Supraorbital morphology and social dynamics in human evolution

Ricardo Miguel Godinho,Paul O’Higgins,Penny Spikins

doi:10.1038/s41559-018-0528-0

Ricardo Miguel Godinho, Paul O’Higgins + Show 1 more

Open Access

https://doi.org/10.1038/s41559-018-0528-0

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Abstract

Uniquely, with respect to Middle Pleistocene hominins, anatomically modern humans do not possess marked browridges, and have a more vertical forehead with mobile eyebrows that play a key role in social signalling and communication. The presence and variability of browridges in archaic Homo species and their absence in ourselves have led to debate concerning their morphogenesis and function, with two main hypotheses being put forward: that browridge morphology is the result of the spatial relationship between the orbits and the brain case; and that browridge morphology is significantly impacted by biting mechanics. Here, we virtually manipulate the browridge morphology of an archaic hominin (Kabwe 1), showing that it is much larger than the minimum required to fulfil spatial demands and that browridge size has little impact on mechanical performance during biting. As browridge morphology in this fossil is not driven by spatial and mechanical requirements alone, the role of the supraorbital region in social communication is a potentially significant factor. We propose that conversion of the large browridges of our immediate ancestors to a more vertical frontal bone in modern humans allowed highly mobile eyebrows to display subtle affiliative emotions.

Highlights

With respect to Middle Pleistocene hominins, anatomically modern humans do not possess marked browridges, and have a more vertical forehead with mobile eyebrows that play a key role in social signalling and communication
Through virtual modelling and manipulation of the Kabwe 1 cranium, we show that the browridge is much larger than the minimum size required to accommodate the disjunction between the orbits and frontal bone
Visual examination of strains experienced by the cranium indicates a slight increase in the strain magnitudes experienced by the lateral margins of the ridges and over the frontal bone with decreasing browridge size

Summary

Introduction

With respect to Middle Pleistocene hominins, anatomically modern humans do not possess marked browridges, and have a more vertical forehead with mobile eyebrows that play a key role in social signalling and communication. This association between cranial gracilization, prosociality and self-domestication has been hypothesized for bonobos who, relative to chimpanzees, present a gracile cranium[13] with smaller browridges[14] and prosocial behaviour, and are hypothesized as self-domesticated[11,12] This suggests a selective trade-off between expressing dynamic affiliative signals and permanent competitive signals that affects the shape and size of the cranium in general and the browridge in particular. More affiliative-based social relationships in bonobos, with frequent consolation[15], are associated with both a reduced browridge and greater attention to the eye area in social communication[16] than in common chimpanzees Despite this association, it should be noted that bonobos are significantly smaller than chimpanzees[13] and that, as predicted by the allometric hypothesis[6], browridges are expected to be proportionally smaller. We note that its vector of action changes to be more vertical and this means the eyebrows have the potential to move vertically over a relatively larger area, and to be more readily observed and more mobile (Supplementary Fig. 1)

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