Also realize that the grain (seeds of grasses and herbs) people were eating 100,000 years ago was, genetically and phenotypically, very different than the heavily hybridized, genetically modified, nutrient-poor, pesticide-laced grain people eat today.
I have no doubt that 100,000 years ago people were eating grain. What I'm skeptical of is the idea they were using it as a staple, and that it made up a meaningful proportion of their diet. The idea that grain consumption began with agriculture is silly, no dietary change that substantive could happen so suddenly. The relationships with cultivated plants that would allow seeds to become a substantial part of a people's diet certainly took tens of thousands of years to develop.
I suspect that the reason so many people are intolerant of grains today - and ironically why "Paleo Diet" aficionados claim we should avoid them - is because we've overconsumed them without preparing them properly (soaking and fermenting them) for many generations and have overloaded our bodies with their antinutrients and damaged our epigenetic phenotypes.