For the last 6 million years, humans or their ancestors have been developing flaws in their genes at a surprisingly fast clip, a new study says. But they've apparently removed some of the buildup through sex.

Every newborn over that time span has had an average of two or three new harmful mutations, researchers calculated. Such flaws don't cause disease, though they detract from a person's overall fitness, said the study published in Thursday's issue of the journal Nature.

The study done by Adam Eyre-Walker of the University of Sussex in England and Peter Keightley of the University of Edinburgh in Scotland offers the first reliable estimates on such mutations, said gene expert James F. Crow of the University of Wisconsin in Madison.

He said in a telephone interview that the estimated flaw rate is surprisingly high and implies ``a rather large load (of flaws) for us to be dealing with.''

So how has the human race survived all that pollution in the gene pool?

Some mutations have apparently been eliminated by people who carry them never having offspring, the researchers said. Flaws that remain might be too minor to make a real difference.

Keightley speculated that human ancestors who carried the most flaws were least likely to have children. That would eliminate mutations in batches and might still be going on today.

Crow said the new work strengthens the argument that most organisms reproduce sexually because it lets them get rid of gene flaws more efficiently.

Modern medicine, by saving lives and overcoming infertility, lets people reproduce now who couldn't have in the past. So it presumably lets more bad mutations live on.

Does this mean the human race will become dangerously loaded down with genetic flaws? Don't worry, Keightley and Crow said.

``I think it's something to think about, but it has an awfully long fuse,'' Crow said.

The mutations almost certainly have very tiny effects, such as a predisposition to headaches, stomach upsets and weak eyesight, especially later in life. It would take centuries or thousands of years of modern health care to build up to a significant level of harm, Crow said.

The researchers calculated the rate of new deleterious mutations by looking for differences in the details of 46 genes in humans and chimpanzees. That let them estimate rates of mutation since the evolutionary split between the ancestors of people and of chimps.