June 2, 2004
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A clue to how humans evolved from apes may be in our heads according to some scientists. As this ScienCentral News video reports, a glitch in a single gene may have paved the way.

Clues to Chew

At first glance, the differences between humans and other primates appear clear. Aside from hair and climbing skill, however, our big brains set us apart. Researchers Hansell Stedman and Nancy Minugh-Purvis of the University of Pennsylvania have stumbled on a slip up in a single gene found in our jaw muscles that they say may have triggered this divergence by enabling the brain case to expand. Their findings, published in Nature, have spurred a lively debate among anthropologists and evolutionary biologists alike, giving them a new reason to ruminate our possible origins.

While studying human muscle disease, Stedman, a gastrointestinal surgeon, found a new version of a gene that encodes for a muscle-fueling protein called myosin. "Myosin is the most abundant protein in muscle," explains Stedman. "It's the motor protein that generates all the force. The body is able to make a wide range of different myosins, and each one has a different gene. The surprise came in finding that one of them…winds up having a mutation that cripples its ability to make a functional myosin…in all humans, as far as we can tell."

Stedman teamed up with Minugh-Purvis, an expert in developmental biology, to study this newly discovered version of the myosin gene, called MYH16. They compared random DNA samples from people from five continents with samples from seven nonhuman primate species. They reported finding MYH16 in human jaw muscle, but they found a different version of this gene in other primates. "Somewhere in recent human evolution, there was a mutation…so that now, all living descendents of that breeding population have inherited the defective form of this gene," says Stedman. He went on to explain that such a genetic defect would essentially weaken biting muscles in the jaw, likely causing them to grow smaller. Today, modern-day humans can bite with hundreds of pounds of force, but nonhuman primates can chomp with tons of force.

Expanding the Mind

Using "molecular clock" analysis, Stedman dated myosin's DNA divergence to roughly 2.4 million years ago—around the same time that some say the first human ancestors evolved from nonhuman primates. "This is a very key moment in human evolution in the sense that it is about that point in time when human brain size…really begins to increase," explains Minugh-Purvis. "And at that same date, the jaws and the dentition or teeth begin to decrease in size….Both of which would implicate these biting muscles."

In humans, the biting, or temporalis, muscles attach at points on the brain case and the lower jaw. Nonhuman primates have larger temporalis muscles that connect to large bony protrusions on the head. In both cases, the muscle crosses over expansion joints in the skull. "And when that muscle contracts, it can close the jaw….But it can also pull on the sides of the brain case," says Minugh-Purvis. "We feel that the power of contraction across these growth plates in the skull may very well have discouraged an increase in brain size."

But skeptics say Stedman and Minugh-Purvis have found a weak cause-and-effect relationship. Ralph Holloway, a physical anthropologist at Columbia University says, "there's no evidence that the chewing musculature constrains brain growth. It is the brain that grows, and that is the matrix for how the skull grows." Meanwhile, molecular anthropologist Todd Disotell of New York University questions the timing of the MYH16 mutation. "Their whole argument hinges on this estimated date," he says. "They are correlating that with another event—which there's not much agreement on—to come up with a nice, simple story. Evolutionary biology rarely works so nice and cleanly.”

Regardless of the correlation to human brain size, the fact remains that this genetic change occurred and produced a novel form of the myosin-coding gene in humans. "The fact that this muscle is a different size in humans arguably directly links to this mutation," says Stedman. "The bottom line is that this gene is the one that's present in modern-day humans." Most importantly, hypotheses like Stedman's will continue to further the discussion of human origins.

This study was published in Nature on March 25, 2004. It was funded by the National Institutes of Health, Muscular Dystrophy Association, Association Francaise contre les Myopathies, Veterans Administration and Genzyme Corporation.

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