Ontogeny of robusticity of craniofacial traits in modern humans: A study of South American populations

Abstract

To date, differences in craniofacial robusticity among modern and fossil humans have been primarily addressed by analyzing adult individuals; thus, the developmental basis of such differentiation remains poorly understood. This article aims to analyze the ontogenetic development of craniofacial robusticity in human populations from South America. Geometric morphometric methods were used to describe cranial traits in lateral view by using landmarks and semilandmarks. We compare the patterns of variation among populations obtained with subadults and adults to determine whether population-specific differences are evident at early postnatal ontogeny, compare ontogenetic allometric trajectories to ascertain whether changes in the ontogeny of shape contribute to the differentiation of adult morphologies, and estimate the amount of size change that occurs during growth along each population-specific trajectory. The results obtained indicate that the pattern of interpopulation variation in shape and size is already established at the age of 5 years, meaning that processes acting early during ontogeny contribute to the adult variation. The ontogenetic allometric trajectories are not parallel among all samples, suggesting the divergence in the size-related shape changes. Finally, the extension of ontogenetic trajectories also seems to contribute to shape variation observed among adults. Fossil species of the genus Homo are characterized by high levels of craniofacial robusticity, which is defined by a diverse array of morphological traits. Robusticity is attributed to crania that possess large superstructures such as brow ridges, mastoid processes, occipital torus, and nuchal crest (Lahr and Wright, 1996), large external dimensions, thick vault bones (Lieberman, 1996), or rugged muscle markings. There is a marked trend toward a reduction in robusticity in several regions of the world, and most modern humans have gracile crania. However, some hunter-gatherer populations from continental Patagonia and Tierra del Fuego (southern South America) as recent as late Holocene (ca. 3,000 years 14 C B.P. to historical times), as well as Australian aborigines, display numerous robust traits that depart from the majority of modern populations (Lahr, 1995;Lahr and Wright, 1996;Lalueza et al., 1997;Sardi, 2002;Bernal et al., 2006;Curnoe and Thorne, 2006). Their crania have been identified as the most robust of any modern population due to the pronounced development of supraorbital ridges, glabellar regions and supercilliary arches (Lahr and Wright, 1996;Bernal et al., 2006).The patterns of craniofacial robusticity in modern and fossil humans have been mainly assessed by analyzing adult individuals, and thus the developmental basis of such differentiation remains untested. Because morphological variation within and between populations appears during ontogeny, the study of ontogenetic series could provide important clues regarding the kinds of changes responsible for size and shape patterns observed among adult crania. The role of ...

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“…The results of this study seem to contrast those obtained in a previous study aimed at analysing the ontogenetic development of cranial robusticity in human populations from South America (Gonzalez et al,2010). In that study we evaluated the ontogenetic allometric trajectories of a subset of cranial traits commonly used to compare the level of robusticity and found that their trajectories were not parallel among all samples, suggesting a divergence in size‐related shape changes.…”

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“…Nevertheless, the masticatory regions have been found to be equally reliable for inferring population history patterns (von Cramon-Taubadel, 2009a). Although environmental plasticity and adaptation to diet can be difficult to disentangle (O'Higgins et al, 2006), it has been shown in humans that typical diet related morphology of the mandible can already be traced in children before they start on their adult foods (Fukase and Suwa, 2008) and that population differentiation in craniofacial shape is already detectable at an early ontogenetic stage (e.g., Viðarsd ottir et al, 2002;Viðarsd ottir and Cobb, 2004;Gonzalez et al, 2010).…”