Abstract
The modal number of lumbar vertebrae in modern humans is five. It varies between three and four in extant African apes (mean=3.5). Because both chimpanzees (Pan troglodytes) and gorillas (Gorilla gorilla) possess the same distributions of thoracic, lumbar, and sacral vertebrae, it has been assumed from parsimony that the last common ancestor (LCA) of African apes and humans possessed a similarly short lower back. This βshortβbacked LCAβ scenario has recently been viewed favorably in an analysis of the intraβ and interspecific variation in axial formulas observed among African apes and humans (Pilbeam, 2004. J Exp Zool 302B:241β267). However, the number of bonobo (Pan paniscus) specimens in that study was small (N=17). Here we reconsider vertebral type and number in the LCA in light of an expanded P. paniscus sample as well as evidence provided by the human fossil record. The precaudal (preβcoccygeal) axial column of bonobos differs from those of chimpanzees and gorillas in displaying one additional vertebra as well as significantly different combinations of sacral, lumbar, and thoracic vertebrae. These findings, along with the sixβsegmented lumbar column of early Australopithecus and early Homo, suggest that the LCA possessed a long axial column and long lumbar spine and that reduction in the lumbar column occurred independently in humans and in each ape clade, and continued after separation of the two species of Pan as well. Such an explanation is strongly congruent with additional details of lumbar column reduction and lower back stabilization in African apes. J. Exp. Zool. (Mol. Dev. Evol.) 314B:123β134, 2010. Β© 2009 WileyβLiss, Inc. [This article was published online on 17 August 2009. An error was subsequently identified and the article was corrected on 8 September 2009.]