Letters to Nature

Nature 428, 415-418 (25 March 2004) | doi:10.1038/nature02358; Received 5 April 2003; Accepted 20 January 2004

Myosin gene mutation correlates with anatomical changes in the human lineage

Hansell H. Stedman1,3, Benjamin W. Kozyak1, Anthony Nelson1, Danielle M. Thesier2, Leonard T. Su1, David W. Low1,5, Charles R. Bridges1, Joseph B. Shrager1,3, Nancy Minugh-Purvis2,4,5 & Marilyn A. Mitchell1

  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. Stedman1,3 Email: hstedman@mail.med.upenn.edu

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 Hominidae1, 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 Homo 4. 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.

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