Some of the basics on aging and evolution are covered again in this Psychology Today article: "How does it happen that, after having accomplished the miraculous success that led us from a single cell at conception through birth and then to sexual maturity and productive adulthood ... the developmental program formed by biological evolution fails even to maintain the accomplishments of its own work? ... humans need to have a life expectancy of only twenty-five to ensure continuance of the species. We are well equipped to reproduce as teens, and a life expectancy of twenty-five left us with enough young-elders to pass on the full amount of culture needed to survive and evolve on the African plain to our current biological form. Not only old age, but middle-age appears to be totally irrelevant to survival. ... The mutation accumulation theory embodies the idea that [a] mutant gene that kills children will not be passed on to the next generation, but a negative gene - e.g., Alzheimer's disease - will be neutral to natural selection. Over time, these genes [will] survive and accumulate in the human population. Related to mutation accumulation is the antagonistic pleiotropy theory, which is the idea that some genes that have a survival value for reproduction carry within themselves negative effects as we age. Pleiotropic genes have more than one effect--in aging, antagonistic effects."

Link: http://blogs.psychologytoday.com/print/3712