidence.

In 2015, mining excavations in Malapa, South Africa, revealed fossil vertebrae trapped in cement-like rock called breccia. Analysis revealed the vertebrae to be two million years old, from the lower back of a female Australopithecus sediba, a relative of modern humans first discovered at the same site in 2008.

Together with previously discovered vertebrae, these fossils form one of the most complete lower backs in the early hominid fossil record, giving us insight into the movements of our ancient cousin.

Silhoutte of ape-like creature hanging from tree branch
Australopithecus sediba silhouette showing the newly-found vertebrae (coloured) along with other skeletal remains from the species. Credit: NYU & Wits University

“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,” says lead author Scott Williams, a paleoanthropologist from New York University, US, and Wits University in South Africa.

“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.”

These vertebrae were therefore treated with utmost care. The risk of damage was reduced by scanning the delicate fossils with a micro-CT scanner, creating 3D virtual models of them which can be viewed here.

The vertebrae were then reunited with the rest of the spine, previously found at the site, belonging to a single female specimen nicknamed “Issa” (meaning protector in Swahili).

Other research based on Issa’s incomplete spine had hypothesised that A. sediba had a straight spine, like Neanderthals and other extinct early hominins, as opposed to the inwardly curved spine of modern humans which indicates strong adaptations to bipedalism.

Now, with a complete and well-preserved spine, the research team could see that Issa has a more curved spine than any other australopithecines yet discovered.

This shows that Issa’s species were adapted to walking on two legs, as the team reports in their paper in the journal eLife.

Co-author Gabrielle Russo of Stony Brook University in the US notes, however, that “while the presence of lordosis [curvature] 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 behaviours”.

Diagram showing which bones of the specimen are available, mainly arm and pelvic bones
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 colour on the right between previously known elements in grey. Credit: NYU & Wits University

In other words, Issa and her species were still climbers.

This means that A. sediba was a transitional form of ancient humans, existing in the space between when hominins spent their entire lives in the trees and when hominins swung down to the ground and began to walk on two legs full time.

Previous research has looked at the upper limbs, pelvis and lower limbs to gain insight into these transitionary adaptations.

“The spine ties this all together,” says palaeoanthropologist Cody Prang of Texas A&M, US, who was not involved in the study.

“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.”

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