Imagine that our ancestors lived in areas that were highly contaminated with heavy metals and that we developed enzymes that allowed us to survive by eating food contaminated with heavy metals. Would that necessarily translate to a diet rich in heavy metals as being our optimal diet?

Is it not possible that our ancestors developed genes for dealing with starch not because starch is good for us, but because, unlike our other primate relatives, humans ranged far and wide and choosing an area to live was not primarily based on the area having adequate supplies of a limited selection of food.

We ate what we could. We watched what other animals ate and gave it a try, especially when our normal food stuffs were scarce. Those that could do a better job of metabolizing those unusual substances had a slightly better chance of surviving and reproducing. Over time, we developed the ability to metabolize a much broader range of substance. That does NOT mean that just because we CAN metabolize them, that they are good for us, just that we can survive on them long enough to have kids.