In the 1870's, the paleontologist O.C. Marsh published a description of newly discovered horse fossils from North America. At the time, very few transitional fossils were known, apart from Archeopteryx. The sequence of horse fossils that Marsh described (and that T.H. Huxley popularized) was a striking example of evolution taking place in a single lineage. Here, one could see the fossil species "Eohippus" transformed into an almost totally different-looking (and very familiar) descendent, Equus, through a series of clear intermediates. Biologists and interested laypeople were justifiably excited. Some years later, the American Museum of Natural History assembled a famous exhibit of these fossil horses, designed to show gradual evolution from "Eohippus" (now called Hyracotherium) to modern Equus. Such exhibits focussed attention on the horse family not only as evidence for evolution per se, but also specifically as a model of gradual, straight-line evolution, with Equusbeing the "goal" of equine evolution. This story of the horse family was soon included in all biology textbooks. As new fossils were discovered, though, it became clear that the old model of horse evolution was a serious oversimplification. The ancestors of the modern horse were roughly what that series showed, and were clear evidence that evolution had occurred. But it was misleading to portray horse evolution in that smooth straight line, for two reasons: # First, horse evolution didn't proceed in a straight line. We now know of many other branches of horse evolution. Our familiar Equus is merely one twig on a once-flourishing bush of equine species. We only have the illusion of straight-line evolution because Equus is the only twig that survived. (See Gould's essay "Life's Little Joke" in Bully for Brontosaurus for more on this topic.) # Second, horse evolution was not smooth and gradual. Different traits evolved at different rates, didn't always evolve together, and occasionally reversed "direction". Also, horse species did not always come into being by gradual transformation ("anagenesis") of their ancestors; instead, sometimes new species "split off" from ancestors ("cladogenesis") and then co-existed with those ancestors for some time. Some species arose gradually, others suddenly. Overall, the horse family demonstrates the diversity of evolutionary mechanisms, and it would be misleading -- and would be a real pity -- to reduce it to an oversimplified straight-line diagram. With this in mind, I'll take you through a tour of the major genera of the horse family, Equidae. CAUTION: I will place emphasis on those genera that led to the modern Equus. Do not be misled into thinking that Equus was the target of evolution! Bear in mind that there are other major branches of the horse tree that I will mention only in passing. (See the horse tree for a lovely ASCII depiction.) Small preface: All equids (members of the family Equidae) are perissodactyls -- members of the order of hoofed animals that bear their weight on the central 3rd toe. (Other perissodactyls are tapirs and rhinos, and possibly hyraxes.) The most modern equids (descendents of Parahippus) are called "equines". Strictly speaking, only the very modern genus Equus contains "horses", but I will call all equids "horses" rather indiscriminately. Most horse species, including all the ancestors of Equus, arose in North America. visit http://www.talkorigins.org/faqs/horses/horse_evol.html for more info

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