From glyphweb;
Little is known for certain of the beginnings of the Orcs, the footsoldiers of the Enemy. It is said that they were in origin corrupted Elves captured by Melkor before the beginning of the First Age. In appearance, Orcs were squat, swarthy creatures. Most of them preferred the darkness, being blinded by the light of the Sun, but the kinds bred later in the Third Age such as the Uruk-hai could endure the daylight.
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I'm sure the "evil" part is comfortably covered by the "serving Morgoth" bit.