Chin Up: Ape-Human Split Was a Jaw-Breaker

The evolutionary split between early humans and ancestral apes may have begun with a tiny mutation in a gene for jaw muscles -- a lucky break that allowed the skull to grow and make room for the enormous brain that would eventually become the hallmark of Homo sapiens.

That's the controversial conclusion of researchers from the University of Pennsylvania, whose discovery of the mutation, announced today, has reignited the long- smoldering debate over how modern humans evolved.

The Penn team's work suggests that early primate skulls -- much like the skulls of modern gorillas and chimpanzees -- were literally muscle-bound by powerful jaw muscles and cramped by the big bony spurs that anchored them. Only when a quirk of nature produced mutants with radically smaller jaw muscles was the skull free to expand from one generation to the next.

Rest of the story via Washington Post

Nature 428, 415 - 418 (25 March 2004); doi:10.1038/nature02358

Myosin gene mutation correlates with anatomical changes in the human lineage

HANSELL H. STEDMAN et al.

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 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.

Link

Posted by Dienekes at March 24, 2004 08:17 PM | PermaLink