Dan ...have you by any chance read a recent book titled '1424 the year the Chinese discovered America'?
Its most interesting and makes alot of plausible and factually based assumptions about a period of discovery pre-dating the European epoch, and there are parts that I am sure dovetail well with the 1491 book which I will now seek out.....oh, and I don't like ardent environmentals [don't want to be OT]