What were the original factions?

I remember when I first started playing 40K way back when, I think the only factions were Imperium, Chaos, Tyranids, Orks, and Eldar. Also in my very foggy memory, at the time Imperium and Space Marines were maybe separate factions at the time? I could be mistaken about that. I think Dark Eldar and Necrons came next, if my faulty memory is to be trusted, but 99% sure they weren't around when I started.

Was that the OG layout, or were there other earlier factions that got phased out? How trustworthy is my recall here?