Nature vs. Nurture
The inevitable . . . er . . . spirited discussion brought on by the veganism posts features a lot of people trying to justify their choices by claiming that they are really the "natural" choices. Leaving aside the fraught question of what "natural" means--is agriculture natural? How about using fire and tools?--both sides make factually incorrect claims. No matter what the vegans say, we are evolved to eat meat--but no matter what the carnivores say, we aren't evolved to eat anything like the quantities that we currently consume, and in any timeframe that you might be imagining as "natural", the normal diet of the overwhelming majority of people involved very, very little muscle protein.
The bigger puzzle is why everyone is treating "natural" as a synonym for "good" or "right". Death in childbirth is natural; so is rape. These are not a good guide for either our behavior or our standards of living.