The human diet needing B12 is a sign that we need to eat animal products.
Atheists like Neil Degrasse Tyson attempt to demonstrate that "evolution is dumb" because we have all this extra vasculature in our legs that we graft onto the small, constricted vasculature to the heart during a bypass.
But wait, which is it? Is our physiology prescriptive or is it not, as in the case of atherosclerosis, or any other case where modern medicine needs to correct something from biology?
You can get B12 from human flesh. Does it mean that this is a sign that I should be "listening to biology" and kill and eat other people? If not, other than "I'm making a exception for specifically humans", why not?
You can't infer what is moral from a vague interpretation of a "sign". If we get to reinterpret what it ethical from vague pronouncements of what we "think" our biology is "telling" us, we can start to justify nearly anything.