http://en.wikipedia.org/wiki/MileagePlus

Doesn't totally address your questions but this is all I could find. I do know there were only 2 levels before, many years before. Premier (25K) and Premier Executive (50K). Later they added Premier Executive 1K (100K), they also had a secret VIP levels(s) which led to the GS level which was not really part of the Mileage Plus program. You used to be able to be a 1P (Premier Executive) GS. Then the merger happened and they changed the names and added Platinum for 75K and GS became more of a real level but still no mileage requirement, just high fare spend, and a lot of it. Don't know the years these changes happened other than 2012's changes to what we have now.