1. Department of Surgery, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
2. Cell and Developmental Biology, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
3. the Pennsylvania Muscle Institute, School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
4. Department of Anatomy and Cell Biology, School of Dental Medicine, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
5. Division of Plastic Surgery, The Children's Hospital of Philadelphia, Philadelphia, Pennsylvania 19104, USA

Correspondence to: Hansell H. Stedman^1,3 Email: hstedman@mail.med.upenn.edu

Abstract

Powerful masticatory muscles are found in most primates, including chimpanzees and gorillas, and were part of a prominent adaptation of Australopithecus and Paranthropus, extinct genera of the family Hominidae^{1, 2}. In contrast, masticatory muscles are considerably smaller in both modern and fossil members of Homo. The evolving hominid masticatory apparatus—traceable to a Late Miocene, chimpanzee-like morphology³—shifted towards a pattern of gracilization nearly simultaneously with accelerated encephalization in early Homo⁴. Here, we show that the gene encoding the predominant myosin heavy chain (MYH) expressed in these muscles was inactivated by a frameshifting mutation after the lineages leading to humans and chimpanzees diverged. Loss of this protein isoform is associated with marked size reductions in individual muscle fibres and entire masticatory muscles. Using the coding sequence for the myosin rod domains as a molecular clock, we estimate that this mutation appeared approximately 2.4 million years ago, predating the appearance of modern human body size⁵ and emigration of Homo from Africa⁶. This represents the first proteomic distinction between humans and chimpanzees that can be correlated with a traceable anatomic imprint in the fossil record.

1. Department of Surgery, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
2. Cell and Developmental Biology, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
3. the Pennsylvania Muscle Institute, School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
4. Department of Anatomy and Cell Biology, School of Dental Medicine, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
5. Division of Plastic Surgery, The Children's Hospital of Philadelphia, Philadelphia, Pennsylvania 19104, USA

Correspondence to: Hansell H. Stedman^1,3 Email: hstedman@mail.med.upenn.edu

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