Vertebral Morphology in Hominoids II: The Lumbar Spine

Abstract

The evolution of hominoids was accompanied by a transformation of the primate body plan from a monkey-like ancestral condition to one characterized by a derived suite of postcranial features. While diagnostic hominoid features are found throughout the postcranial skeleton, the trunk, and especially the lumbar region, is one of the most functionally important and immediately noticeable aspects of hominoid anatomy. Hominoids have a reduced number of lumbar vertebrae that are generally distinguished from those of their cercopithecoid close relatives with respect to dimensions of lumbar vertebral bodies and pedicles, position and orientation of transverse processes, shape and orientation of spinous processes, and orientation of zygapophyses at the thoracolumbar transition. Traditional functional interpretations emphasize that these features stiffen the lower body and limit sagittal mobility during orthogrady and/or forelimb-dominated behaviors. More recent comparative research on nonhominoid primates and other mammals support that hominoid lumbar features confer axial stability in a variety of positional behaviors, while studies of experimental biomechanics have revealed more rotational capabilities in the hominoid trunk than previously thought, and analyses of back musculature offer new information about fiber-type differences between hominoids and other primates. Multiple models concerning reconstruction of the trunk morphology and behavior of the crown hominoid ancestor, and the sequence in which aspects of the hominoid trunk evolved, have surfaced over the last decade. Given the numerous chapters in this volume devoted to human spinal morphology and adaptations to bipedalism, our discussion is focused primarily on lumbar anatomy shared by all hominoids.