We the mutants
On the trail of genes that made us human
Over the past few million years, humans became the chatty, complex creatures that we are today, while other primates remained in the trees. Fossils have helped only so much in figuring out how Homo sapiens became so odd. Increasingly, the best clues are coming from our own DNA.
Language, art, and sophisticated tools are key to who we are, and all emerged in the past 200,000 years. Some anthropologists have speculated that mutations were responsible. "It's the most economical explanation," says Richard Klein of Stanford University. Researchers have now zeroed in on one mutation, in a gene that has been linked to language.
People with damage to this gene, called FOXP2, have trouble articulating words and understanding grammar. Svante Paabo, an anthropologist at the Max Planck Institute in Leipzig, Germany, compared normal human FOXP2 genes with those of chimpanzees and other mammals, and found that the human version is slightly different. His group then used mathematical analysis to calculate that the variation cropped up within the past 200,000 years, suggesting it might have led to speech.
Swelled head. Long before that, our ancestors had developed big brains, and single mutations might have spurred that change as well. Last week, in the Proceedings of the National Academy of Sciences, scientists reported identifying a mutation that appeared about 2.7 million years ago, just before early human brains doubled in size. The mutation knocks out the gene for a form of sialic acid, a sugar that coats cells. Chimps and most animals have this form; humans have none. The researchers also tested fossilized bone from a Neanderthal, a big-brained species related to humans that died out roughly 30,000 years ago, and found that it lacked the acid, too.
Lead researcher Ajit Varki of the University of California-San Diego says he started looking at sialic acid several years ago because it's scarce in animal brains, though abundant elsewhere in their bodies. "There's some reason you don't want this in a mammalian brain," says Varki, who speculates it might interfere with brain development.
The idea that single genetic changes fostered our emergence remains tentative, but Paabo and others are seeking more clues. They are now sequencing the genome of chimpanzees--our closest living relatives. Comparing it with our own genome should make it easier to find other genetic mutations that could account for human nature.
This story appears in the September 16, 2002 print edition of U.S. News & World Report.
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