From smaller jaws to larger brains?

Stedman

Stedman contemplates skulls with research team colleague Nancy Minugh-Purvis, instructor in anatomy and cell and developmental biology.

Photo by Daniel R.Burke

Associate Professor of Surgery Hansell Stedman has discovered a clue that may help solve a long-running evolutionary puzzle—how the brain was able to expand, thus distinguishing humans from their primate predecessors.

He and his colleagues in the School of Medicine have found a genetic mutation that makes the jaw muscles of humans significantly smaller and weaker than those of primates. Since news of the discovery appeared in the March issue of Nature, media outlets worldwide have trumpeted the find as the “missing link” between apes and humans.

Stedman himself won’t go that far. “The reason the world seized on this has vastly more to do with its potential implications than it has to do precisely with what we’ve definitely proven,” he said.

What he says his team has proven is that all humans share a common genetic mutation—two base pairs of genes encoded for the muscle protein myosin, which all non-human primates have, are missing from their jaw muscles.

But wait a minute. What’s a surgeon doing working in the province of biologists and paleontologists?

“The intriguing part of this is that the original discovery would likely have been passed over for a significant bit of time if it had not been for the surgical background,” said Stedman. “Surgeons are used to thinking in terms of pathology and how one abnormality in one organ system might affect other organs around it.”

Stedman’s team made their discovery in the course of their work on muscular dystrophy. They have been working for several years on using gene transfer to restore normal muscle function. This led them to look at myosin, which governs muscle contraction. In the course of this work, they ran across the missing genes that led to the weakening of the human jaw.

As for why this mutation spread, Stedman dismisses the standard explanation that attributes it to dietary changes. Instead, he points out that primate jaws are used as weapons when they fight to establish dominance. He suspects that changes already taking place in the brain at the time may have helped the mutation along. At some time in ancient prehistory, he said, “It’s possible that an especially cunning male chimp might have been able to outwit a more powerful one, and if this mutation lifted an evolutionary constraint on brain growth, you can see where this all leads.”

Stedman said his team intends to explore the possibility that this mutation may have led to changes in the skull that made room for a larger brain. “We are simply not in a position to make any dogmatic assertions about how all of this happened, but we’re only half joking when we call this the room for thought mutation,” he said.

Originally published on April 15, 2004