Several species of animals have figured prominently in aging research. Most of this research has focused on the genetic basis of aging. Aging is a process that involves both a decline in the function of an organism and a greater risk of the diseases associated with aging. Researchers have identified animal genes that govern life span, either by increasing the risk of those diseases of aging or by conferring some protection against them, permitting an extension of the life span. Some of the most well studied organisms include:

Yeast
Roundworms
Fruit flies
Mice
Nonhuman primates

Baker's yeast (Saccharomyces cerevisiae) is a much studied primitive organism. Although yeast is not an animal per se, it shares so much in common with animals, at least on a cellular and genetic basis, that it serves as a useful model. Because of its short life span, yeast makes an ideal research subject into the determinants of that life span. Researchers have known for some time that restricting caloric intake will increase life span in a number of animal species; recent work has shown that this is also true in yeast. Mammals have genes that correspond to some of those associated with increases in longevity in yeast, and an understanding of the workings of the yeast genes should further our understanding of the mammalian ones.

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Caenorhabditis elegans is a roundworm with a 20-day life span, making it a good subject for research. Many of its genes have been identified and their function elucidated. Among the genetic controls over its life span that have been studied are a series of interacting proteins that act like insulin and that control reproduction and longevity, and a mechanism controlled by a group of genes called clock genes. These regulate metabolism in the roundworm and affect longevity. The roundworm genes that have been identified as conferring increased longevity do so by granting resistance to external stresses, such as high temperatures, radiation and oxidative damage. Oxidative damage results when the toxic byproducts of oxygen metabolism damage the components of cells (see Oxidative Damage Research Center).

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Drosophila melanogaster, or the fruit fly, is a favorite subject for studies on longevity. Researchers have identified one gene that they have named Methuselah, which can increase fruit fly life span by 35%. In recent months, a number of studies on a fruit fly gene called Indy (for "I'm Not Dead Yet") have been published. Various mutations in that gene result in a doubling of the average life span, without any loss of fertility or physical activity. Loss of the mutation returns life span to a normal length. The protein that the Indy gene produces is closely related to a human protein active in energy production. Because the fruit fly has genes such as Indy that produce proteins very much like human proteins, it makes an excellent animal model for aging research.

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Mice are favorite subjects of scientists researching human aging. As mammals, they are more closely related to us than yeast, worms or fruit flies, and their relatively short life span makes them easier to study than longer-lived mammals. Some of the laboratory mice that are frequently studied are the Ames dwarf mouse, the growth hormone transgenic mouse and the SOD2 mutant mouse. The latter strain seems to be particularly susceptible to damage from oxidative stress. Toxic byproducts of energy production harm the DNA of the mitochondria of those mice. Mitochondria are tiny powerhouses within cells intimately involved in energy production. Damage to mitochondrial DNA has been implicated as a likely contributor to aging (see the Mitochondrial Aging Information Center).

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The discovery that fruit flies and roundworms carry genes that impact their longevity is exciting, particularly because many of those genes have human counterparts. However, the fact remains that fruit flies and roundworms are not human; how applicable the information gleaned from research on them is to humans has yet to be seen. But our DNA is virtually indistinguishable from that of nonhuman primates such as monkeys and apes. Indeed, 98% of our DNA is identical to that of our nearest animal cousins. The National Institute on Aging is sponsoring an extensive series of experiments into aging and longevity using primate models including rhesus and squirrel monkeys. Primate studies are ongoing in neurobiology, skeletal changes, reproductive aging and other age-related diseases such as heart disease and diabetes. Results from studies of caloric restriction and its impact on aging in primates are starting to be available.

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