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...

One contribution of 17 to a discussion meeting issue 'Major transitions in human evolution'. Homo erectus was the first hominin to exhibit extensive range expansion. This extraordinary departure from Africa, especially into more temperate climates of Eurasia, has been variously related to technological, energetic and foraging shifts. The temporal and regional anatomical variation in H. erectus suggests that a high level of developmental plasticity, a key factor in the ability of H. sapiens to occupy a variety of habitats, may also have been present in H. erectus. Developmental plasticity, the ability to modify development in response to environmental conditions, results in differences in size, shape and dimorphism across populations that relate in part to levels of resource sufficiency and extrinsic mortality. These differences predict not only regional variations but also overall smaller adult sizes and lower levels of dimorphism in instances of resource scarcity and high predator load. We consider the metric variation in 35 human and non-human primate 'populations' from known environmental contexts and 14 time-and space-restricted paleodemes of H. erectus and other fossil Homo. Human and non-human primates exhibit more similar patterns of variation than expected, with plasticity evident, but in differing patterns by sex across populations. The fossil samples show less evidence of variation than expected, although H. erectus varies more than Neandertals.This article is part of the themed issue 'Major transitions in human evolution'.

Orientation of the iliac blades is a key feature that appears to distinguish extant apes from monkeys. Iliac morphology is hypothesized to reflect variation in thoracic shape that, in turn, reflects adaptations for shoulder and forearm function in anthropoids. Iliac orientation is traditionally measured relative to the acetabulum, whereas functional explanations pertain to its orientation relative to the cardinal anatomical planes. We investigated iliac orientation relative to a median plane using digital models of hipbones registered to landmark data from articulated pelves. We fit planes to the iliac surfaces, midline, and acetabulum, and investigated linear metrics that characterize geometric relationships of the iliac margins. Our results demonstrate that extant hominoid ilia are not rotated into a coronal plane from a more sagittal position in basal apes and monkeys but that the apparent rotation is the result of geometric changes within the ilia. The whole ilium and its gluteal surface are more coronally oriented in apes, but apes and monkeys do not differ in orientation of the iliac fossa. The angular differences in the whole blade and gluteal surface primarily reflect a narrower iliac tuberosity set closer to the midline in extant apes, reflecting a decrease in erector spinae muscle mass associated with stiffening of the lumbar spine. Mediolateral breadth across the ventral dorsal iliac spines is only slightly greater in extant apes than in monkeys. These results demonstrate that spinal musculature and mobility have a more significant effect on pelvic morphology than does shoulder orientation, as had been previously hypothesized. Anat Rec, 300:810-827, 2017. © 2017 Wiley Periodicals, Inc.