We have a range of adaptations for foods, and what
doesn't keep us from reproducing, we have tolerated.
A lot of what makes us sick doesn't actually kill us
before we reproduce. Only that which affects reproduction
really cuts it as natural selection acting on the
genome. A good example of a disease that we have
tolerated because of suboptimal diet is arthritis,
which has a lot of food components to it, but
usually doesn't catch up with us until after the
babies have been born.
Would I rather live as a grandmother with arthritis
from eating corn, or starve to death and have no
kids? The people who could put up with the inconven-
iences of sub-optimal diets are the ones who
survived. Now that we live in a sophisticated
society where 45 year olds need to stay alive
to pay for their kid's college, these 'old age'
diseases like coronaries and stroke and cancer
are starting to affect our kids prosperity,
or lack of it, more directly.

Anyone who has seen bears in a garbage dump or a dog
at a table knows other species are capable of eating
'new,' but not necessarlily 'better' diets as well.