I thought that this was very interesting. You can trace the foundation of this transition to Intel all the way back to 1999! Main article: http://arstechnica.com/reviews/os/macos-x-dp2.ars/
Lines up with what Steve says, that they've been setting up OS X to run on multiple platforms since the beginning. I read most of the series of articles on the development of X. It's interesting to see how they went from classic Mac OS to what is now the best OS out there, if not ever. MP
And if you go back to 1998, it looks like this... If you really want to go back to the start of this, you'd be going back to NEXTSTEP 3.1 (released on May 25, 1993) which was the first version to run on 486 based PCs.