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Is the Human Genome Going Downhill?

For millions of years, our genomes have been collecting mutations at an alarming rate, researchers write in today's issue of Nature. That begs an intriguing riddle: If our DNA is so prone to mistakes, why we are alive at all?

Because our genetic blueprints are among the largest known, scientists have long suspected that humans are particularly vulnerable to mutations. And because we have few offspring, such mutations may not be weeded out effectively, leaving our species with an ever more ramshackle genome. Geneticist Peter Keightley of the University of Edinburgh in the U.K. decided to test this theory. With evolutionary biologist Adam Eyre-Walker of the University of Sussex in Brighton, U.K., he examined DNA sequences for 46 human genes. Comparing these sequences to equivalent ones in chimpanzees and more distant relatives like gibbons and baboons, the duo could find out which mutations had occurred on the evolutionary branch leading to humans.

The researchers divided the genetic errors into two groups: "silent" mutations that wouldn't alter a gene's protein, and "nonsilent" mutations that lead to variant proteins. They discovered that silent mutations were 60% more likely to be passed to offspring than nonsilent ones. But mutation is a haphazard process, and both brands are equally likely to occur; the nonsilent mutations are just more likely to be discarded by natural selection.

Analyzing the data, the researchers stumbled upon an apparent paradox: that natural selection has eliminated at least two new mutations per person every generation, a feat possible only if almost everybody dies childless. To explain why humanity still exists, the researchers suppose that harmful mutations tend to become lethal--and are weeded out--only when they occur in bunches. That way, people that carry just a few nonsilent mutations wouldn't suffer any consequences. But if that theory is true, Keightley says, the bulk of nonsilent mutations that persist may undermine our health over the millennia--although he has no idea whether there is a coming genetic apocalypse of sorts for the human race.

If Keightley's theory is right, technology may compensate for many harmful mutations, says Richard Lenski, an evolutionary biologist who studies bacterial mutation rates at Michigan State University in East Lansing. "It is an important study, which provides a lot of food for thought," he says.