I think human beings are the descendents of various different animals, not just one. Furthermore, I believe it is possible to tell what animal you are descendent from based on your particular physical characteristics and behaviors. For instance, my friend John's ancestors were definitely apes, because he has a lot of back hair. My ex-girlfriend had evolved from rabbits, because she slept with a lot of different people (rabbits are whores. It's a proven fact).
As for myself, my ancestors are most definitely horses.