So, what should we eat? The past does not reveal a simple answer, ever. The best we can hope for is that it might shine a useful but flickering light into the darkness of our understanding. Our bodies are filled with layers of evolutionary histories; both recent and ancient adaptations influence how and who you are in every way, including what happens to the food you eat. The recent adaptations of our bodies differ from one person to the next, whether because of unique versions of genes or unique microbes, but our bodies are all fully-equipped to deal with meat (which is relatively easy) and natural sugars (also easy, if not always beneficial), and harder to digest plant material, what often gets called fiber. Our ancient evolutionary history influences how we deal with these foods, as does our stone age past, as do the changes that occurred to some but not all peoples as agriculture arose.
Rob Dunn, on Paleo diets