Spinal Evolution PDF Download

The thoracolumbar spine is crucial for understanding primate evolution and the origins and unique adoption of human bipedalism. Both great apes and humans have stabilized their spine with the posterior shift of the transitional vertebra and reduction of non-ribbed lumbar vertebra. There is debate regarding whether these features are homologous or homoplasious, and thus whether bipedalism evolved from a short stiff back like great apes or a longer lumbar column more similar to monkeys and Miocene hominoids. Genetic modification of Hox9 in mice results in the independent modification of rib count and transitional vertebra placemen and genetic modification of Hox11 in mice results in a cranial homeotic shift at the lumbosacral border and position of the sacrum. These transitions mimic the shift of the transitional vertebra and lumbosacral boundary in hominoids. The proposed study addresses important questions that will influence interpretation of differences between extant primates, as well as fossil specimens, by providing a basic understanding for the role of developmental influences on functionally important vertebral features. Since skeletal development is a highly conserved process across tetrapods, conclusions about the development of vertebrae in mice will be broadly applicable to interpretations across mammals. In this analysis, I compare the morphology of the primate transitional vertebra in museum collections and experimentally modified mice. Quantitative, qualitative, and geometric morphometric analyses were conducted on Hox9 and Hox11 modified mice and primate museum specimen thoracic and lumbar columns using micro-CT and 3D surface scanning and a large osteological dataset. In the first part of this dissertation, I analyzed Hox9 and Hox11 modified mice to investigate developmental association or independence of vertebral characters including articular facets, spinous processes, transverse processes, and costal processes. The results of this study indicated that the different Hox clusters have distinct roles on different morphology and in different areas of the vertebral column. Hoxa9, -b9, -c9 and -d9 paralogs had specific and largely separate roles in specifying vertebral identity. Hoxa9 mutations in mice impacted rib placement and count. Hoxb9 and Hoxc9 modifications in isolation affected the placement of the transitional vertebra and the position of spinous process orientation change. Hoxd9 modifications instead affected the sacrocaudal boundary and the number of laterally fused segments contributing of the sacrum. Hoxd11 and combined modifications of Hoxa11 and Hoxd11 resulted in a partial or complete lumbosacral transformation and an anterior shift of the placement of the sacrum within the pelvis. The results of this study indicate that articular facet orientation can be altered independent of costal identity and that articular facet orientation and spinous process orientation shift together whereas transverse process orientation shifts once with the transitional vertebra and a second time over the rib boundary. In the second part of this dissertation, I analyzed a 3D dataset of primate vertebrae to identify characters associated with vertebral column mobility and stiffening. I scanned primate vertebral columns from the 8th thoracic to the 3rd lumbar using either a NextEngine 3D surface scanner or an Artec Space Spider Surface Scanner. From the scans, the vertebrae were scored for thoracic and lumbar characteristics and the angles of the spinous processes were measured to assess spinous process shape to determine which vertebral characters are linked over the articular facet and costal transitions. Additionally, I investigated whether lumbar and sacral count affects sacral position and vertebral entrapment. These data were compared to the Hox mouse models to assess the developmental independence of transitional and costal characters in primates. The results of this study showed that like the Hox mice, articular facet orientation and spinous process orientation were correlated and that there was a shift in transverse process orientation both at the articular facet transition and the costal transition, when they were not in unison. This demonstrates that classic Schultz definitions may not correspond to the developmental boundaries that determine vertebral morphology. Furthermore, humans and chimpanzees displayed a similar cranial shift in sacrum placement to the mice when lumbar count was increased. This has implications for spinal mobility and demonstrates an additional method for either reducing or increasing lumbar entrapment and changing the number of lumbar vertebra contributing to locomotion. In the third part of this dissertation, I analyzed the 3D primate dataset using geometric morphometrics to assess differences between pre- and post-transitional vertebrae within the context of hominoid evolution. The dataset is divided into analyses of the entire section of the vertebral column sampled (T8L2), individual analyses of the transitional vertebra and first two post-transitional vertebrae, individual species analyses of the vertebral column, and finally individual species analyses of individual articular facet surfaces independent of orientation. Overall, differences were seen in shape and orientation across the transitional vertebra between species and this often correlated with locomotor pattern and where mobility or stiffening was needed in the spine. These projects address two important questions: 1) whether patterns of the thoracolumbar and lumbosacral transitions are similar across apes, and 2) the developmental independence of various thoracic, lumbar, and sacral vertebral features. Throughout hominid evolution there has been a posterior shift in the position of the transitional vertebra and there has been an alteration in the placement of the sacrum relative to the iliac crest. The results of these studies show that alteration of Hox genes in mice can result in similar patterns seen in primates and may underlie the anatomical trajectory seen in hominid evolution. Furthermore, the differences in the transition of vertebral characters among great apes suggests that reduction and stiffening of the lumbar column may have evolved independently.