Archaeologists revealed the spine of a young Australopithecus afarensis, a hominin who died some 3 million years ago in what is today Ethiopia, being 2.5 years old at the time of death.
The skeleton of this small female A. aferensis was discovered in Dikika, Ethiopia, in 2000. Since then, researchers have been painstakingly chipping her bones out of hard sandstone, trying not to damage them. They’ve already learned that this 3.3-million-year-old human ancestor was bipedal but also climbed trees. The Australopithecus was nicknamed “Selam,” for the Amharic word for “peace“. The remains show a markedly different transition between the upper and lower back, one that may have given a boost for bipedal walking. Before the find, it was not known whether earliest ancestors of modern humans had the same pattern and the same numbers of vertebrae. Modern apes, chimpanzees and gorillas have 13 pairs of ribs compared with modern humans’ 12. Modern humans also have lower backs that are longer than those of other great apes. Humanity’s more flexible lower backs are more suitable for upright walking. Selam’s vertebrae are each only about 1.2 centimetres across. X-ray of the fossil revealed a a spine with 12 ribs and 12 thoracic vertebrae, just like the spine of modern humans.
Vertebrae and ribs are small, delicate bones that don’t preserve well in the fossil record. A few partial skeletons of Australopithecus aferensis, Australopithecus africanus, Australopithecus sediba and Homo erectus have provided some hints as to what hominin backbones looked like, but were so fragmentary that researchers haven’t been sure how many vertebrae made up their upper back. The difference between the modern human is in the thoracolumbar transition, or the anatomical changes in the vertebrae from the upper to lower back. These changes occur at the facet joints, where ligaments that allow for flexion and rotation attach the bones together. In modern humans, these facet joints subtly change shape and orientation at the 12th thoracic vertebrae, the lowest one that links up with a rib. In Australopithecus afarensis this anatomical change happened at the 11th thoracic vertebrae, the one above the last rib-bearing bone. This is the exact same pattern seen in the few other early hominin partial backbones that have been preserved.
(after Zeresenay Alemseged, University of Chicago, Live Science & Heritage Daily)