No it's not sufficient. It can't be because it wouldn't explain why it is going up just now. We should slowly be moving away from our ancestors.
Furthermore there is the issue of neuroplasticity that the brain changes through learning. Also studies of meditation, psychotherapy, rehabilitation, and a number of other things have shown a multitude of things other than taking drugs can change brain chemistry itself over time. In fact asking people to think about certain things lead to verifiable changes in some studies, these were instant, short-term studies but presumably it would have a lasting effect if those sorts of thoughts became routine.
People don't have identical reward systems. The scientist who spends all day everyday in a laboratory going over experiments has a very very different reward system from an obese methhead who lives in a trailer park (and a far more healthy one at that!). While it would be very difficult (and in the former case undesirable) to condition oneself away from these things every day every decision we make is adding to conditioning for what comes in the future. So it comes down ultimately to personal responsibility. Take responsibility. You know gorging on junk food won't bring you lasting satisfaction so find other things and yes it will feel rough for awhile but put up with it and in time more worthy beneficial things will stimulate your reward system and you'll feel good again without the bad habits.