Yesterday I mused on why we get old and die. From the point of view of evolution, we only need to live long enough to pass on our genes. Everything else is gravy.
Consider the brown marsupial mouse of Australia, a shrewlike creature that does it and dies with remarkable alacrity.
At the appropriate time of year, biological clocks tell male marsupial mice that it is time to mate. Hormones gush -- not a now-and-then trickle, as in humans, but a sudden spate. Docile juveniles, less than a year old, are forthwith turned into sex-crazed adolescents. Their appetite for sex displaces all other concerns, including food, drink, grooming, sleep, and the avoidance of predators.
After a few frenzied days of non-stop copulation, the haggard male marsupial mice expire - every one of them! - having essentially gone from youth to old age in a flash. Their work is done. The next generation is assured. The females of the species can now manage quite well without them, thank you.
And that, my friends, is the gist of the story from the point of view of natural selection. Humans are programmed for a similar sort of existence, although on a rather more extended schedule -- just ask any adolescent. At which point culture intervenes. With increasing success we manage to take a licking and keep on kicking long past our peak reproductive years, in the meantime finding time for lots of other fun and games, like art, and science, and religion, and -- and meditating on the short, urgent life of the brown marsupial mouse of Australia.