A Horse is a Horse, Of Course, Of Course…As Long As You Know What a Horse Is: Part 1

Next week, I’m going to get back to actual misconceptions for Misconception Monday posts, I promise—but I am a “completer-finisher,” according to some workplace personality test I once took, so I need to round out this trio. Last stop on my mini-tour of you-can’t-show-that-in-textbooks-anymore topics: Horses!

Here’s the scoop: The horse fossil record extends back about 55 million years. Around 1870, paleontologist O. C. Marsh described a fossil horse, roughly dog-sized, then known as “Eohippus.” Eohippus (today known as Hyracotherium) represented the far end of a line of fossils that seemed to show a gorgeous and gradual transition from tiny, 4-toed (on the front limbs), low-toothed, forest-dwelling horses to the mighty, 1-toed, high-toothed, plains-dwelling modern horses.

Marsh's diagram for Huxley's talk on horse evolution. (public domain via Wikimedia Commons) “Darwin’s Bulldog” Thomas Henry Huxley visited Marsh at Yale University in 1876 and was so impressed by his collection of horse fossils (most from the Americas, which turned Huxley’s Europe-centric view of horse evolution on its head) that he rewrote a speech he was to give in New York on the topic. Marsh developed a visual aid for Huxley’s lecture, upon which the American Museum of Natural History based an exhibit. Both the diagram and the exhibit were designed to show the trends toward modern equinity: increasing size, decreasing number of toes, increasing height of the crown of the tooth. There was no branching in this version of events, rather, one horse evolved in to the next horse form until the pinnacle of horse evolution was achieved—the living horse. The world had a perfect example of gradual, directional, evolutionary change. Diagrams of the progression appeared in every biology book—including the one John Scopes got into so much trouble for teaching from.

Stephen Jay Gould wrote about this in an essay called “Life’s Little Joke.” Gould explains that at the time, Cope and Huxley had the trunk of the evolutionary lineage (Hyracotherium) and the “surviving twig”: modern horses. The mistake the scientists made was in relegating known fossils that did not fit the pattern to “side branches” of little to no consequence to overall horse evolution. Huxley went even further, and claimed that vertebrates as a whole followed a similar straight-line progression. The scala naturae had struck again.

The problem is that paleontologists kept finding fossil horses. Herds of them. In fact, Hyracotherium vasacciensis at the American Museum of Natural History in New York. (Daderot via Wikimedia Commons) today, horses are one of the best-documented groups in the fossil record. And it turns out that the once supposedly inconsequential side branches were numerous and richly diverse. The simple straight-line example of evolution wasn’t so straight after all. As collections grew and scientists were able to shed their preconceptions, it became clear that the evolution of horses was a bush made up of numerous branching lineages, not a tree with one directional trunk. Some branches of the bush showed a general trend toward decreasing size, not increasing, and horses with three toes co-existed with horses with one toe.

The diagram of horse evolution today is often shown in all of its diverse glory. And I think, for the most part, no one has any problem with it. But if you search for “horse evolution” you’ll still find plenty of the archaic-style diagrams and plenty of people who claim (incorrectly) that those simple diagrams represent the scientific understanding of horses today. These same people then go on to use the diagrams as another example of scientific folly.

I’m not particularly interested in defending evolution against those that pick a fight with a 140-year-old diagram, so I won’t. Instead, I’m going to argue that with a few tweaks, diagrams showing a limited view of horse evolution, trends and all, are totally okay. Stay tuned for part 2, later this week, to find out why.