floresiensis tended to have smaller bodies ( Grabowski, Hatala, Jungers, & Richmond, 2015). erectus being relatively large-bodied, while Au. Rather, current data point away from a gradual increase and toward species-specific trends in body mass, with Au. Relative to brain size, body size does not appear to have changed as drastically over the course of hominin evolution. Estimates of hominin cranial capacities, commonly used as a proxy for brain size, range from a diminutive 375 cc for Australopithecus afarensis ( Holloway, 1995) to the current worldwide average of 1,350 cc in modern humans ( Beals et al., 1984 Lieberman, 2011): a 3.6 fold increase. Increased brain size is a critical feature of human evolution. However, acknowledging the significant influence of the growing brain on skull morphology does not provide a mechanistic explanation for how the skull changed onto-genetically or over evolutionary time in response to increases in brain size. Here we suggest complementing such existing datasets with experiments focused on mechanisms responsible for producing the observed patterns to more thoroughly understand the role of encephalization in shaping the modern human skull.ĭevelopment shifts in brain ontogeny and embryonic brain-skull interactions that led to large-scale shifts in the size and position of the different cranial skeletal modules (e.g., Bastir et al., 2010 Bruner, 2004, 2007 Lieberman, 2011 Lieberman, Krovitz, & McBratney-Owen, 2004 Lieberman, McBratney, & Krovitz, 2002 Martínez-Abadías, Esparza, et al., 2012 Weidenreich, 1941).īiomechanical forces related to mastication and respiration are generally thought to significantly impact the oral, nasal, and pharyngeal cavities and ultimately the facial skeleton, while changes in brain growth and size are considered to have more generalized, profound effects on all regions of the skull ( Bastir & Rosas, 2016 Lieberman, 2011 Moss & Young, 1960). Fossil and comparative primate data have thoroughly described the patterns of association between brain size and skull morphology. We argue that understanding how changes in human skull morphology could have resulted from increased encephalization requires the direct testing of hypotheses relating to interaction of embryonic development of the bones of the skull and the brain. Most prior studies have used fossil or comparative primate data to establish correlations between brain size and cranial form, but the mechanistic basis for how changes in brain size impact the overall shape of the skull resulting in these cranial traits remains obscure and has only rarely been investigated critically. Important evolutionary hypotheses, such as the spatial packing hypothesis, assert that increases in relative brain size (encephalization) have caused alterations to the modern human skull, resulting in a suite of traits unique among extant primates, including a domed cranial vault, highly flexed cranial base, and retracted facial skeleton. Dramatic changes in cranial capacity have characterized human evolution.
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