User:Jtwsaddress42/Projects/Project 4/Sections/Chapter 8/Expansion of the brain - Mysosin frameshift mutation and the appearance of Homo

In 2004, Hansell H. Stedman and his colleagues at the University of Pennsylvania were searching for mutations related to muscular dystrophies when they discovered a species-specific frameshift mutation of a highly conserved primate gene, MYH16. The MYH16 gene encodes a heavy myosin chain that is present in the jaw muscles of primates and is responsible for powerful contractile muscle movement. In primates, the MYH16 gene encoded myosin chain is highly conserved and expressed in the temporalis and masseter muscles that connect the jaw to anchor points on the sagital crest in primates. The frame-shift mutation that Stedman's group discovered is universal to humans, but apparently is disruptive to most other primates due to the masticatory demands of their diet and therefore has been highly conserved within the family.

Stedman and his group propose that the MYH16 frameshift mutation diminshed the pressure that the temporalis and masseter muscles placed on the developing sagital crest, thereby releasing the somatic selective pressure required to induce its formation in the first place. This in turn, allowed for an expansion of the braincase starting approximately 2.4 mya, as the structural constraint on braincase expansion, the sagital crest, became vestigial and ultimately disappeared from ontogeny.