An appeal to "ancestral diet" is really an appeal to metaphysics, since none of us were there. Personally, I'd rather appeal to human A and P. But ultimately everyone's views about any given topic are affected by our beliefs about our origins. For example, my belief in the Bible agrees with the physiological fact that animal proteins are practically indigestible because they are so complex and tightly bound that our enzymes have such great difficulty unwinding and disassembled them into their constituent amino acids that comparatively few are able to actually be used; meanwhile they sit overlong in our guts, getting stuck to the walls of our colons, and putrifying; and since there are many animal fats trapped among these proteins, they go rancid too; and when rancid fats find their way into the bloodstream, the body deals with them by surrounding the molecules with water (so they are isolated) and storing them in your tissues. In YOUR tissues, not my tissues.