Hm. I admit that in the historical picture, it makes a lot of sense. There's no fundamental reason why things have to die of old age, and the first microscopic "damage model" of ageing cited turned out to not fit the facts. And in a certain gene-driven viewpoint, it makes total sense that if you want the body to do something new as it ages, you need additional genes to make it do the new stuff.
But we get to take advantage of modern knowledge about how microscopic damage accumulates (see e.g. Aubrey de Grey's stuff) to go beyond genes a bit, and when you see stuff like the metabolic rate and lifespan being really close to inversely correlated (the seed of the idea dates back to the 1930s, but I think the connection between metabolic rate and lifespan may be more recent), it's good confirmation.
So, I guess what I'm actually confident about is that a damage model of ageing is correct for most animals, and that we're not like octopuses or annual flowering plants. But that doesn't actually mean that our ageing can't be an adaptation - it would just mean that rather than dying at some specific time, the human body plan ages on some time scale that made sense for some long-gone ancestor, e.g. an early multicellular animal. This does have a slight falsifiability problem, though - either we simply never stumbled upon and spread the genes that would fix our damage, or we did stumble upon those genes, but didn't spread them because ageing is good for us. Both leave essentially identical fossils and genomes behind.
One piece of evidence may be mutations that would have changed the time scale of ageing without granting immortality - it certainly seems unlikely that all these different species are still best served by dying with the proportionality to metabolic rate set down by our ancestor.
At a recent Reddit AMA, Eric Lander, a professor of biology who played an important part in the Human Genome Project, answered this question:
His response:
This seems to me, at first blush, to exhibit the Evolution of Species Fairy fallacy. Evolution doesn't work to benefit species, populations, or the "next generation". If a mutation arises that increases longevity, and has no other downsides, then animals with that mutation should become more common in the gene pool, because they die less often. I remember reading that the effect would not be very strong, because most animals don't die of old age. But why would there be the opposite effect?
I am loath to attribute a very basic error to a distinguished professor of biology. Is there another explanation? Is the claim that evolution selects for mortality true?
Note: Eric went on to add:
This seems to be blatant rationalization of a preconceived idea that death is good. (I doubt he truly believes that extra progress is worth everybody dying.) So perhaps his first statement is also a form of rationalization. But it seems improbable to me that he would make such a statement about biology if he didn't think it well-founded. More likely there's something I'm misunderstanding.
ETA: one of the first Google results is this page at nature.com, The Evolution of Aging by Daniel Fabian, which goes into some depth on the subject. The bottom line is that it agrees with my expectation that evolution does not select for mortality. Choice quotes:
How could a distinguished professor of biology, a leader of the HGP and advisor to the US President, get something so elementary wrong, when even a biology undergrad dropout like myself notices this seems wrong?
ETA #2: Gwern points to the Wikipedia article on Evolution of Ageing, which lists several competing theories of the evolution of aging (and therefore mortality). This shows the subject is more complex than I had thought and there may be good reason to believe mortality is selected for by evolution (or at least is reliably linked to something else that is selected).
I should be glad that I didn't discover an obvious error being committed by a distinguished professional, even if he may be ultimately wrong!