There is an excellent reason why many religions, including Christianity, place human beings in a unique position among the species (we have souls, as the gorilla hunter said.)

If we are not unique, not privileged among the species, if it is a matter of degree, then we have no right to use animals for any purpose. No horses for riding, no mules for plowing, no dogs for shepherding...or in a modern sense, no animals for drug testing. Give up on leather shoes, or become a vegan--sure. But I wonder how many of the PETA crowd refuse medical care?

You can draw a line and include humans and great apes on this side of the "special" category and every other species on the other side. But on what basis--because they look like us? Intelligence? How about the grey parrot or the dolphin? How intelligent is enough? Shall we label the profoundly mentally retarded non human? (Thank you, Adolph Hitler!) Shall we pick our genetic similarities? So why do chimps and gorillas past muster, and rhesus monkeys (valuable for medical research) do not? And if they do, how about lemurs?