In Greek myth the winged horse Pegasus was actually ridden by the hero Bellerophon rather than by Perseus, yet Shakespeare’s words neatly capture the striking combination of supernatural power and tractability that is the horse. This chapter picks up these themes by developing three topics: it describes the evolution of the modern horse, Equus caballus, identifies key features of its biology relevant to subsequent discussions, and reviews the history of human–horse interaction in the Old World, emphasizing the horse’s domestication and subsequent spread. Horses and their relatives, the wild asses and zebras, were once seen as an almost paradigmatic example of how evolution works, although more recent research has shown that their history is more complex and multi-branched than originally thought. Along with tapirs and rhinoceroses, they belong to the taxonomic order Perissodactyla, the odd-toed division of the ungulates or hoofed mammals. The superficial similarities that they share with even-toed antelopes, which belong to the order Artiodactyla, are thus largely the result of evolution converging on similar body plans. In fact, some genetic studies suggest that perissodactyls are closer to carnivores than to the artiodactyls. Like modern tapirs and rhinoceroses, the earliest horses were three-toed, but for the past 40 million years or so all have borne their weight on just the third toe, with ligaments, rather than a fleshy pad, for support. Subsequently, the central metapodial (the bones connecting the digits to the wrist or ankle) was considerably elongated to form a long, slender lower limb and the second and fourth digits were minimized, though still giving support when galloping and jumping. Beginning around 10 million years ago, in the late Miocene period, the remaining side toes were reduced to splints and the animal’s weight came to be carried entirely on a single enlarged hoof. The first perissodactyls were browsers, not grazers. Some 45–34 million years ago, however, temperatures fell at higher latitudes and climate became more seasonal: successful ungulates evolved new adaptations, including the first appearance of both ruminants (which ferment their food in a specialized foregut) and new kinds of ancestral horses such as Mesohippus and its successor Miohippus.
Keeping Pace with the History of Evolving Runtime Models
