> Wouldn't we have evolved in an environment with constant stress?

Well, humans did not reach an old age if you look back to history. Only in recent times, say, 2000 years or so, or, if you want to be strict, the last 100 years, did humans reach very old ages. Back in older days, 30 years was considered old. Then 40 or so.

I heard that is mostly a myth due to misunderstanding what life expectancy means. It's an average over populations, pulled down mostly by enormous infant mortality. If you made it to 15 then there was a good chance you'll make it to 60.