But almost everything about the idea of it seems to be almost a violation of nature. Nature isn't perfect. Cancerous cells grow out of control and take over the normal functions of the body. Immune systems aren't equipped to deal with every pathogen that seeks to invade and dominate. Yet, although it lacks perfection, nature is balanced in its pure form.
Nature seeks and finds it's equilibrium through trial and error, and has always been successful.
Even if the changes that it undergoes are fatal, extinction and evolution work slowly together, and until nature created humans, it was a process that went unchallenged. Common sense and the smallest of coherent children would be able to point out that it is exceedingly odd to put a pig liver, or baboon heart, into a human. Logically, scientifically, the human body is going to say no. It deals with these cells through a system of recognition and destruction. The Hyperacute rejection of Compliment is an efficient killer, it clogs the blood supply, sometimes calling in phagocytes to finish off the job, or sometimes completing things itself, by bursting the cell membrane.