The species Equus evolved in the Americas, died out here around 8,000-10,000 years ago. Obviously, early Equines did not need boats to cross over into Asia, but they did. They were eventually domesticated by humans around 5000 years ago. They came full circle when they arrived back in the Americas with Christopher Columbus in 1492.
Human beings have always been creative, insightful and knowledgeable about how to manipulate the natural world to their benefit.
I would pay money to see anyone attempt it today, using nothing that wasn't available 40,000 years ago.
Haven't you ever heard of Thor Heyerdahl? In 1947, in order to test his theory about how Polynesia and the Hawaiian Islands may have been populated, Heyerdahl built a replica of an aboriginal balsa raft (named the "Kon-Tiki") using only materials available to ancient Peruvians. Heyerdahl and five companions left Callio, Peru and crossed 4300 miles in 101 days to reach the Raroia atoll in the Tuamotu Archipelago. Despite skepticisim, the seaworthiness of the aboriginal raft was thus proven and showed that the ancient Peruvians could have reached Polynesia in this manner.
Of course I have. But I'm not sure it's relevant to 40,000 years ago or 15,000 years ago, and the direction is exactly opposite of where the migration would have had to come from.
You're not going to be able to make a "Kon-Tiki" argument for any pollination of humans into South America. Or maybe you can.
I don't care who is right or wrong. I want to know what happened.
By the way, the horse thing has always puzzled me.