As anyone who has owned a pet knows, animals possess many characteristics that come across as strikingly “human.” Moreover, as scientists learn more about such creatures as bees, dolphins, gorillas, wolves, and crows, many of the traits once considered uniquely and distinctively human have been shown to occur among animals as well. Sophisticated forms of communication, the ability to distinguish self from other, a wide range of emotional states, logical reasoning and abstraction, the use of tools, the ability to learn, complex kinship and social relationships – all these behaviors (and many others) have been discovered among a wide variety of animal species.  All this research suggests that, in many of these domains, it is inaccurate to think of animals and humans as creatures separated by a vast qualitative gulf. Instead, we should see them as sentient beings arrayed along a single continuum of complexity and sophistication. According to this view, our difference from animals is only a matter of degrees and gradations, not of fundamental ontological status. 
This argument has two significant problems, however.  First and foremost, it flies in the face of our moral intuitions about the absolute “threshold of inviolability” inherent in the concept of human dignity. (See Appendix G.) It also downplays excessively the layered nature of reality, the synergistic process through which individual traits of humans come together, interacting dynamically to potentiate each other and generate something qualitatively new: our personhood. I take up these issues at greater length in a forthcoming book.
 See the entries under the Bibliography heading of “Animal Enhancement.”
See Jeffrey Schaler, ed., Peter Singer Under Fire (Open Court, 2009).