I saw a presentation from an evolutionary biologist who may the point that we die because there is no evolutionary pressure not to die. Once we've birthed and raised out young, we are no longer relevant from an evolutionary perspective. He tested this with fruit flies by separating the young by gender for increasingly longer periods of time effectively delaying breeding so only the longer lived specimens could live to breed. By doing that over many generations, I believe he tripled the life expectancy of his test group.