Charles Darwin began to formulate his theory of evolution by natural selection (see Mammal Ancestry) while on a five-year (1831 to 1836) journey around the globe aboard the HMS Beagle. While on this cruise of discovery, Darwin stopped in South America and unearthed a fossil of an extinct form of horse. How could a horse have lived in the Americas long before the Spaniards introduced the animal during their 16th-century conquests? More than a century would pass before scientists figured out what was behind Darwin's discovery and were able to show how the horse — and other mammals — came to be distributed across the continents.

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