Uh, two towers made it pretty clear the Uruk-hai were cross bred- orcs with humans. This made them larger, more disciplined and more tolerant of sunlight.

Silmarillion says that the original orcs came from elves who were corrupted by Morgoth, way back when. (Morgoth is to Sauron as the Valar are to Gandalf. Sauron, Gandalf and Saurumon were all Maia;)