The fitness "cost" isn't the "net cost." Pretty much everything our bodies do has a cost. Our organs are all consuming a tremendous amount of energy. Our guts h

The fitness "cost" isn't the "net cost." Pretty much everything our bodies do has a cost. Our organs are all consuming a tremendous amount of energy. Our guts harbor bacteria that can absolutely overwhelm the body if they get out. It takes us more than a decade of growth to reach sexual maturity. You can only determine the value by taking into account the benefit. Populations that could consume milk, an easy and plentiful supply of calories and calcium, did better off than some of their local neighbors who didn't.