I had lots of evolution indoctrination during study for my zoology degree. It helps me understand the following.
Most people are aware of the health benefits of being physically active. The question arises then why people so easily fall into sedentary habits. The idea developed here is that sedentary behavior is part of a suite of behaviors to reduce levels of physical activity that were strongly selected in the evolutionary past, likely because high levels of physical activity had direct negative consequences for survival. However, hunter-gatherer populations could not reduce activity indefinitely because of the need to be active to hunt for, and gather food. Hence they never experienced low levels of activity that are damaging to health, and no corresponding mechanism avoiding low activity evolved. Consequently, gene variants promoting efficiency of activity and increased sedentariness were never selected against. Modern society facilitates reduced activity by providing many options to become less active and divorcing food intake from the need to be active. Choosing the less active option is hard wired in the genes; this explains why being sedentary is so common, and why reversing it is so difficult.
Source: An Evolutionary Perspective on Sedentary Behavior. – PubMed – NCBI
Steve Parker, M.D.