[davem]

Short answer is that Tolkien altered his opinions on the subject. Originally they were corrupted Elves, but later he decided they were corrupted Men, or even a kind of organic 'robot'. During the period of LotR they were Elves but as with many things in the 'Legendarium' as he called it (ie the mythology as a whole) he altered & developed his ideas throughout his life.

This is one of those questions that would take an essay to answer in full, but the corrupted Elves idea was the one that Tolkien held to for most of his life, & the one that makes most sense in terms of the mythology.

EDIT

What you'll find with Tolkien's work is that the deeper you look into it for definitive answers the more you'll find confusions & contradictions. Only the main themes of the Legendarium found a fixed form - but the details & even some of the larger & more important ideas were subject to alteration - even published works like LotR & the Hobbit were revised after publication & changes were made.

Of course, if you want definitive answers you're in the wrong place - we like to argue too much here.

Word of warning - watch out for Lalwende - she's a troublemaker & will just get you into bother if you listen to her.