Asked by: James Gartland, Warrington
It’s more helpful to think of age as a side effect of evolution by natural selection. All organisms are subject to environmental dangers outside their control, and even for an organism that never ‘ages’, this will result in a decreasing probability of surviving to successively greater ages. It thus makes sense to invest effort in reproduction at a young age – even at the expense of reducing lifespan. After all, even a body that can perfectly repair itself at a cellular level might be eaten by a lion tomorrow.
However, this means that genetic mutations that only show symptoms later in life are less likely to be weeded from the gene pool by natural selection, because those genes have already been passed on by then. According to this theory, animals with more predators (and thus high extrinsic mortality) should have shorter intrinsic lifespans, even in predator-free conditions. And this appears to be true.
The little brown bat Myotis lucifugus has almost no predators and lives for up to 30 years; a mouse has enough predators that it rarely lives more than five months in the wild, but even in the lab will die of old age within three years.