I think there was a potential draft of the story for C&C 3 and Tiberium, where the Scrin were actually human:
Which was then altered to look like this:
This is official art from C&C 3. Both pieces come from the same artist. Worst of all is the former is what appears to be a final, colorized version, so this means it likely wasn't just a "what if" idea, they put a lot of thought and development into it, before it was scrapped and the aliens became "generic Hollywood aliens".
Now, remember, the C&C 3 and Tiberium "story bible" did take inspiration from the scrapped TS Story Bible (like the Tacitus being recovered in 2028), as well as some elements seen here:
Kane was originally in contact with the Scrin, at one point, and he was the member of an ancient, highly advanced human civilization. Not too dissimilar from what Westwood had in mind, from what I can recall (well, the ancient civilization, that Westwood had in mind, was from Mars, at one point, but that was scrapped).
We'll never know the answer...But it ties in everything pretty neatly, if Scrin are humans.
1. They've evolved to become a cult of addiction instead of a race (Kane's Wrath Intel Database). They were humans and we're all prone to addiction. Reason why we don't see "the addicts is because they're humans and most of the Scrin harvesting force are just bio-mechanical constructs made by advanced nanotech. This can also explain why it's so easy for them to mind-control Humans (who's to say an alien brain and biology are both compatible with Human brains?) and to turn them into "Scrin" (Traveler-59's Cultists (and they're called by a typical Nod name!))
2. They know Kane cause he was their prophet, in the past. They were part of that same ancient civilization.
3. The "Brotherhood" that was the "enemy" of "The Scrin" (as mentioned in that GDI Intel Database entry) can only be Nod. There was also the word "Ascended" thrown in there.
(Read, under "InOps" here, find the entry titled "Alien Origins, Connections to the Tacitus": http://cnc.wikia.com/wiki/Intelligence_Database)
This explains how Kane knows of the Scrin and understands their technology and seems to know their origins. The Scrin were originally on Earth...Not as invaders, but rather, they were Humans, whom were later exiled (or simply left). This is why Kane can regenerate so well and so easily compared to us mortals, why he can give his DNA to others (C&C4...But let's ignore that one). This explains why they came back for Earth, specifically. Kane is also seemingly immune to Tiberium (C&C3) and this explains why some mutants exist, why some humans are seemingly not dead, but only mutated after Tiberium exposure (think of it like Marvel's Inhumans and Tiberium is Terrigen Crystals / Terrigen Mists) - Those mutants were part of the "ancient advanced human race" that roamed Earth millennia before (or a Human/Ancient hybrid). This isn't too dissimilar of the concept of the Nephalem in Diablo 3, the Precursors/Isu from Assassin's Creed (or a potential Human-ancient hybrid, like what the Assassins and Templars are) or the half-God/half-human hybrids of Greek mythology. Heck, even Halo has its lore settled on the concept of there having been an ancient space-faring Human race 100 000 years ago, just close enough to today for humans to have had the chance to fully evolve to Homo Sapiens, but distant enough so that any trace of that civilization can have completely vanished, without leaving a trace, so to explain as to how we don't have far advanced technology today, and to avoid contradicting how civilization truly started 10 000 years ago for us.
So...Yeah, that would've been a cool twist. Think they're aliens? Nope! They're humans. Oh, also CABAL is part of them, because he was reborn from an ancient Scrin AI. So technically, the 3rd faction is not only the Scrin, but also CABAL and The Forgotten. It's all 3! That would've been mind-blowing. Imagine the devs telling us whenever we asked if it was The Scrin, CABAL or the Forgotten, and the devs would always say "Yes", but we'd be like "they're trolling us! Which one is it?!" and then it turns out they were telling us the truth.