Vertebral numbers and human evolution

Abstract

Ever since Tyson (1699), anatomists have noted and compared differences in the regional numbers of vertebrae among humans and other hominoids. Subsequent workers interpreted these differences in phylogenetic, functional, and behavioral frameworks and speculated on the history of vertebral numbers during human evolution. Even in a modern phylogenetic framework and with greatly expanded sample sizes of hominoid species, researchers' conclusions vary drastically, positing that hominins evolved from either a "long-backed" (numerically long lumbar column) or a "short-backed" (numerically short lumbar column) ancestor. We show that these disparate interpretations are due in part to the use of different criteria for what defines a lumbar vertebra, but argue that, regardless of which lumbar definition is used, hominins are similar to their great ape relatives in possessing a short trunk, a rare occurrence in mammals and one that defines the clade Hominoidea. Furthermore, we address the recent claim that the early hominin thoracolumbar configuration is not distinct from that of modern humans and conclude that early hominins show evidence of "cranial shifting," which might explain the anomalous morphology of several early hominin fossils. Finally, we evaluate the competing hypotheses on numbers of vertebrae and argue that the current data support a hominin ancestor with an African ape-like short trunk and lower back. Am J Phys Anthropol 159:S19-S36, V C 2016 American Association of Physical AnthropologistsThe vertebral column is a defining feature of the Vertebrata. It facilitates locomotion via lateral bending in many fish, amphibians, and reptiles, and sagittal bending (flexion and extension) in many mammals (Howell, 1944;Slijper, 1946;Hildebrand, 1974;Schilling and Hackert, 2006). In some mammals (with "dorsostable," as opposed to "dorsomobile" vertebrae; Gambaryan, 1974), the vertebral column may restrict spinal mobility via various mechanisms, including stabilizing modifications of the centrum or neural arch structures (e.g., tall spinous processes, dorsal origin and orientation of the lumbar transverse processes), pelvic involvement, trunk (epaxial and hypaxial) muscle reorganization, and reduction in the number of free-moving vertebrae (Slijper, 1946;Richter, 1970;Gambaryan, 1974;Filler, 1986;Halpert et al., 1987;Sanders, 1995;Lovejoy and McCollum, 2010;Williams, 2012b;Galis et al., 2014). Hominoid primates are dorsostable mammals that have shortened the trunk. Further stiffening of the lower back in Hominoidea has evolved via flattening the thoracic articular facet joints, restricting extension through changes in spinous processes shape and angulation, entrapping the lumbar vertebrae between extended iliac blades, and by reducing and reorganizing the epaxial muscles from their primary role as powerful, strap-like extensors to that of stabilizers (Slijper, 1946;Jungers, 1984;Ward, 1993;Sanders, 1995;Lovejoy and McCollum, 2010;Williams and Russo, 2015). The non-human great apes (gorillas, chimpanzees, bonobo...

New fossil remains of Homo naledi from the Lesedi Chamber, South Africa

“…Overall, H. naledi resembles more primitive species of Homo such as H. erectus, H. habilis, or H. rudolfensis much more than it resembles archaic or modern humans (Berger et al, 2015; Laird et al, 2017; Marchi et al, 2017; Feuerriegel et al, 2017; Williams et al, 2016; Schroeder et al, 2017). H. naledi does, however, possess a number of derived features that are otherwise known only from modern humans and Neandertals (Supplementary file 1; Figures 34 and 35; Berger et al, 2015; Dembo et al, 2016; Kivell et al, 2015; Harcourt-Smith et al, 2015; Williams et al, 2016). Some of these derived features, including features of the wrist, cannot be assessed in H. erectus because no fossils of the relevant bones exist for this species (Kivell et al, 2015).…”

Neck function in early hominins and suspensory primates: Insights from the uncinate process

“…Unique commonalities in axial, shoulder, torso and forelimb morphology in extant apes and Ateles suggests they may be best explained by shared forelimb suspensory positional behaviors (Benton, ; Erikson, ; Gebo, ; Lockwood, ). Similarly, commonalities in spinal configuration have long been known to link atelids and hominoids, as Benton () classified these two groups together as “short‐backed primates,” whereas all other primates were described as “long‐backed” (see Williams, Middleton, Villamil, & Shattuck, for discussion). Although supplemented by the prehensile tail, Ateles engages in suspensory positions in up to 47% of postural behavior, and consistently demonstrates effective brachiation with brief bouts of free‐flight and pendular motion featuring long, fluid strides (Mittermeier, ).…”

Testing biomechanical models of human lumbar lordosis variability

“…The human lumbar spine exhibits several adaptations to cope with the biomechanical demands of bipedal locomotion and orthograde posture. Compared with closely related living species, the human lumbar spine is relatively longer and contains more vertebral elements with a modal pattern of five functional lumbar vertebrae compared to three or four in most great apes (Pilbeam, ; Williams et al, ). Elongation of the human lumbar spine provides more trunk mobility, aided in part by having a greater number of vertebrae with sagittally oriented zygapophyses (rather than coronally oriented zygapophyses in the thoracic spine) and reduced “entrapment” of the caudal‐most lumbar vertebrae between the iliac blades, a feature present in great apes (Lovejoy and McCollum, ; Lovejoy, ; Pal and Routal, ; Russo, ; Shapiro, ; Whitcome, ; Williams, ).…”