Craniofacial skeletal response to encephalization: How do we know what we think we know?

Abstract

Dramatic changes in cranial capacity have characterized human evolution. 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. 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. 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. Fossil and comparative primate data have thoroughly described the patterns of association between brain size and skull morphology. 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.

Modern human changes in regulatory regions implicated in cortical development

“…Table S7). These terms have appeared prominently in the human evolution literature in the context of neoteny and delay brain maturation, brain growth and the craniofacial phenotype in between species comparisons [6,[30][31][32]. Two chromatin regulators with mSNC in regulatory regions are present in these two ontology terms and are associated to neurodevelopmental disorders: KDM6A (mSNC in promoter), which associates to the H3K4 methyltransferase complex [27], and is mutated in patients with Kabuki syndrome [33]; and PHC1 (mSNC in promoter), a component of the repressive complex PRC1 [27], found in patients with primary microcephaly-11 [18].…”

Abnormal cranium development in children and adolescents affected by syndromes or diseases associated with neurodysfunction

“…As disturbances in the differentiation of body proportions were not taken into account, relative or absolute microcephaly/macrocephaly was not distinguished. There is a relationship between the size of the brain and the size of the skull 30 , the body size 31 , and the neurological capacity of organisms 31 . Previous studies have shown that tetraplegia is accompanied by short stature 3 , though current studies have shown that tetraplegia is accompanied by microcephaly, which confirms the theory of encephalysis.…”

Variation of bony labyrinthine morphology in Mio−Plio−Pleistocene and modern anthropoids

“…According to this inference, the bony labyrinth pattern common to the extant great apes may be a secondary effect of independent acquisition of large brains. Understanding the actual developmental mechanisms and processes that produce variation of craniofacial structures remains a major challenge (Lesciotto & Richtsmeier, 2019), and a better understanding of developmental interactions between the brain and labyrinth is needed.…”