http://en.wikipedia.org/wiki/Paleolithic_diet

"Critics of the Paleolithic diet have pointed out a number of flaws with its underlying logic, including the fact that there is abundant evidence that paleolithic humans did in fact eat grains and legumes,[3] that humans are much more nutritionally flexible than previously thought, that the hypothesis that Paleolithic humans were genetically adapted to specific local diets remains to be proven, that the Paleolithic period was extremely long and saw a variety of forms of human settlement and subsistence in a wide variety of changing nutritional landscapes, and that currently very little is known for certain about what Paleolithic humans ate."