Over the past 500 years, the average length of a human life in the developed world has more than doubled while the maximum has remained essentially unchanged. We have eliminated or greatly reduced most of the traditional causes of mortality, including mass killers such as smallpox, measles, influenza, and complications of childbirth. But old age remains incurable and always lethal.

Why? On the face of it, aging looks like poor design. We have been selected by evolution for reproductive success; the longer you live without serious aging, the longer you can keep producing babies. Even if you are no longer fertile, staying alive and healthy allows you to help protect and feed your descendants.

The obvious answer is that if nobody got old and died there would be no place for our descendants to live and nothing left for them to eat. But that confuses individual interest with group interest; although group selection may have played some role in evolution, it is generally agreed that the major driving force was individual selection. If I stay alive, all of my resources go to help my descendants; insofar as I am competing for resources, I am competing mostly with other people's descendants. Besides, we evolved in an environment in which we had not yet dealt with other sources of mortality, so even if people did not age they would still die, and on average almost as young. In traditional societies, only a minority lived long enough for aging to matter.