Someone was complaining recently that humans should not be eating meat and stick strictly to a vegetarian diet.
Notwithstanding that humans have been eating meat for hundreds of thousands of years there was a recent study completed in Austria that found vegetarians were not as healthy as those who consumed meat in their diet and suffered more depression.
There was an article in a science journal some years ago telling the story of someone who arrived at an emergency room in a coma. The emergency room physicians assumed the patient had been using drugs but nothing of that sort showed up in his blood. One doctor had a Eureka moment and had the patient checked for B-12 and sure enough, the patient had none. Given a shot of B-12, the patient soon came out of his coma at which time he revealed that he had been a vegetarian for 28 years, suffering only minor brain damage from his vegetarian diet.