Abstract
Functional morphology of the atlas reflects multiple aspects of an organism’s biology. More specifically, its shape indicates patterns of head mobility, while the size of its vascular foramina reflects blood flow to the brain. Anatomy and function of the early hominin atlas, and thus, its evolutionary history, are poorly documented because of a paucity of fossilized material. Meticulous excavation, cleaning and high-resolution micro-CT scanning of the StW 573 (‘Little Foot’) skull has revealed the most complete early hominin atlas yet found, having been cemented by breccia in its displaced and flipped over position on the cranial base anterolateral to the foramen magnum. Description and landmark-free morphometric analyses of the StW 573 atlas, along with other less complete hominin atlases from Sterkfontein (StW 679) and Hadar (AL 333-83), confirm the presence of an arboreal component in the positional repertoire of Australopithecus. Finally, assessment of the cross-sectional areas of the transverse foramina of the atlas and the left carotid canal in StW 573 further suggests there may have been lower metabolic costs for cerebral tissues in this hominin than have been attributed to extant humans and may support the idea that blood perfusion of these tissues increased over the course of hominin evolution.
Highlights
Functional morphology of the atlas reflects multiple aspects of an organism’s biology
Recent studies have revealed that the cross-sectional area of the transverse foramina in the cervical spine could be used as a reliable proxy for estimating blood flow and brain metabolism in euarchontans, especially when combined with an assessment of dimensions of the carotid foramina[24]
Through comparative study of these two fossils, we quantitatively investigate the shape of the hominin atlas, with a particular emphasis on the articular facets, and comment on the selective pressures that may have been operating on head mobility in Australopithecus
Summary
Functional morphology of the atlas reflects multiple aspects of an organism’s biology. Combined analysis of the superior and inferior articular facets has the potential to provide essential information about the range of head motions in Australopithecus, which has only been superficially assessed due to the scarcity and fragmentary nature of cervical vertebrae in the hominin fossil record[9,10]. A non-human hominoid-like atlas in Australopithecus (e.g., concave superior and inferior articular facets) might suggest a range of head motions that differs from humans[6,7,11,12], indicating that arboreal activities were fundamental components of the postural and locomotor repertoire of Australopithecus. Assessments of total encephalic arterial flow (i.e., in arteries running through the cervical vertebrae and the carotid canal) could help in estimating blood flow and potential variation in brain energetic demands characterizing early hominins, and could confirm (or not) a relatively recent emergence of the human-like metabolic pattern in the hominin lineage (i.e., having a brain that accounts for a relatively high percentage of the estimated basal metabolic rate)[20,24]
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