I have no idea how cavemen lived, and it doesn't interest me all that much either, but my guess is that they weren't as sedentary and they weren't as proficient

I have no idea how cavemen lived, and it doesn't interest me all that much either, but my guess is that they weren't as sedentary and they weren't as proficient in the art of preserving food. Overall, it should have been quite difficult for most of them to create a caloric surplus, day after day. What I do know is that bodymass is lost and gained with deficits and surpluses of calories. Transient spikes in insulin and stuff like that are irrelevant. This is well established in nutritional science.