Numbers of Vertebrae in Hominoid Evolution

Vertebral formulae, the combination of regional numbers of vertebrae making up the bony spine, vary across vertebrates and within hominoid primates. Reconstructing the ancestral vertebral formulae throughout hominoid evolution has proved a challenge due to limited fossil evidence and disagreement among researchers. Proposed “long-backed” and “short-backed” ancestors have implications for the evolution of bipedalism and human evolutionary history generally. Here, we analyze a large dataset of hominoid vertebral formulae, including previously unstudied species and subspecies. We find more variation within and between species than expected, particularly in hylobatids (gibbons or lesser apes) and in gorilla and chimpanzee subspecies. Our results suggest that combined thoracic and lumbar numbers of vertebrae are somewhat phylogenetically structured, with outgroup taxa (two species of Old World monkeys, or cercopithecoids) retaining the primitive number of 19 thoracolumbar vertebrae, hylobatids generally possessing 18 thoracolumbar vertebrae, and hominids (great apes and humans) having 17 or 16 thoracolumbar vertebrae. When compared to cercopithecoids, and to putative stem hominoids, extant hominoids show evidence for homeotic change at both the lumbosacral (e.g., decrease in lumbar vertebrae; increase in sacral segments) and in the position of the transitional vertebrae. Homeotic changes are probably also responsible for the differences between African apes and modern humans, with differences in the number of thoracic and lumbar within a 17-segment thoracolumbar framework.