Like, I just was thinking about how lots of pet species will just eat as much food as you give them to the point of making themselves sick, and keeping them at a healthy weight requires not giving them access to too much food. Obviously some humans have problems with this, but imagine how bad things would be if everyone were basically psychologically incapable of not eating food when we had access to it even when we’d had enough, given our dramatically higher access to food due to agriculture.
I mean a lot of fat people are old. Gain three pounds a year for thirty years and that’s sixty pounds of excess before you’re even sixty.