What is the true origin of the orcs?

The most accepted and solid theory is that Orcs come from Elves. That's what all the published novels say (as far as I remember). The other theories survived in some of Tolkien's notes, but he never made the effort of including them in any story, I think. However, I don't see any reason for rejecting the other theories alltogether. They "could" be true, why not? Except for the theory that puts men as the origin of the first orcs. That's completely impossible in the version of the story that survived, because when men awoke there was already plenty of orcs.

Update: 29th May 2014

The late essay Of Dwarves and Men contains what seem to have been Tolkien's final thoughts on the matter. Christopher Tolkien dates this to at least 1969, based on strong evidence ("It was written on printed papers supplied by Allen and Unwin, of which the latest date is September 1969") and the bulk of it was printed in History of Middle-earth 12, but the relevant section was extracted to form part of the Drúedain material in Unfinished Tales.

An author's note (note 5) to this material states:

To the unfriendly who, not knowing them well, declared that Morgoth must have bred the Orcs from such a stock the Eldar answered: "Doubtless Morgoth, since he can make no living thing, bred Orcs from various kinds of Men, but the Drúedain must have escaped his Shadow; for their laughter and the laughter of Orcs are as different as is the light of Aman from the darkness of Angband." But some thought, nonetheless, that there had been a remote kinship, which accounted for their special enmity. Orcs and Drûgs each regarded the other as renegades.

This, of course, never made it into the main stories and was ommitted from the published Silmarillion because of that.

The only final interpretation that seems reasonable to me is that:

• There is no known "true origin" for Orcs within Middle-earth; what we're presented with is a bunch of in-universe theories.
• One of those theories is that Orcs were bred from corrupted Elves.
• Another is that they were bred from Men.
• And some believe that the Drúedain were a likely candidate.

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Original answer - February 2013

There are a number of problems with all of the origin stories that may explain why Tolkien never settled on a definitive one himself.

If Orcs come from corrupted Elves, are they immortal within Arda? What happens when they are killed? Do they go to the Halls of Mandos, and can an Orc repent and be restored to his Elf-dom?

If Orcs come from Men, what of the timeline problems? How did Morgoth manage to breed so many in such a short time? You can see Tolkien grasping in this direction with some of his later revisions, moving the origin of Men further back, for example.

Problems with Morgoth not being able to create life himself have already been discussed, but at a stretch could be handled by the story of Morgoth disseminating his power throughout Arda.

Ultimately this was never resolved so there is no "true" origin. Orcs must be viewed in the same light as Tom Bombadil; one of the world's unknown and unknowable factors, and we must be content to leave it so.

THe problem wih orcs is that of hereditary evil. Morgoth might be able to corrupt individual elves to evil, but how does that evil then pass on, automatically, to the next generation? Is Morgoth powerful enough to introduce damnation to a species: a species born bad with no hope of redemption? That was Tolkien's dilemma, having introduced orcs into Arda. Morgoth cannot create new life (only puppets a la Aule's original dwarves), but - if he corrupts existing life forms how can he make them hereditarily evil through all generations thus putting them outwith Eru's grace? Tolkien's idea of making them into animals addresses this issue (no souls to begin with), but scarcely gels with the undoubted sentience and intelligence of the orcs we encounter in LOTR. Who knows how Tolkien would have resolved this? Maybe Orcs, left to themselves and without the false religions of Melkor and Sauron worship, would have come to Eru's grace in time, had they survived long into the fourth age?

Here is an idea. If you remember from The Silmarillion, when Eru Illuvatar composed a theme in his music, Melkor attempted to always mess up the theme, thereby causing Eru to introduce another theme, which in turn was "messed up." After a time, Eru allowed the Valar to actually see the overall composition materialized, including the elements Melkor had introduced. The Orcs could have been Melkor's "attempts" realized when Eru materialized the composition. Iru did not have to directly create the Orcs, but only indirectly, as the Orcs were originally Melkor's idea. Eru Illuvatar was bound by his own self-imposed limits to materialize everything within the composition, whether good or evil.

During the first war between elfs and orcs, orcs managed to win a couple of time, and I think Tolkien made it clear that elfs were superior to human in most ways. So maybe Morgoth's corrupted elfs were a strong race of Orcs who could fight elfs, and later he corrupted men to create a lower race of Orc. Then the two races could have been crossed (or not) and allowed different army corps and one common race. And we can imagine that the snaga are corrupted hobbits ! :)

I don't believe orcs were corrupted elves although it is ambiguous;for although Tolkien states in a letter (Rhona Beare 1954) that 'trees may go bad as in the Old Forest;Elves may turn into Orcs and if this required the special perversion of Morgoth, still Elves themselves could do evil deeds', in another letter (Peter Hastings 1954) he quotes LOTR Chapt I, Book VI '(the Shadow) cannot make real life of its own. I don't think it gave life to the Orcs, it only twisted them and ruined them'. And earlier in the same letter he specifically defines Orcs as a race of 'rational incarnate creatures' and quotes Treebeard's statement that Morgoth did not 'create' them but made them in counterfeit of certain creatures pre-existing'. In other words, Orcs need not be corrupted from the Elvish race (at least not solely);there were plenty of humanoids available for Morgoth to give them their demonic shape.