Here is a credible idea: “We should not eat things that we are maladapted to eat.” On the face of it, this makes perfect sense: if we are maladapted to eating something (or doing anything, for that matter), then clearly it will do us harm. That’s true pretty much by definition. Let’s call this argument the adaptational health argument.
Now, let’s shift the emphasis. The meaning of the statement is actually the same, but when restated, it reveals a problem: “We should only eat things that we are adapted to eat.”
Ah. Now, the shoe is on the other foot! What exactly are we adapted to eat? We have to eat something. How do we choose? What if we have been eating things that we are not adapted to eat?