Maybe "free radicals" aren't so dangerous after all. The theory that oxygen molecules in the body play a key role in aging, has been weakened by experiments with yeast. The theory is that free radicals damage cells and DNA and that by reducing the radicals, through diet or supplements lifespan can be increased. Rats and fish have lived longer on reduced diets. But when researchers at MIT speeded up the metabolism of yeast cells, creating free radicals, the cells actually lived 20 percent longer. Scientists at Harvard medical School who commented on the results, agree the study does raise doubts. But, they point out that yeast cells and humans are different. The study appears in the journal Nature.