The vertebral formula of the last common ancestor of African apes and humans

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 "short-backed LCA" scenario has recently been viewed favorably in an analysis of the intra- and interspecific variation in axial formulas observed among African apes and humans (Pilbeam, 2004. J Exp Zool 302B:241-267). 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 (pre-coccygeal) 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 six-segmented 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.

Etiology of lumbar lordosis and its pathophysiology: a review of the evolution of lumbar lordosis, and the mechanics and biology of lumbar degeneration

“…Skeletons of Australopithecus showed lumbar spines that likely comprised 6 lumbar vertebrae. 117,128,203,207 In the subsequent course of evolution the human lumbar spine was reduced by 1 vertebra when that vertebra was captured in the pelvic girdle and became part of the sacrum. In a review of the Galloway Osteological Collection (Kampala, Uganda), 4% of modern humans continue to have a sixth lumbar vertebra.…”

“…128 The short lumbar spines of great apes are very stiff and have limited flexion. 201 This differential evolution of human and old world primate lumbar spines suggests a functional purpose for 5 lumbar vertebrae in humans, not an evolutionary failure.…”

Spinopelvic pathways to bipedality: why no hominids ever relied on a bent-hip–bent-knee gait

“…African apes are unable to lordose their lumbar spines, and therefore must flex both the hip and knee joints in order to position their centre of mass over the point of ground contact (Lovejoy 2005a). Lumbar immobility in Pan and Gorilla is a consequence of their possession of only three to four lumbar vertebrae and the 'entrapment' of the most caudal lumbar vertebra(e) between cranially extended ilia (Stevens 2004;Stevens & Lovejoy 2004;Lovejoy 2005a;McCollum et al 2009). Although all three African ape species share these features, there is now considerable evidence indicating that they have not been retained from the common ancestor shared with the human clade.…”

An evolutionary hypothesis to explain the role of deconditioning in low back pain prevalence in humans

“…Larger bodied semi-terrestrial species including extant great apes and dryopithecines instead have a shorter lumbar spine of 3 to 4 lumbar vertebrae [144,[150][151][152]. The transition to bipedalism was accompanied by a lumbar spine consisting of 5 to 6 vertebrae, though there is some disagreement as to whether this constituted a lengthening from shorter backed species or a shortening from longer backed arboreal quadrupeds [133,150,[152][153][154][155][156][157][158][159][160][161][162][163][164]. Nevertheless these anatomical arrangements of vertebral number were also accompanied by structural changes to the vertebral bodies themselves.…”

“…For example there is some disagreement regarding the stages of locomotion presented [131][132][133] with this debate centring on whether the last common ancestor (LCA) of modern humans and the great apes walked with a bent-hip-bent-knee gait when bipedal and whether their lumbar spine was long and flexible (like OWMs) or short and stable (like in the great apes) [133,150,[152][153][154][155][156][157][158][159][160][161][162][163][164]. It is beyond the scope of this article to examine this area of debate but it should be noted that the final circumstance, a long flexible yet unstable lumbar spine in humans being related to LBP, can be supported irrespective of whether an intermediary involving a short back LCA stage in locomotor evolution and its corresponding anatomical evolution occurred [147,150].…”