There is no reason to suppose that other animals lack emotion. Emotion is quite a primitive feature, certainly prior to reason or intellect, and fundamental emotions such as fear and attachment are often essential for survival. We see this even in human development from infancy through to adulthood. Emotion is what prompts many behaviours in animals, from the most basic to the more complex, including the fact that elephants, for example, have been observed to mourn their dead fellows.
The idea that other animals lack emotion or intellect, that they don’t qualify for personhood, has primarily been used as an excuse for humans to abuse them and exploit them without feeling guilt for harming a “person” or a being of any importance.
But anyone who has ever kept companion animals or spent any time at all around other animals could not fail to notice the distinctive signs of sentience, emotion and intelligence. Evolution (which is actually a fact, by the way; the ‘theory’ part comes into play in discerning how it happened - that it happened has been in no doubt amongst biologists, even before Darwin) establishes beyond any reasonable doubt that we are fundamentally related to all other living things, and most closely to other mammals, especially other primates. There is no reason to suppose that there is some thick black line that separates humans from all other organisms. We might value other humans more than other animals (in general), but that is because we are human - pretty much all animals display recognition and preference, to some degree, for their own kind.
When it comes to issues of life and death, of our treatment of other living beings, we cannot afford to take these things lightly just because we are dealing with nonhuman animals. There’s little doubt that mistreatment of other animals correlates with a greater likelihood of mistreatment of other humans as well. And if we use a lack of intelligence, for example, as an excuse for cruelty to nonhuman animals, what’s to stop us from using the same excuse for humans with intellectual disabilities, or infant humans who have yet to develop their intellectual capacity, or elderly humans with dementia who have lost their mental capacity? Refusal to respect nonhuman animals based on the fact that they are not human is speciesism, plain and simple, and is not rationally or ethically justifiable. As Jeremy Bentham, the father of Utilitarian ethics, eloquently put it, “The question is not, ‘Can they reason?’ nor ‘Can they speak?’ but ‘Can they suffer?’”.
