Until about three million years ago the ancestors of Homo sapiens were mostly vegetarians, and they most likely wandered in groups from site to site where fruit, tubers, and other vegetable food could be harvested. Their brains were only slightly larger than those of modern chimpanzees. By no later than half a million years ago, however, groups of the ancestral species Homo erectus were maintaining campsites with controlled fire — the equivalent of nests — from which they foraged and returned with food, including a substantial portion of meat. Their brain size had increased to midsize, between that of chimpanzees and modern Homo sapiens. The trend appears to have begun one to two million years previously, when the earlier prehuman ancestor Homo habilis turned increasingly to meat in its diet. With groups crowded together at a single site, and an advantage added by cooperative nest building and hunting, social intelligence grew, along with the centers of memory and reasoning in the prefrontal cortex.
The debate over “group selectionism” will rage on, but I think this much is beyond dispute: if our ancestors had not consumed meat — and subsequently learned to cook it — our brains would never have gotten big enough to morally reject it.