Placement of the diaphragmatic vertebra in catarrhines: Implications for the evolution of dorsostability in hominoids and bipedalism in hominins

Abstract

A fundamental adaptation to orthograde posture and locomotion amongst living hominoid primates is a numerically reduced lumbar column, which acts to stiffen the lower back and reduce injuries to the intervertebral discs. A related and functionally complementary strategy of spinal stability is a caudal position of the diaphragmatic vertebra relative to the primitive condition found in nonhominoid primates and most other mammals. The diaphragmatic vertebra marks the transition in vertebral articular facet (zygapophysis) orientation, which either resists (prediaphragmatic) or allows (postdiaphragmatic) trunk movement in the sagittal plane (i.e., flexion and extension). Unlike most mammals, which have dorsomobile spines (long lumbar columns and cranially placed diaphragmatic vertebrae) for running and leaping, hominoids possess dorsostable spines (short lumbar columns and caudally placed diaphragmatic vertebrae) adapted to orthogrady and antipronogrady. In contrast to humans and other extant hominoids, all known early hominin partial vertebral columns demonstrate cranial displacement of the diaphragmatic vertebra. To address this difference, variation in diaphragmatic placement is assessed in a large sample of catarrhine primates. I show that while hominoids are characterized by modal common placement of diaphragmatic and last rib‐bearing vertebrae in general, interspecific differences in intraspecific patterns of variation exist. In particular, humans and chimpanzees show nearly identical patterns of diaphragmatic placement. A scenario of hominin evolution is proposed in which early hominins evolved cranial displacement from the ancestral hominid condition of common placement to achieve effective lumbar lordosis during the evolution of bipedal locomotion. Am J Phys Anthropol, 2012. © 2012 Wiley Periodicals, Inc.

Evolution of the hominoid vertebral column: The long and the short of it

“…Sts 14 is the most complete South African early hominin vertebral column, preserving 15 consecutive thoracolumbar vertebrae and a partial sacrum . A vertebra at the thoracolumbar border (Sts 14f) is asymmetrical in that it bears a fully functional rib facet on one side (with an associated last rib) and a short, lumbar transverse process‐like structure on the left side . The latter process is unusual because it is not fully fused to the vertebra and bears a transverse foramen, which are morphologies often associated with short, ankylosed ribs.…”

“…In all extant hominoids, the transitional vertebra is modally located at the thoraco‐lumbar border, specifically at the level of the last rib‐bearing vertebra, whereas this transition occurs more cranially in the thoracic column in nonhominoid primates and many other mammals (Fig. ) . Sagittally oriented zygapophyses (medially facing superior and laterally facing inferior articular facets) constrain rotation and lateral bending, but may allow enhanced flexion and extension of the spine (but see Haeusler, Frater, and Bonneau).…”

“…Sagittally oriented zygapophyses (medially facing superior and laterally facing inferior articular facets) constrain rotation and lateral bending, but may allow enhanced flexion and extension of the spine (but see Haeusler, Frater, and Bonneau). On the other hand, flat, coronally oriented zygapophyses (dorsally facing superior and ventrally facing inferior articular facets) allow lateral bending and rotation, but provide some resistance to sagittal plan motion . Mammalian quadrupeds with spines that actively contribute to locomotion through sagittal plane flexion‐extension during running and leaping (like many primates, rodents, and carnivorans) possess cranially positioned transitional vertebrae, and thus long post‐transitional spines, extending the region over which sagittal movements can occur .…”

“…Mammalian quadrupeds with spines that actively contribute to locomotion through sagittal plane flexion‐extension during running and leaping (like many primates, rodents, and carnivorans) possess cranially positioned transitional vertebrae, and thus long post‐transitional spines, extending the region over which sagittal movements can occur . Mammals characterized by stiff‐backed running (such as perissodactyls and large‐bodied artiodactyls) or other locomotor modes that require rigid lower backs (such as suspension and ape‐like vertical climbing) have more caudally positioned transitional vertebrae and, accordingly, short post‐transitional regions that reduce the length of the column over which bending moments can occur . Running and leaping mammals may also benefit from the rotational stability that a cranially positioned transitional vertebra provides over a long lower back, whereas rotation and lateral bending of the spine occur during and may facilitate forelimb‐dominated locomotor behaviors…”

“…The transitional vertebra (bearing flat, coronally oriented prezygapophyses and curved, more sagittally oriented postzygapophyses) is the tenth thoracic vertebra (T10) in Macaca in a series of 13 thoracic vertebrae; it is T12, the last thoracic vertebra, in Homo . Therefore, the former demonstrates cranial displacement of the transitional vertebra relative to the last rib‐bearing vertebra by three elements, while the latter shows common placement of transitional and last rib‐bearing vertebrae . Coronally oriented zygapophyses are highlighted with straight dashed lines and arrows and sagittally oriented zygapophyses are indicated with dashed circles and curved arrows.…”