Together with previously discovered bones from the same individual — nicknamed “Issa,” which is Swahili for protector — the fossilized remains form one of the most complete lower backs ever discovered in the early hominid record and give an indication as to how this human relative would have moved through the world.
The researchers said the newly studied lower back fossils were a missing link that proved early hominins used their upper limbs to climb like apes, and their lower limbs to walk like humans.
The fossils were first discovered in 2015 during excavations of a mining trackway running next to the site of Malapa in the Cradle of Humankind World Heritage Site, near Johannesburg.
They were then virtually prepared — to avoid the risk of damage — and reunited with fossils recovered during earlier work at the site and found to articulate with the spine of the fossil skeleton, part of the original specimens of Australopithecus sediba first described in 2010.
The discovery also established that like humans, sediba had only five lumbar vertebrae.
“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 Professor Lee Berger, an author on the study and leader of the Malapa project.
The excellent preservation of Issa helped to show that curvature of sediba’s spine was more extreme than any other Australopithecus yet discovered — that kind of spine curvature is typically seen in modern humans and demonstrates strong adaptations to bipedalism.
“While the presence of lordosis (the inward curve of the lumbar spine) 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 Professor Gabrielle Russo of Stony Brook University, another author on the study.
Arboreal behaviors refer to climbing and living in trees.
“The spine ties this all together,” added study author Professor Thomas Cody Prang of Texas A&M University, 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 Australopithecus sediba was a transitional form of ancient human relative and its spine is clearly intermediate in shape between those of modern humans and great apes — meaning that the species would have possessed both human and ape-like traits in its movements.