What Makes us Different?

Not very much, when you look at our DNA. But those few tiny changes made all the difference in the world

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Even before the chimp genome was published, researchers had begun teasing out our genetic differences. As long ago as 1998, for example, glycobiologist Ajit Varki and colleagues at the University of California, San Diego, reported that humans have an altered form of a molecule called sialic acid on the surface of their cells. This variant is coded for by a single gene, which is damaged in humans. Since sialic acids act in part as a docking site for many pathogens, like malaria and influenza, this may explain why people are more susceptible to these diseases than, say, chimpanzees are.

A few years later, a team led by Pääbo announced that the human version of a gene called FOXP2, which plays a role in our ability to develop speech and language, evolved within the past 200,000 years--after anatomically modern humans first appeared. By comparing the protein coded by the human FOXP2 gene with the same protein in various great apes and in mice, they discovered that the amino-acid sequence that makes up the human variant differs from that of the chimp in just two locations out of a total of 715--an extraordinarily small change that may nevertheless explain the emergence of all aspects of human speech, from a baby's first words to a Robin Williams monologue. And indeed, humans with a defective FOXP2 gene have trouble articulating words and understanding grammar.

Then, in 2004, a team led by Hansell Stedman of the University of Pennsylvania identified a tiny mutation in a gene on chromosome 7 that affects the production of myosin, the protein that enables muscle tissue to contract. The mutant gene prevents the expression of a myosin variant, known as MYH16, in the jaw muscles used in biting and chewing. Since the same mutation occurs in all of the modern human populations the researchers tested--but not in seven species of nonhuman primates, including chimps--the researchers suggest that lack of MYH16 made it possible for our ancestors to evolve smaller jaw muscles some 2 million years ago. That loss in muscle strength, they say, allowed the braincase and brain to grow larger. It's a controversial claim, one disputed by anthropologist C. Owen Lovejoy of Kent State University. "Brains don't expand because they were permitted to do so," he says. "They expand because they were selected"--because they conferred extra reproductive success on their owners, perhaps by allowing them to hunt more effectively than the competition.

BEYOND THE GENES

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