Our bodies, from a biological perspective, are essentially the same as those of the early humans that lived a variety of hunter-gatherer lifestyles 300.000 years ago. In terms of human evolution, we’re all the same specie: Homo Sapiens.
That same evolution shaped our bodies to be extraordinarily well adapted to that ancient lifestyle: our ancestors were proficient at all the physical tasks required to hunt, gather, build shelter, etc.
In turn, our bodies evolved to rely on these physical stimuli to keep them healthy. Calories were very scarce and energy conservation was primordial, so we evolved to spend our calories only when needed. How do our bodies “decide” that? By the physical demands we’re placing on them: it’s the simple “use it or lose it” principle at play.
For example, you want strong bones? Place pressure on them by jumping, lifting and carrying heavy loads (among other activities) so your body recognizes the need for strong bones. Don’t do it, and you might be looking at osteoporosis at a young age…