The body needs activity in order for its cells to release hormones and other things that tell other cells to do things like 'regenerate'. This is basically why physical activity appears to increase the health of people's brains and other organs. 

That said the human body didn't evolve with gyms and hardcore regimented workouts. Even hunting was often 'low-impact', sustained activity, with the majority of gatherers purposely migrating between areas that were especially fertile. Some anthropologists believe the amount of time necessary for a skilled gatherer to obtain everything they needed for a week, was within a matter of 6 or so hours. 

But if you'd like to know more about what we can do to 'hack' these biological mechanisms, most recommend a healthy lifestyle, with fiber and nutrients in your diet, and regular, low-impact physical activity.