The assumption by an apparent majority on this list is that if a food was
available to pre-Neolithic man, he most certainly would have consumed it.
As Todd has said before, "I can't imagine paleo man not taking advantage
of stomach cheese from suckling animals."  And, so the theory goes, if he
consumed it the item was de facto good for them (and us).

Assume for a moment, just for the sake of argument, a toxic substance
available to man - I am arguing that the casein in stomach cheese, alcohol
in rotten fruit, and lectins in beans fall into this category - but
imagine any toxic substance you like.  Natural selection tells us that
succeptible lines choosing to consume this food, whether by real choice or
choice forced by conditions such as famine or choice forced by addiction,
would be selected out of the population.

Problem is that reality may not be quite as black and white as the theory
above.  What happens when those consumed toxic items are only seasonably
available or available in small quantities?  Does a threshold amount, both
volume-wise and periodic-wise, exist above which selection occurs?

I don't believe you can make the jump from mere consumption (which itself
is arguable) to healthfulness.  Especially given evidence that, for
instance, casein is a foreign protein causing all manner of havoc in our
systems and the closer a population is to their hunter gatherer roots, the
more powerful alcohol's negative impact on their well being.  Such a jump
requires an a priori assumption that the stuff is good for you.  The safer
bet, in my opinion, is to call these items neutral at best to our well
being if not outright deleterious.