Welcome to the Barrow-Downs!
The question you ask is less simple than it sounds. In the Silmarillion, it is stated that the race of Orcs likely came from the corruption of Elves by Melkor, the original dark lord, during the First Age (LoTR takes place at the end of the Third Age some 7000 years later). In later writings, Tolkien may have changed his mind and decided that Orcs were not corrupted Elves but rather: 1. corrupted men; 2. corrupted beasts; or 3. some combination of men, Elves, beasts and Maiar (to oversimplify - spirits).
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Beleriand, Beleriand,
the borders of the Elven-land.
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