Answer:
Tolkien himself states that nobody knows how the orcs were made, but it was not by corrupting elves. That in itself strongly suggests that Melkor secretly sang them into existence (probably along with many other evil creatures) in the song that created the universe that all the Ainur sang.
First answer by Eccles-Jordan Trigger. Last edit by Eccles-Jordan Trigger. Contributor trust: 127 [recommend contributor recommended]. Question popularity: 15 [recommend question].