My Year of Darwin 3/28/2014: Horses; here, there, then everywhere

Certainly it is a marvellous fact in the history of the Mammalia, that in South America a native horse should have lived and disappeared, to be succeeded in after ages by the countless herds descended from the few introduced with the Spanish colonists! -Charles Darwin, Voyage of the Beagle

Although the oldest horse ancestor (on the lineage that leads just to modern horses) was found in London, the 50 million years of subsequent horse evolution occurred in North and South America, with many contemporary species living together in North America. Just as humans may have crossed the Bering Straight to reach North America. Modern horse ancestors went the opposite direction to colonize Asia. 

http://www.amnh.org/exhibitions/past-exhibitions/horse/the-evolution-of-horses

A nice web page on horse evolution is (got to love a page called Stratmap) here: http://www.flmnh.ufl.edu/natsci/vertpaleo/fhc/Stratmap1.htm