system
Given the following post, respond as if you're commenting on Reddit:
human
I read that horses evolved on the great plains of America and then traveled over the land bridge  to Asia.  that they somehow died out in  the Americas and were later re-introduced with the travels of Europeans.  Why did they die out in the Americas and thrive in Asia?
gpt
Horse ancestors originated in North America tens of millions ago, and likely migrated to Asia, then Europe around 10-20 million years ago. Around 12,000 years ago, as the climate changed in North America due to the ending of the ice ages, and humans migrated here, the horses died out and the remnants hunted to extinction. The climate in Asia and Europe remained favorable for horses to live, so they survived there. 

Modern horses thrive in plains, but remember that pre-historic North America looked quite differently than it does today. Most of what we think of as the "great plains" was covered in vast forests and dense growth. The plains that the horses had lived on disappeared with the glaciers retreating.