Causes, Mechanisms and Consequences of Early Hominin Encephalisation: A Hypothesis

DOI: 10.14673/HE2015121003
Published in Human Evolution – Vol.30 – 1-2-2015

Keywords: Hominin encephalisation, pleiotropy, genetic correlation, brain: body allometry, energetics, Miocene, Pliocene, Australopithecus.

Abstract
Hominin brain size continues to be the focus of intense research. However, relatively little attention has thus far been given to the causes and mechanisms that led to the initial, albeit moderate, brain size increase among australopiths and/or the reasons why this increase was followed by a relatively long period of stasis until the emergence of Homo. To address this issue I take a comparative phylogenetic approach that investigates the scaling relationship between brain size and body mass in catarrhines. Unlike traditional studies, body mass of the opposite sex is factored into the allometric relationships. The results obtained throw new light on the possible causes underlying early hominin encephalisation.
Phylogenetically controlled analyses reveal a complex pattern, whereby brain size in both sexes is correlated with body mass of the larger sex, i.e. males. Hence, in cases where male body size has been the target of selection, either through sexual or natural selection, females have larger endocranial volumes than expected on the basis of their body masses alone, irrespective of family, dietary ecology or social organisation; this bias is seen for average values also. The scaling coefficients suggest a genetic, i.e. pleiotropic, link between both structures. Against this backdrop it is unsurprising that the sexually dimorphic early australopiths were also slightly encephalised: these hominins probably had brain sizes expected for their body size and sexual dimorphism. The higher energy requirements of their larger brains, together with environmental changes during the late Miocene/early Pliocene, most likely underlie the changes in dietary ecology observed in the hominin fossil record. By switching to a ubiquitous, energy-rich C4 (mainly) plant food niche hominins were able to sustain their larger brains without compromising other aspects of their biology, e.g. time for socialising. This made further brain size increases possible later in their evolution, i.e. with the emergence of Homo. 

Macho, G.A.
Research Laboratory for Archaeology,
Dyson Perrins Building, South Parks Road,
Oxford OX1 3QY, UK.
E-mail: Gabriele.Macho@rlaha.ox.ac.uk

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