ideal:
CP:Nobody has lived beyond 120 or so because that is the genetically programmed upper limit for humans.
What makes you think this? Perhaps Darwin thought the same of a younger age.
Darwin likely did not think the same of an age much younger than that.
There's a difference between life expectancy and life span. There were people in the 1800s who lived past age 100. The average age at death was lower than today because infectious diseases weren't adequately controlled, because there were no or few antibiotics and they weren't as effective as today's, and because trauma medicine wasn't as advanced as today.
On the other hand, since records were still comparatively poor people tended to accept accounts of extreme age more readily than today.
Further, people in the 1800s in part aged at different rates due to their wealth and social position -- a poor man at 70 probably looked more decrepit than a rich one at 70. How can we know this? Photography existed at the time. There are limits to the ability of "lifestyle" factors that can lengthen life and prolong youthful appearance. I have yet to see any person whom I could not tell was 90 or so, though I did meet a man I thought was 90 who was 107, but I've seen people 50 I thought were 70.
If you look at really old writings they all tend to claim that the earliest people lived for centuries, which is myth, but to agree that fortunate people (usually those that kept this or that god's rules) lived to 100 or 110. The difference is that more people have the opportunity to live so long because they don't die of smallpox or staph as children and can get (in Western countries at least) immediate, effective trauma treatment.
There is an observable upper limit to the human life span, and it is around 120. We may alleviate some symptoms and extend our lives artificially so more people attain that and some live beyond it, but the fact that the oldest people (except for legendary and mythical heroes) don't live past it shows that such has long been known and is universal.
Further, it's easily observable that mammals and birds, at least, have species specific life spans. If not, pampered protected dogs who are correctly fed and exercised would not age and die. The same would hold for others. It doesn't happen. Minimal calorie intake has an effect, but they still eventually age and die. (Some reptiles seem to be potentially immortal, but they are always killed by disease or injury or predation.)
It's comforting to believe the life span is infinitely plastic and that if you eat boiled fish or raw cereal or whatever you either won't die or will live to 125, and permissible as long as it doesn't interfere with real efforts to modify the built-in life span.
My guess is that limited lifespans developed because such creatures evolve faster when conditions change. We will soon no longer have to worry about that. But not yet.