It's not about can only humans feel pain and other animals not. It's more: do some animals do not feel pain. I don't think many would argue dogs or even mice do

It's not about can only humans feel pain and other animals not. It's more: do some animals do not feel pain. I don't think many would argue dogs or even mice do not feel pain. I am not a biologist, but from working in labs that do research with model organisms (like drosophila) what I understood is that there is little evolutionary advantage of being able to feel pain if you cannot heal it anyway. Also if there are no nerves in some regions feeling of pain is unlikely. I remember that someone told me that experiments show that drosophila with servered legs don't change their way of walking, and do not limp as animals do that feel pain. So the notion got adopted that they cannot feel pain and experiments on them are ethically save as they do not suffer.