Researchers have made a noteworthy step forward in understanding the causes of aging. By inserting a genetic defect into the cellular mitochondria of mice, life span is shortened and signs of old age are created. This provides the first hard proof for the mitochondial theory of aging - scientists knew that older people have more defects in their mitochondria, but did not know whether this was a cause or effect of aging. This research is a first step and doesn't yet invalidate any of the other theories of aging. From the article: "Since mitochondria also generate free radicals, the latest work may also propel work on the free-radical theory." The next step would be to see if scientists can extend life spans in mice by eliminating defects in the mitochondria.