Similarly, in yeast cells high levels of a gene called sir2 made them live 50% longer than usual.

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The next question was whether a new drug, rather than a crash diet, could activate the sir2 gene.

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He figured drugs stimulating the human version of sir2 (called sirt1) might harness natural defenses against diseases of aging.

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When Guarente's team deleted the sir2 gene from the yeast and then restricted calories, the diet no longer lengthened life span.

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Guarente's yeast lived longer when they were given less food, and the underlying mechanism seemed to be that starvation tripped SIR2.

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Sir2 can turn off whole swaths of genes inside cells, keeping the DNA coiled up in a protected position and less exposed to environmental damage during hard times.

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Almost a decade ago, Leonard Guarente at Massachusetts Institute of Technology found a gene called SIR2 in brewer's yeast that, when mutated, seemed to extend the yeast's life.

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Puigserver and his colleagues found that SIRT1, the mammal version of the SIR2 gene, was turned on in the livers of mice when they were put on a calorie-restricted diet.

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In other studies high levels of the same gene lengthened the lives of worms and fruit flies, a hint that sir2 might be part of an ancient survival program common to many species.

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