This result was surprising, according to Ruvkun, because when worms get old they start to show decay all across their body, just as people do.

"For 50 years we've known about insulin signaling and we've always though that it works by engaging receptors on muscles and fat," he said. "It was always assumed that the tissue where you see the aging is where it would be functioning."

But instead, in the worm--and, Ruvkun suggests, perhaps in people as well--aging seems to be regulated by the brain. Ruvkun says this makes sense, as the entire body ages at the same rate, which might not be possible if aging were not regulated centrally.

The idea that the brain may control aging is attractive from an evolutionary perspective too. Closely related species sometimes age at very different rates. Perhaps small changes in brain insulin systems allow for these changes.

Ruvkun believes that the interaction between insulin and "free radicals" may explain its powerful effect on aging. Too many of these dangerous by-products of cellular metabolism can be lethal for an animal, but insulin can cause cells to stop producing enzymes which scavenge free radicals. If this process is turned off, he speculates, animals may live longer because they continue to eliminate free radicals.

But some scientists caution about reading too much about mammalian aging into results from the worm.

"One has to be careful about generalizing extensively from observations in the roundworm," said Dr. Richard Weindruch, who studies aging in mice at the University of Wisconsin. "It's not clear to what extent these findings represent the situation in mammals."

The role insulin plays in aging in mice is not yet known, but when mice are placed on a restricted diet, which lowers their insulin levels, they live longer.