It seems what is true of the Old World is also true of the New World. Catastrophic events caused major human migrations. At https://www.livescience.com/archaeology/indigenous-mexicans-migrated-to-california-5200-years-ago-likely-bringing-their-languages-with-them-ancient-dna-reveals … genetic resarch using DNA from teeth and bones of ancient people in central and southern California, and in northern and western Mexico, shows up two major burst of migration. Thes coincide with major events. One at 3200-3000BC, the Piora Oscillation, and the other 4300 years ago = 2300BC. The latter one involved the introduction of maize farming but the first one involved a movement of hunter gatherer peoples.