Evolution of the hominoid vertebral column.

摘要

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 vertebral evolution invoke very different roles for homology and homoplasy in the evolution of vertebral formulae in living and extinct hominoids. These processes are fundamental to the emergence of morphological structures and reflect similarity by common descent (homology) or similarity by independent evolution (homoplasy). Although the "short backs," reflecting reduced lumbar regions, of living hominoids have traditionally been interpreted as homologies and shared derived characters (synapomorphies) of the ape and human clade, recent studies of variation in extant hominoid vertebral formulae have challenged this hypothesis. Instead, a "long-back" model, in which primitive, long lumbar regions are retained throughout hominoid evolution and are reduced independently in six lineages of modern hominoids, is proposed. The recently described skeleton of Ardipithecus ramidus is interpreted to support the long-back model. Here, larger samples are collected and placed in a larger phylogenetic context than previous studies. Analyses of over 8,000 mammal specimens, representing all major groups and focusing on anthropoid primates, allow for the reconstruction of ancestral vertebral formulae throughout mammalian evolution and a determination of the uniqueness of hominoid vertebral formulae. This survey, in combination with analyses of intraspecific diversity and interspecific similarity, suggests that reduced lumbar regions are homologous in extant hominoids. Furthermore, hominoid vertebral formulae are unique among primates and relatively unique among mammals in general. Hominins likely evolved five lumbar vertebrae from a short-backed ancestor with an "African ape-like" vertebral profile. By the appearance of Australopithecus, hominins evolved a cranial placement of the diaphragmatic (one that bears a change in articular facet orientation) vertebra, which generates a functionally longer lower spine while maintaining five lumbar vertebrae. In light of these findings, it is proposed that bipedalism evolved in a party arboreal, partly terrestrial African ape-like locomotor context.