Emergence and optimization of upright posture among hominiform hominoids and the evolutionary pathophysiology of back pain.
The lordotic region of the lumbar spine is a significant focus of pain and dysfunction in the human body, and its susceptibility to disorders may reflect its substantial reconfiguration during the course of human evolution. The basic anatomy of the lumbar vertebra in Old World Monkeys and Early Miocene apes, or proconsulids, retains typical mammalian architecture. The lumbar vertebra in humans is different in the repositioning of the lumbar transverse process dorsal to the vertebral body rather than originating on the body itself and in the loss of the styloid process that is adjacent to the facets in other primates. These two features appeared in Morotopithecus bishopi 21.6 million years ago, suggesting that this ape is the founder of an upright hominiform lineage. The iliocostalis lumborum muscles migrated onto the iliac crest approximately 18 million years ago, becoming a powerful lateral flexor muscle of the trunk. The posterior superior iliac spine shifted far dorsal to the longissimus insertion in the genus Homo between 1 and 2 million years ago, making this muscle a powerful extensor of the lumbar spine. Functionally, the establishment of strong muscular flexors and extensors adds dynamic compressive stresses to the lumbar disks and also makes these muscles susceptible to strain.