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In Pre-Humans, Weaker Jaw Muscle, Bigger Brain

By Cheryl Simon Silver

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Chimps
Evolution

About 2.4 million years ago, pre-humans diverged from their ape-like brethren, losing their prominent jaws and developing the larger skulls associated with physical and, later, intellectual advancements. Now, a team of scientists reports that it all started with a mutation that caused weaker jaw muscles in some of the apes.

Modern macaque, gorilla, and humans skulls.
Hansell Stedman and colleagues at the University of Pennsylvania, and at The Children’s Hospital of Philadelphia, write in Nature that the apes with the mutation formed much smaller jaw muscles, caused by a mutation in the myosin gene that freed the skull to expand. The skulls of the apes lacking the mutation continued to be strapped by massive jaw muscles, leaving no room to grow a substantially larger brain.

The gene, MYH16, is active in jaw muscles of both humans and monkeys. The mutation in the human gene, however, keeps the relevant protein from building up in, and strengthening, the jaw muscles of humans.

The authors present compelling, and provocative, arguments that the mutation was responsible for the rapid appearance of “human-like” characteristics soon after the mutation occurred.

The authors’ interpretations of the finding are already reigniting the simmering debate over the evolution of modern humans. Most analyses to date have focused on fossil samples, but these ancient skeletal remnants are exceedingly rare.

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It also vividly demonstrates how recent advances in genomics—including the ability to compare the genes of one organism to those of another—are transforming whole realms of science.

As Pete Currie, of the Victor Chang Cardiac Research Institute in Sydney, Australia, notes in his commentary in Nature, Stedman and colleagues describe “what may be the first functional genetic difference between humans and apes.” As such, it suggests that “the genetic basis of human evolution can and will be defined.”

The complete sequences of the human genome, and the nearly complete sequence of the chimpanzee genome, revealed that the genes of the two species differ by only one percent. Now the details of that one percent are coming to light, filling in gaps and changing our understanding of how humans evolved.

Stedman, Hansell H. et al. Myosin gene mutation correlates with anatomical changes in the human lineage. Nature 428, 415-418. (March 25, 2004)

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