As far as is known, it was really the other way round. The first and second groups of "indiginous" Americans migrated from north to south, both having come from Asia. Mexico is part of what is now called North America. The Distinction between North, South and Central America is somewhat artificial, just like that between Asia, African and Europe. There's really only 4 continents, and that's if you count Antartica and Australia. In the far far past, IIRC, there was only one.