Conserved patterns and locomotor-related evolutionary constraints in the hominoid vertebral column

The evolution of the hominoid lineage is characterized by pervasive
homoplasy, notably in regions such as the vertebral column, which plays a
central role in body support and locomotion. Few isolated and fewer
associated vertebrae are known for most fossil hominoid taxa, but
identified specimens indicate potentially high levels of convergence in
terms of both form and number. Homoplasy thus complicates attempts to
identify the anatomy of the last common ancestor of hominins and other
taxa and stymies reconstructions of evolutionary scenarios. One way to
clarify the role of homoplasy is by investigating constraints via
phenotypic integration, which assesses covariation among traits, shapes
evolutionary pathways, and itself evolves in response to selection. We
assessed phenotypic integration and evolvability across the subaxial
(cervical, thoracic, lumbar, sacral) vertebral column of macaques (n =
96), gibbons (n = 77), chimpanzees (n = 92), and modern humans (n = 151).
We found a mid-cervical cluster that may have shifted cranially in
hominoids, a persistent thoracic cluster that is most marked in
chimpanzees, and an expanded lumbosacral cluster in hominoids that is most
expanded in gibbons. Our results highlight the highly conserved nature of
the vertebral column. Taxa appear to exploit existing patterns of
integration and ontogenetic processes to shift, expand, or reduce cluster
boundaries. Gibbons appear to be the most highly derived taxon in our
sample, possibly in response to their highly specialized locomotion.