If you put your fingers on the side of your head, just above your ears, and move your jaw you will be able to feel your temporalis muscles doing their thing.
Compared to our primate cousins our temporalis muscles are puny - approximately one eighth the size.
The reason for the difference is a mutation of the MYH16 gene in humans that produces a protein called myosin heavy chain 16. In primates like the gorilla this protein produces the powerful chewing pressure of the jaw.
Our closest hominid relative, the chimpanzee have an intact MYH16 gene. Since the rate of mutation can be determined, Hansell Stedman and his team at the University of Pennsylvania have calculated that the mutation that disabled the gene in our line happened between 2.1 and 2.7 million years ago.
The large temporalis muscle has to anchor to very thick and strong skull bones. A loss of MYH16 in our ancestors permitted the brain-case to become thinner and larger allowing for the expansion of the hominin brain which is one of the defining features of Homo sapiens. The genus Homo first appeared around 2 million years ago. Add to that the thought that a reduction in the jaw muscles would likely lead to finer control of the mandible that is required for speech.
The story of the loss of our big temporalis muscle is undoubtedly more complex and interconnected with other changes. However the relic of this gene in our genome is another piece of evidence of our common ancestry with all living things.
| Part 1 - Protein Functional Redundancy .................... | Part 5 - Vitamin C |
| Part 2 - DNA Functional Redundancy | Part 6 - Human Chromosome 2 |
| Part 3 - ERVs | Part 7 - Human Egg Yolk Gene |
| Part 4 - Smelly Genes | Part 8 - Jumping Genes |