A comparative morphometric study of the hominoid lumbar spine
This thesis investigates the size/shape variation in the lumbar spine of extant and fossil hominoids. As a novelty, 3D coordinate data sets were obtained from the last five consecutive presacral vertebrae for comparative analyses. Size/shape variation of single vertebrae and patterns of metameric size/shape variation along the lumbar spine are investigated. Large samples of populations of Homo sapiens, Gorilla gorilla, Pan troglodytes, and Pongo pygmaeus are investigated. The fossil sample includes Australopithecus afarensis, Australopithecus africanus, and Homo ergaster. Statistical shape analysis was conducted using geometric morphometric methods. Comparison 1 (Chapter III) explores sexual dimorphism in size and shape within each of the modern taxa. Results indicate that Pan shows neither vertebral size nor shape dimorphism. Gorilla, and probably Pongo (small sample size) are highly sexually dimorphic in size and less so in shape. Homo sapiens shows less size dimorphism than Gorilla but a markedly larger shape dimorphism. Thus despite sexual dimorphism of locomotor repertoires within great apes there are few indications of these in vertebral shape. In contrast, humans with no dimorphism in locomotor repertoire, show shape dimorphism in the lumbar spine related to sexual differences in pelvic shape and consequent differences in bipedal kinematics. Comparison 2 (Chapter IV) investigates inter-specific size/shape variations between extant hominoids. It is found that differences in shape between the taxa corroborate the functional relationships already described in the literature. Further, the differences between the taxa in shape are not congruent with the consensus molecular phylogeny. Comparison 3 (Chapter V) focuses on the fossils. Results indicate that A. africanus and A. afarensis lumbar vertebrae are most similar to each other. In comparison with modern taxa, they are most similar in shape to those of modern humans and less so to great apes. Homo ergaster falls within the range of modern humans. This thesis concludes that lumbar vertebral morphology shows interesting intra-specific patterns of scaling and of sexual dimorphism that appear to vary according to function between apes and between apes and humans. The australopithecines show similarities in shape with modern humans, indicating that despite inter-specific differences in pelvic shape, there are key adaptations in the lumbar spine which guarantee an energetically efficient bipedalism that was developed as early as 3 million years ago in the hominin lineage. However, humans and australopithecines differ in that the latter show no adaptations in the vertebral column to bipedal endurance running. Chapter VI concludes with a protocol for the analysis of future fossil vertebral discoveries.