@nemo Wait. We didn't think that before?
In Texas History class (yes it's a full year in middle school) we learned about the Plains Indians (forgive me if there is better terminology now; it's been 25 years since I was in that class). And in order to have fully developed economy/society revolving around horses by the mid 1700s, they had to have horses well before that. If you'd asked me yesterday when the Comanche and Apache people got horses, I'd have guessed sometime between 1550 and 1650.
What could possibly have made previous experts conclude it was later than that?
Heck, the Spanish governor of New Mexico made a trip to California in 1605 (left an inscription at El Morro). Clearly they had horses. Their indigenous guides had horses. And horses breed FAST. So if southwestern tribes acquired a few, of course they would spread (and some would escape and breed too). And high plains are *right there*. What mechanism did scientists propose to explain why this wouldn't happen?