MYH16 Masticatory Gene Mutation Correlates with the Evolution of Human Morphological Cranial Capacity

MYH16 Masticatory Gene Mutation Correlates with the Evolution of Human Morphological Cranial Capacity

In the late Miocene, Homo sapiens encephalization accelerated simultaneously with an anatomic craniofacial transformation, associated with a shift towards a gracilization pattern of diet. Hansell H. Stedman has discovered a deleterious masticatory gene mutation responsible for jaw muscle development that is found in all modern human populations. This gene is functionally conserved in all non-human primates such as chimpanzees and extinct Hominidae species such as Australopithecus andParanthropus, and is only found in its mutated, non-functional state in humans. The myosin heavy chain (MYH16) inactivation occurred after the divergence of the phylogenic lineages leading to humans and chimpanzees, predating the appearance of the Homo species. The frameshift mutation is a functional defect, causing hypotrophy of the type II fibres of the masticatory apparatus, yet through natural selection evolution has ‘fixed’ this deficiency in the lineage leading directly to Homo sapiens. Stedman, working with Nancy Minugh-Purvis, proposes an evolutionary theory that the dramatic eightfold decrease of the masticatory muscles had a pleiotropic effect on Homo sapiens cranial morphology. Stedman and Minugh-Purvis propose this first identifiable molecular distinction between humans and apes removed an evolutionary constraint on encephalization of the Homo species, correlated with traceable morphogenesis in the fossil record, thus providing a mechanistic genetic basis for human evolution.