What do you have in common with your cave-dwelling ancestors of the paleolithic era? Your genes. As you are about to discover, this is crucial to your ability to achieve fitness.
You see, we have a "thrifty gene". Actually, there are probably many such genes, but for the point of this article just one will suffice.
In a fascinating compilation study published in the Journal of Applied Physiology (Vol. 93, Issue 1, 3-30, July 2002), Booth et. al. set out to prove a hypothesis -- that physical inactivity leads to chronic disease. Sounds like common sense...but then again, common sense is rare these days when it comes to health.
What is most interesting about this study was the level of exercise that Booth et. al. used to establish an "activity baseline".