The argument is probably made that since footprints (from people supposedly here over ten thousand years ago) are rare as it is, footprints from more northern regions simply disappeared; also there was the Laurentian ice sheet which largely melted (though why people would be trekking over ice?), cutting off much of the land now available in the north. BTW, proponents of a European (ancient) settlement of the New World often agree with multiple waves of immigration, so those in Mexico and Chile could be from another hypothetical colonization: one from Oceania (Pacific Island region).
Interesting. Thanks for the info.
susie