AUSTRALOPITHECUS SEDIBA WALKED LIKE A HUMAN, BUT CLIMBED LIKE AN APE

Lumbar spine fossils found at the Malapa site in South Africa are the "missing link" resolving a decades-long debate, showing that early hominids used their upper limbs to climb like apes and their lower limbs to walk like humans.

 

Source: CENIEH

 

Daniel García Martínez, from the Anthropology Unit of the Complutense University of Madrid (UCM) and affiliated member of the National Centre for Research on Human Evolution (CENIEH) is part of the international team of scientists that has just published an article in the journal E-LIFE on the discovery of two-million-year-old fossil vertebrae from an extinct species, Australopithecus sediba. These vertebrae can be considered the "missing link" resolving a decades-long debate, showing that early hominids used their upper limbs to climb like apes and their lower limbs to walk like humans.

 

The recovery in a cement-like bedrock of new vertebrae from the spinal column of a single Australopithecus sediba individual found in 2015 at the Malapa site (Johannesburg, South Africa), together with the vertebrae discovered in 2008, form one of the most complete lumbar spines in fossil records and give insight into how this ancient human ancestor walked and climbed.

 

The discovery also establishes that, like humans, sediba had only five lumbar vertebrae. "The lumbar region is critical to understanding the nature of bipedalism in our early ancestors and understanding how well suited they were to walking on two legs," said lead author Scott Williams of New York University. (USA) and the Witwatersrand University (South Africa).

 

Issa, the protector

 

The fossils were virtually reconstructed, after being scanned using micro-computed tomography, thus eliminating the risk of damaging the delicate bones. Once virtually reconstructed, the vertebrae were added to previously recovered fossils, which perfectly fit with the spine of fossil skeleton MH 2, part of the original type specimens of Australopithecus sediba first described in 2010.

 

The female skeleton MH 2, dubbed "Issa" (protector in Swahili) by the researchers, is one of the first two hominid skeletons to preserve both a relatively complete lower spine and dentition from the same individual, providing certainty about the species to which the vertebral column belongs.

 

“While Issa was already one of the most complete skeletons of an ancient hominid ever discovered, now these vertebrae complete her lower back and make her lumbar region a contender for not only the most complete hominid, but also probably the best preserved. This combination of integrity and preservation offered the team an unprecedented look at the lower back anatomy of the species,” says Lee Berger, co-author of the study and leader of the Malapa project.

 

Transitional hominid

 

According to the study, the lordosis, i.e. the excessive curvature of the lower part of the back, of Australopithecus sediba is more extreme than that of any other australopithecus discovered so far, second only to that observed in the Turkana child's spine (Homo erectus) in Kenya, 1.6 million years old, and that of some modern humans.

 

Regarding the integration of the lumbar spine with other regions of the skeleton, Daniel García Martínez says that "the ability to use the tree environment for locomotion is also observed in some other anatomical regions, such as in its narrow upper thorax”. In turn, Markus Bastir, from the National Museum of Natural Sciences (MNCN) points out that "these sediba results fit very well with our other transitional hominin torso reconstructions, where we also see mosaic evolution in other related anatomical systems."

 

Previous studies on the upper limbs, pelvis, and lower limbs of this ancient species noted mixed adaptations across the skeleton in sediba, indicating its transitional nature between walking like a human and climbing like an ape.

 

“Sediba is a transitional form of an ancient human ancestor and its spine is clearly intermediate in shape between those of modern humans (and Neanderthals) and great apes. Issa walked like a human, but could climb like a monkey", concludes Berger, who in 2008, together with his nine-year-old son, Matthew, discovered the first remains of what would be a new species: Australopithecus sediba