That's quite possible.

There are probably a good percentage of people
on the planet that thrive on a grain/legume diet.
After all, in 10,000 years a lot of adaption and
natural selection can take place.

I agree that it may be possible that the diet
at either end of the spectrum may work for
some poeple.  Either eat a meat-fat diet,
or a agricultural diet, but not both.  This may
work very well for the 'geneticaly more advanced'
people that actualy have the genes to handle it.

It also may be deadly for the poeple that do
not have the genes to handle it.  Thus the
selective pressure.

How can we get good data on this?
Seems to me that the evidence taht comes from
the Vegans shows that after many years on
a Vegan diet that the body finaly breaks down.
My next-door neighbors are Vegan and are very thin
an malnourished looking.  (They think they look healthy)

R. Keene