An obvious limitation of the laboratory record is that there are few human data: One cannot experiment on humans for both ethical and practical reasons. There are four species of multicellular animals that account for most of the recent research into longevity extension. Two of those "model systems," the mouse and the rat, are mammals commonly used in biomedical research. The other two are invertebrates beloved of geneticists: the fruit fly and the worm. Also, some laboratories focus on the use of in vitro cell cultures with which to investigate the biology of the individual cells of the mammalian organism. Modular organization and common descent ensures that the genes each of these organisms carries are homologous to the genes humans carry and often have similar if not identical functions. For example, some 62 percent of the genes that are recognized to cause human diseases are known to exist in flies and to give rise to similar disorders when mutated. By investigating these model organisms, human beings investigate themselves by proxy.
PATTERNS OF AGING. When people intervene in the aging process, how can they tell if they are successful? Obviously, by extending longevity, but it turns out that there are at least three different manners of extending longevity, and only one of them is likely to be useful (Arking et al., 2002b). Compared with their normal-lived controls, experimental animals can live long by (1) increasing their early survival
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