[Boromir88]

It really comes down to what you want to believe. It's clear that Tolkien's very LAST thoughts were that Orcs were taken in and corrupted men. But, Tolkien also played with the idea of Orcs coming form slime and rocks (which later he rejects), then it changes into being corrupted elves, and then switches all over the place. You can even make an argument that some orcs were Maiar that had taken an Orc form (Boldog for instance).

Let's start with the Elf theory. This occurs in the Silmarillion:

But of those unhappy ones who were ensnared by Melkor little is known of a certainty. For who of the living has descended into the pits of Utumno, or has explored the darkness of the counsels of Melkor? Yet this is held true by the wise of Eressëa, that all those of the Quendi who came into the hands of Melkor, ere Utumno was broken, were put there in prison, and by slow arts of cruelty were corrupted and enslaved; and thus did Melkor breed the hideous race of the Orcs in envy and mockery of the Elves, of whom they were afterwards the bitterest foes.~Of the Coming of Elves

There are a few things about this quote...

1) First off just notice "little is known of a certainty..." therefor, it can already be tossed up into the air and debated endlessly.

2) Also notice "this is held true by the wise of Eressea..." Orcs being from corrupted elves is what the "Wise of Eressea," believe, it's not necessarily what Tolkien felt. As Tolkien tells us with Treebeard in Letter 153:

Treebeard is a character in my story not me; though he has a great memory and some earthly wisdom, he is not one of the wise and there is quite a lot he does not know or understand.

The "Wise of Eressea" therefor, are characters in Tolkien and not necessarily what Tolkien feels about Orcs.

With that being said we do have this from HoME:

It remains therefore terribly possible there was an Elvish strain in the Orcs. These may then even have been mated with beasts (sterile!) - and later Men. Their life-span would be diminished. And dying they would go to Mandos and be held in prison till the End.~Morgoth's Ring, Myth's Transformed: Text VIII

So at this time (which is dated in 1955) Tolkien still felt like Orcs were originally started as Elves. And over the years as they began to mate with men and beasts the elvish blood-line diminished as well as their life span.

It's clear that in Tolkien's last thoughts he felt like Orcs would be best to be originated from corrupted Men, and if I'm not mistaken he even alters when Men first appear to fit this theory:

Let's first start out of The Annals of Aman (which is dated to be 1959 and also appears in Morgoth's Ring: Myth's transformed, Text X:

Alter this. Orcs are not Elvish

Tolkien continues on with the theory into Text X of Morgoth's Ring (1969):

Other originally independent creatures, and Men among them (but neither Elves nor Dwarves) could be reduced to a like condition. But ’puppets’, with no independent life or will, would simply cease to move or do anything at all when the will of their maker was brought to nothing.

Tolkien talks about Orcs being nearly complete puppets, and here he says with Men they could be reduced this this form of "puppetness," but it is impossible for Dwarves or Elves to be reduced to this state.

So, that's pretty much all the information there is and it comes down to what matter do you want to believe. An argument for them originating from Men is easier, because those were Tolkien's final thoughts on the matter and he even goes as far as to want to change that idea. If you're like me, who believes in both ideas. However, it would be hard to hold to the theory that Orcs are and always were corrupted Elves.