The lack of well-preserved spines from early human ancestors has prevented anthropologists from getting a clear picture of how our backs evolved. A new discovery has changed that, as much as any single fossil can.
Backpain is so common because our spines are ill-suited to the modern lifestyle, but where did we go wrong? Is the problem that we spend too much time sitting down, or have our spines not caught up with our new-fangled practice of walking upright? Although anatomists accept the problem has more to do with the former, we'd still like to know more about how our spines have adapted to us coming down from the trees.
The discovery of a 3.3-million-year-old skeleton from Dikika, Ethiopia, has filled in a little of this missing knowledge. The fossil of the 2.5-year-old child, described in Proceedings of the National Academy of Sciences, has been named Selam, meaning peace in Amharic.
Selam was found in 2000, but removing the encasing sandstone without damaging the precious bones has been a slow process. The authors note Selam represents “the only known complete hominin cervical and thoracic vertebral column before 60,000 years ago.” That's a lot of human evolution to be represented by one specimen. Moreover, Professor Zeresenay Alemseged of the University of Chicago, who found Selam, said in a statement: "This type of preservation is unprecedented, particularly in a young individual whose vertebrae are not yet fully fused."
Humans have 12 thoracic vertebrae, which form the middle section of the spinal column. Most African apes have 13. We compensate with extra vertebrae in the lower back, assisting our fully upright stance. One of the big questions about spinal evolution was which category Australopithecines fell into as they made the awkward shift from tree-dwellers. Selam, an Australopithecus afarensis, had 12 thoracic vertebrae, indicating this feature of human anatomy is very old.
Likewise Selam had an extended waist and more lower back vertebrae for easier running. "This structure and its modification through time is one of the key events in the history of human evolution," Alemseged said.
Even today, humans vary in our transition between the vertebrae of the lower and middle spine. In most people, it happens over 2-3 vertebrae, while a minority have a more sudden change. Considering this, a single specimen may not reveal the full diversity of our ancestors' spines. Nevertheless, three other fossil Australopithecus spines and a 1.6-million-year-old Homo erectus skeleton, while incomplete, show single vertebrae transitions, higher up than in most modern human spines. Salem fits this pattern, confirming that what is now a rarity evolved at least 3.3 million years ago, and was once widespread.