Humans are built to move. We evolved under conditions that required daily intense physical activity, and even among individuals with lower physical potential, that hard-earned genotype is still ours today. The modern sedentary lifestyle leads to the inactivation of the genes related to physical performance, attributes that were once critical for survival and which are still critical for the correct, healthy expression of the genotype. The genes are still there, they just aren't doing anything because the body is not stressed enough to cause a physiological adaptation requiring their activation. The sedentary person's heart, lungs, muscles, bones, nerves and brain all operate far below the level at which they evolved to function, and at which they still function best.
Mark Rippetoe, Practical Programming for Strength Training
evolution, fitness, training