Reply #43: Our physiological need for vitamin B12 is the biggest evidence

[SarahB]

Without vitamin B12, we develop pernicious anemia and our red blood cells cannot function to carry oxygen. The only way to get B12 is through some type of animal product barring supplements (a fairly recent historical development). Simply put, humans would have died out long ago is veganism was the norm.
Now, that being said, I have nothing against vegans or vegetarians. That's their choice and there are supplements that they can take and effectively remain healthy with their eating style which on the other side can have health benefits as well.