One could argue the story could start at any moment, but the energy crisis is only when it gets "interesting." I always thought it was absurd that more shakeup happens in a few years than as much happened in four million just because they went to Earth. It's why the four million years thing is so stupid to me. It's such an unfathomably long time. You'd get the same effect with a few hundred years of war without the leaps of logic.