This is a study of the numerical composition of the vertebral column, the central structure of the vertebrate body plan and one that plays an instrumental role in locomotion and posture. Recent models of hominoid evolution invoke very different roles for homology (similarity by common descent) and homoplasy (similarity by convergence or parallelism) in the evolution of living and extinct hominoid primates. The recently described remarkable skeleton of Ardipithecus ramidus was interpreted in support of extensive homoplasy in hominoid postcranial evolution. Here, hominoid vertebral formulae (regional numbers of vertebrae) are placed in a large phylogenetic context of over 8,000 mammal specimens, representing all major groups and focusing on anthropoid primates. This survey, in combination with analyses of intraspecific diversity and interspecific similarity, suggests that reduced lumbar regions are homologous in extant hominoids. Furthermore, it is proposed that bipedalism evolved in a partly arboreal, partly terrestrial African ape-like locomotor context.