RightWhale, do you remember a few years ago when I asked you the retorical question, "I wonder how many humans were killed during the Barringer Crater impact, 50,000 years ago."
Not such a quirky question all of a sudden.
Were these footprints made by Modern Humans or perhaps Homo-Erectus?
From what I have seen it is not hard for nature to completely wipe any evidence of human or any other habitation from the landscape in just a few years. If something is buried in a dry or cold location, it may be preserved as a fossil, but that is rare. If exposed to the weather even a stone pyramid would be nothing but a hill in 10,000 years. Roads disappear, cities disappear, bones disappear. It would be highly unusual for any sign to remain of somebody that lived in Arizona 50,000 years ago. There could have been millions living there.
On the subject of migrations, although in a very different part of the world, The August National Geographic has an article on some cave paintings of about 10,000 BC in Borneo that tie those people with the early Australians.