'Australopithecus sediba' walked like a human, but climbed like an ape

Fossils from the lumbar column found at the South African site of Malapa are the "missing link" closing a decades-old debate by showing that the earliest hominins used their upper limbs to climb like apes and the lower ones to walk like humans

Daniel García Martínez, of the Anthropology Unit at the Universidad Complutense de Madrid (UCM) and an affiliated researcher at the Centro Nacional de Investigación sobre la Evolución Humana (CENIEH), is part of the international team of scientists that has just published a paper in the journal e-Life on the discovery of fossil vertebrae two million years old from an extinct species, Australopithecus sediba. These vertebrae can be considered the "missing link" closing a decades-old debate by showing that the earliest hominins used their upper limbs to climb like apes and the lower ones to walk like humans.

The recovery in a consolidated cement-like rock of new vertebrae from the spine of a single individual of Australopithecus sediba, found in 2015 at the site of Malapa (Johannesburg, South Africa), taken together with other vertebrae discovered in 2008, make up one of the most complete lumbar columns in the fossil record and give an idea of how this ancient human relative walked and climbed.

The discovery also establishes that, just as in humans, A. sediba only had five lumbar vertebrae. "The lumbar region is fundamental to understanding the nature of bipedalism in our earliest ancestors and how well adapted they were to walking on two legs", explains the lead author of the study, Scott Williams, of New York University (USA) and University of the Witwatersrand (South Africa).

Issa, the protector

The fossils were reconstructed virtually following scanning by micro-computed tomography, thus eliminating the risk of damaging the delicate bones. Once the virtual reconstruction was complete, the vertebrae were added digitally to the fossils previously recovered and they articulated perfectly with the spinal column of the fossil skeleton MH 2, part of the original type specimens of Australopithecus sediba, described for the first time in 2010.

The female skeleton MH 2, nicknamed "Issa" (protector in Swahili) by the researchers, is one of the first two hominin skeletons conserving both a relatively complete lower back and dentition from the same individual, making it possible to be certain about the species from which the spinal column came.

“Although Issa was already one of the most complete skeletons of an ancient hominin ever discovered, these new vertebrae now complete the lower back and make its lumbar region a contender, not just for the title of most complete hominin, but also probably the best-preserved. This combination of integrity and preservation gave the team an unprecedented insight into the lower back anatomy of the species”, says Lee Berger, co-author of the study and leader of the Malapa project.

Transition hominin

According to the present study, the lordosis, which is the excessive curvature of the lower part of the back, in Australopithecus sediba is more pronounced than in any other Australopithecus discovered to date, and surpassed only by that observed in the spinal column of the Turkana Boy (Homo erectus) from Kenya, dated to 1.6 million years old, and certain modern humans.

As for the integration of the lumbar column with other parts of the skeleton, García Martínez states that “the capacity to use trees for locomotion is also observed in certain other anatomical regions such as in its narrow upper thorax”. For his part, Markus Bastir, of the Museo Nacional de Ciencias Naturales (MNCN), points out that “these results for A. sediba are a very good match with our other torso reconstructions for transition hominins, where we also see mosaic evolution in other related anatomical systems”.

Previous studies of the upper limbs, pelvis and lower limbs of this ancient species had already suggested mixed adaptations through the A. sediba skeleton, indicating its transitional nature, between walking like a human and climbing like an ape.

A. sediba is a transitional form of an ancient relative of humans and its spine has a clearly intermediate shape between that of modern humans (and Neandertals) and the great apes. Issa walked like a human, but could climb like a monkey", concludes Berger who, together with his nine-year-old son Matthew, discovered the first remains of what was to become the new species Australopithecus sediba in 2008.