American Oxen

Ok, so I just thought of a great ATL with a very simple POD. What if 11,500 years ago a small population of Euceratherium or "Shrub Ox" survive the Quaternary extinction event in the Central Valley of California, while harsh conditions reduce their population to only a few hundred individuals, those that survive (smaller than their Pleistocene ancestors) end up filling the roll of Aurochs to Pre-Colombian civilizations, providing a platform for an American species of dairy, beef, and working cattle, and thus giving Native Americans the means to develop much more advanced agrarian societies. HOWEVER, as CGP Grey outlined in his video "AmericaPox: The Missing Plauge" farming societies with large domesticated animals, tended to be a breeding ground for Plagues. 



SO, the survival of the Shrub Ox would lead to Native American Plagues. That means that in 1492 when Columbus landed in Haiti, he would find two things: An Aztec colony with cities and farms reminiscent of Mediterranean Europe AND a new disease that would come with him back to continental Europe while his diseases would spread across the New World (faster than in OTL thanks to shipping and trade caravans), reducing the GLOBAL population by upwards of 90%.

NOTE: Natives COULD conceivably travel to Europe in this TL, and there's those very good theories of Egyptians and Chinese exploration of the New World, to say nothing of the Vikings, but we're gonna assume that since the Americans would only be interested in traveling North to South, they'd have no reason to launch a Westward expedition across the Sea to find a new rout to a known source of goods.