The troll and elven lineage was the idea that night elves descended from trolls. This was eventually confirmed in World of Warcraft: The Magazine, Issue 5. In game, It does state that the night elves are descended from "a primitive tribe of nocturnal humanoids" who were feral and nomadic (a few sources call this tribe the "Kaldorei".)
The original information to support the claim can be found in the in-game text "The Twin Empires", which states:
- "...ancient texts speak of a small faction of trolls that broke off from the Amani Empire and founded their own colony in the heart of the dark continent.
- There, these brave pioneers discovered the cosmic Well of Eternity which transformed them into beings of immense power. Some legends suggest that these adventurous trolls were the first Night Elves, though this theory has never been proven."
Alternatively other legends imply that night elves descended from the offspring of Elune, she herself appearing to be night elf in form.
According to an article on World of Warcraft's website which discusses the issue, there is no evidence that proves either explanation, and truth is unknown. Much the same is said in in-game books and in the RPG as well.
Other possible races
It is stated in Lands of Conflict that primitive humans then known as Azotha were nomadic and nearly as primitive and barbaric as the trolls. They would also be a good candidate for ancestor of the night elves. However humans are descended from Vrykul and this leaves little place for speculation that they may have further evolved into night elves.