Book Review: The Rise of Horses

Almost two decades ago vertebrate paleontologist Bruce MacFadden published his monograph Fossil Horses, an instant classic that was as much about new approaches in paleontology as the equids considered in the book. For over a century the family history of horses had been depicted as some of the best, most-accessible evidence for evolution the fossil record had to offer, and MacFadden's book provided an excellent synthesis of what had been discovered. Since the publication of Fossil Horses, however, no other books have appeared to follow-up on what MacFadden presented. Brief nods and short summaries have appeared here and there, but it seemed that MacFadden's book had the last word on horse evolution.

That is why I was excited when I found out that Johns Hopkins University Press would be publishing a translation of Jens Franzen's The Rise of Horses (previously published as Die Urpferde der Morgenröte: Ursprung und Evolution der Pferde). It was intended for a popular audience rather than an academic one, but I still was looking forward to seeing how Franzen summarized what we now know about horse evolution. As soon as the book arrived I dropped everything else and opened to the first glossy page.

Like other popular treatments of horse evolution, such as G.G. Simpson's Horses, Franzen's book begins with an overview of our interactions with horses. They have been food, our partners in war, beasts of burden, and athletic teammates, but where, exactly, did they come from?

As has been known since the late 19th century, the first horses evolved over 55 million years ago in North America, but Franzen does not begin with the familiar little "dawn horse" Eohippus. Instead he sticks closer to his area of expertise; the exceptionally preserved fossils of Germany's Messel pit. This site was the home of the petrified primate "Ida" (which Franzen helped describe), and during her time fossil horses (Eurohippus) and their close relatives (Propalaeotherium) browsed among the lush undergrowth of the ancient forest. These were not the ancestors of the modern horse, but Franzen's summary of the Messel discoveries and his own work is entertaining.

Unfortunately, however, the succeeding chapters become mired by a disorganized presentation. Rather than providing historical context by introducing readers to the array of known fossil horses, Franzen instead covers functional aspects of their evolution first, such as changes in teeth and number of toes. The transitions seen in these features are thus presented out of their historical context and may make them more difficult to understand. Additionally, Franzen ascribes these changes to the ill-defined consequences of what he calls the "Frankfurt theory of evolution" in which the internal drive towards energy efficiency is more important to evolutionary change than natural selection. I had never heard of this concept before, and I must admit that Franzen's description of it only left me confused.

Franzen's treatment of trends in horse evolution is also perplexing in that he presents it in a linear fashion, only to later state that the actual pattern was much more bushy. I guess the old habit of showing horses moving up an evolutionary ladder dies hard. Likewise, Franzen states that horses followed "Cope's Rule", or exhibited a trend towards becoming larger over time. This is unfortunate because it largely ignores the work of Bruce MacFadden in showing that horses do not, in fact, show a unidirectional trend towards larger size. For the majority of their evolutionary history horses stayed small, and even after these small types were wiped out a little more than 10 million years ago several lineages of larger horses became dwarfed.

What makes these discussions more difficult to understand is that The Rise of Horses does not present an outline of horse evolution until the closing chapters of the book. The types of fossil horses, the habitats in which they lived, and what their closest relatives were do not get addressed until after the "trends" in horse evolution are covered. By saving the history of horse evolution until the end, readers without a background in vertebrate paleontology might feel a little lost in the middle of the book. I recommend reading the last chapters first and then going back.

Other small errors and inconsistencies also detract from the book's overall appeal. The Rise of Horses is not a trainwreck, not by any means, but it simply does not live up to its full potential. (Perhaps Franzen could have benefited from being teamed up with an experienced science writer.) Although the opening chapters on Messel are interesting the rest of the book left me disappointed. The disorganization and small errors made it more difficult to read than was necessary, and overall The Rise of Horses does not contain enough new information to make me prefer it over MacFadden's book.

Then again, I cannot blame Franzen for covering familiar material. While it has not stopped entirely, research into fossil horses has slowed in recent years. There are only a handful of paleontologists who study them anymore despite lingering questions about how horses evolved. By combining techniques from paleontology, genetics, and evo-devo scientists could better understand how modern horses came to be as they are, but the interest in doing so appears to have flagged. I sincerely hope this changes. We will be in a sorry state indeed of The Rise of Horses remains the most up-to-date book on horse evolution twenty years hence.

[Thanks to Johns Hopkins University Press for providing me with a review copy of The Rise of Horses/]