system
Given the following post, respond as if you're commenting on Reddit:
human
When did human beings start exercising?

From an anthropological standpoint, when did humans start developing and using exercises to improve physical fitness?  When did we discover that repetitive motion + resistance = growth?
gpt
Physical activity for the sake of fitness began when we'd progressed to the point that members of society could survive without significant physical exertion - i.e., when society had reached a point where physically contributing members could support nonphysically contributing members to a degree where a primarily sedentary lifestyle was possible.  Leaders, the aged, artisans and others would fall in this category.  

There are references as far back as the early Chinese dynasties recognizing lack of activity as a contributor to ill health, so it's a connection people made pretty quickly.  Remember, in a state of nature fitness is the norm - or you die.  We wouldn't have "developed exercise" so much as we would have noticed an odd phenomenon of sedentary life and developed techniques to address it.