To give some examples of what I'm talking about, Arthur De Vany, who's
currently writing a book which he has excerpted at
http://www.socsci.uci.edu/econ/personnel/devany/Essay.html, makes the
following statement:

   "The only universal characteristic of ancestral and living hunter
    gatherer diets is the almost complete absence of simple carbohydrates.
    There were no >simple carbohydrates like sugar and pasta. Fruits were
    tough and fibrous, >not the refined, sweet stuff we have today. The
    closest thing to a simple >carb was honey, rare and guarded by wild
    bees. There were no grain or cereal sources of carbs in the ancestral
    diet."

(From "Evolutionary Fitness:  What Evolution Teaches Us About How to Live
and Stay Healthy  Copyright 1995 by Arthur De Vany, Ph.D. and excerpted at
the above URL.)

I don't want to pick on De Vany, who is far from alone in making this kind
of claim.  But this somewhat common belief appears to be false.  There's
more to cereal grains than that they are high in simple carbohydrate; they
also contain forms of protein (which can be generically referred to as
glutens) which are completely foreign to the human animal, and which have
been shown to cause serious disease in at least some individuals.
Allergies to cereal grains seem to be among the most common of food
allergies, too.  As Hoggan mentions, there is at least some evidence that
the proteins in grains may be implicated in far more serious autoimmune
responses than just food allergies.  Ray Audette has often suggested that
obesity is an autoimmune disorder, and has also suggested that what he
calls "foreign proteins" in grains and beans may be a primary culprit in a
number of diseases that hunter/gatherers never suffer from.

One might also wonder about dairy.  I know of no evidence that the
ancestral human diet would ever include dairy.  In fact, although I eat
dairy products myself, you could argue that the consumption of dairy is
just about the most bizarre thing that modern humans do.  Dairy products
contain lactose, and compellingly, about 75% of the world population is
lactose-intolerant.  But lactose isn't the only foreign substance; casein
is also in all dairy products, and is another substance which would be
foreign to the human digestive tract, and which appears to cause at least
some individuals problems.  I know of no rigorous evidence that casein is a
major danger, but it does seem logical to posit that substances unique to
foods that would never have been eaten by humans  in nature should be
looked at with suspicion.