When talking evolution it's not just humans, and human behaviour. The fasting metabolism, hunger hormone system etc. is shared through pretty much all of the animal kingdom. We had it before we left the seas. Fish don't stockpile food, they store it in adipose tissue with about exactly the same mechanism as we do, there might not be much food around, that means increased competition, that means you need to be active, not lethargic, when hungry, and the level of exertion experienced during that fasting time will be taken by the body as the signal how much to bulk up, that's why growth hormones are highly active at that time. We're dealing with a truly ancient mechanism.

You can trust that I read up on the stuff or you can do it yourself or you can trust an army of gymbros to have done it.