Why then hold to any beliefs about right and wrong?
If there is no belief that humans are any different than barnyard animals why should humans behave any differently?
Most of them don't behave any differently. What we do have that animals don't have is imagination. It gives us our ability to create order, and also to destroy on a large scale. But it's just a physical attribute. There's no need to develop these intricate stories and people them with named dieties who have motives and moods. It's human imagination that does that, I suppose, from Native American mythology to Greek mythology to Middle Eastern. But it's the same impulse that allows us to create fantasy worlds with dragons and mages and beautiful princesses. Just a by product of the cerebrum and the frontal lobe.