It was never actually implemented in GC1. As with the current game, the manual isn't always the most reliable guide to the gameplay!
Just as an aside, if there had been a life force resource in GC1, it would have been impossible to balance. Planet Quality was EVERYTHING in GC1 - it helped morale, taxes, production, research. Giving 1 player +50% to PQ across their empire would have basically meant GAME OVER for everyone else.