This tiff about the details of the 1500 hour rule found in the Airline Safety and Federal Aviation Administration Extension Act of 2010 could possibly be settled using Lexis Nexis research at a decent university library, looking at the transcripts of the 2010-111th Congress. It would be a great afternoon research subject for someone who wanted do it. I am betting the 1500 number was just pulled in from historical FAA admin. law anyway, and when you dig deeper there's not much behind it. My indirect reasoning is, an ATP would have been required all along for transport aircraft piloting if the FAA knew pilots with less than that are significantly more accident prone. I don't think they know that, because if they did and yet still allowed low-timers to fly airliners all these years before the 1500 rule, it's a rather large liability for the agency which of course they would tend to avoid.