From the Washington Post: Evolution and middle age

http://www.washingtonpost.com/national/health-science/evolution-has-given-humans-a-huge-advantage-over-most-other-animals-middle-age/2012/03/12/gIQAtVnccS_print.html

Evolution has given humans a huge advantage over most other animals: middle age

By David Bainbridge, Published: March 26

"The claim that middle age evolved faces one obvious objection. For any trait to evolve, natural selection has to act on it generation after generation. Yet we often think of prehistoric life as nasty, brutish and short. Surely too few of our ancestors lived beyond age 40 to allow features of modern-day middle age, such as the deposition of a spare tire around the middle, to have been selected for.
This is a misconception. Although average life expectancy may sometimes have been very low, this does not mean that humans rarely reached the age of 40 during the past 100,000 years. Average life expectancy at birth can be a misleading measure; if infant mortality is high, then the average is skewed dramatically downward, even if people who survive to adulthood have a good chance of living a long, healthy life.
The evidence from skeletal remains suggests that our ancestors frequently lived well into middle age and beyond. Certainly many modern hunter-gatherers live well beyond 40."