Portal 2 was originally a prequel, Valve writers Chet Faliszek and Jay Pinkerton have admitted.
Referencing a leaked job ad from 2008, the Valve pair confirmed to RPS that the developer's original concept for Portal 2 took place before the events of the first game, with Cave Johnson the main villain.
The confirmation suggests that the 60s and 70s sections in the sequel originally took up a much larger portion of Valve's concept for the game.
"The game's been in development for a while. There's a small core team that starts on it, and they start experimenting with ideas, see what's interesting," Faliszek explained.
"The original goal was the have the sense of Portal, the feel and vibe of Portal, and what else happened was open. A lot of those experiments then get integrated into the core experience. People wanted more portals, they wanted GLaDOS, they wanted to be Chell."
Pinkerton added: "At one point two years ago some Cave Johnson dialogue got leaked - so I can now tell you, two years ago Cave was the bad guy in Portal 2 and GLaDOS wasn't in the game. It was a prequel. We liked the character enough that we snuck him into this.
"Originally we were almost seeing how far we could push the line," he added, "and at some point, with saner heads, we realised, you know what, we want to see GLaDOS again. We want Chell back.
"It was us trying to try new things, but then I think realising that we missed a lot of the elements that made the Portal 1 story."
The 2008 job ad described a Cave Johnson who "goes from sidekick to principle antagonist" and "starts to lose his grip on humanity as the story progresses".