For those of you who have not heard of the naturalistic fallacy already you have likely witnessed or expressed this faulty logic yourself. It is quite common for people to become naturalists and experts on animal behaviour when veganism comes up. Fortunately too their expertise allows us to make important judgements about how humans ought to act (pleeeaase tell me you caught the sarcasm there).
The simple fact of the matter is that we do not learn about how we ought to act (morally speaking) by understanding what is natural for our ancestors (most people mean our species here - and they are magically willing to ignore the changes we have made in terms of memetic and temetic evolution). Anyway, go to the fallacies section to see the updates I have made to the naturalistic fallacy section. It is an extremely important fallacy to be aware of. The ignorance of this particular illogical line of reasoning actually acts as a huge roadblock to changing how humans, in general, come to treat nonhuman animals.
All the best. Be sure to keep yourself sharp and educated. It has very important consequences.