In between the big kill-offs, there have also been many smaller, less well-known extinction episodes-the Hemphillian, Frasnian, Famennian, Rancholabrean, and a dozen or so others-which were not so devastating to total species numbers, but often critically hit certain populations. Grazing animals, including horses, were nearly wiped out in the Hemphillian event about five million years ago. Horses declined to a single species, which appears so sporadically in the fossil record as to suggest that for a time it teetered on the brink of oblivion. Imagine a human history without horses, without grazing animals.

- Bill Bryson, "A Short History of Nearly Everything"