Mr. Laser Beam wrote:
^ The Mirror Universe is different. It's not an alternate timeline created by time travel; it always existed. It's a completely separate universe. The same people's births and lives are simultaneously no more, and no less, likely to happen in the MU than they are in any other.
Is there anything from the shows or even books to support that? AFAIK, the books and comics have given several divergence point stories, and William Shatner even pitched an Enterprise episode about the point at which the timelines split.