There was a very interesting article on this topic on The Art of Manliness this week that discusses how we as humans need to exercise to be healthy and that a long list of ailments was unknown to our ancestors who had to do physical work every day just to survive. They mention some of our anatomical features that suggest we were made to run and run fast for a long time. The book Born to Run
($10) goes into this deeper and is about a hidden tribe of Indians in Mexico who are said to be the best runners in the world. And judging by the reviews on Amazon it looks great and I'm ordering it. 
