A small number of species have exceedingly long life spans and show few signs of degenerative aging, so clearly biology is up to the task of continual repair and vigor. Yet the vast majority of species consist of individuals who age, and who will die because of aging should they survive the many other causes of mortality in the wild. Why do we age to death? The present consensus is that the prevalence of aging is the result of an evolutionary arms race to the bottom. Species that age better adapt to changing conditions and thus will take over most evolutionary niches. In effect we age because the world changes. Some thoughts here from a researcher in the field:
In the long run, the ability of a species to evolve is more important than anything else in determining its competitive success. This is true almost by definition: given enough time, the ability to adapt and improve will overtake any initial disadvantage. But evolutionary theory these last 50 years has been quite skeptical of "in the long run". If it is driven to extinction because of a competitive disadvantage in the short run, then what matters if it has the potential to improve, eventually?
This has everything to do with aging. A population with aging has more diversity and a faster turnover compared to a similar population in which death is only due to famine, predators, disease, etc. So - in theory - a population with aging evolves more rapidly than a population that doesn't age. But "the long run" can be thousands of lifetimes, and in the meantime those individuals that die early (of aging) are at a competitive disadvantage compared to those who continue to live, and have that much more time in which to produce offspring.
Can an aging population resist invasion (by longer-lived competitors) and cohere long enough that its superior rate of adaptation turns into a decisive advantage? This is the question that has been at the center of my research the last dozen years. On the one hand, there is abundant evidence that aging is no accident, that it has evolved via natural selection that explicitly favors aging. On the other hand, the theoretical argument casts doubt on the scenario where aging is selected on this basis.
The best resolution I have been able to find for this paradox is that aging has been able to evolve on this basis, and it is because the short-term advantage of unrestrained reproduction has been held in check by a different, faster-acting evolutionary principle than evolvability. Unrestrained reproduction leads to population overshoot, population crash, and extinction. This is a powerful, fast-acting evolutionary force, and populations have had to adapt by tempering individual competitiveness. This has created an environment in which the long-term advantage of aging is relevant, and aging as a population-level adaptation can thrive on this basis.