The Horse @ Utah– July 19, 2014 - 1 THE HORSE The sound is unmistakable: the thundering hooves of a running horse. Horses have been racing across the landscape for more than 50 million years—much longer than our own species has existed. But once horses and humans encountered each other, our two species became powerfully linked. Humans domesticated horses some 6,000 years ago, and over time, we have created more than 200 breeds, from the powerful Clydesdale to the graceful Arabian. As we have shaped horses to suit our needs on battlefields, farms and elsewhere, these animals have shaped human history. They have also captured our imagination and hearts. Millions of people rely on horses as their spirited, dedicated, much adored companions. The Horse @ Utah– July 19, 2014 - 2 ANCIENT ORIGINS The Evolution of Horses Imagine a world in which horses of all colors, shapes and sizes roamed the world, some barely larger than a small dog. That world no longer exists—but once it was real. Today’s horses represent just one tiny twig on an immense family tree that spans millions of years. All the other branches of the horse family, known as Equidae, are now extinct. The earliest known horses evolved 55 million years ago and for much of this time, multiple horse species lived at the same time, often side by side, as seen in this diorama. Caption for touchable cast of Hyracotherium fossil WHAT IS IT? This ancient mammal, called Hyracotherium, is one of the earliest known members of the horse family. It lived around 55 million years ago in the forests of North America and Europe. Hyracotherium had four toes on the front feet and three in the back. Hyracotherium vasacciense Wind River Valley, Wyoming, USA 55 million years old Cast of American Museum of Natural History 4832 The Horse @ Utah– July 19, 2014 - 3 HORSE DIORAMA LABEL DECK #1 Label deck intro ANCIENT HORSES Some 10 million years ago, up to a dozen species of horses roamed the Great Plains of North America. These relatives of the modern horse came in many shapes and sizes. Some lived in the forest, while others preferred open grassland. Here, two large Dinohippus horses can be seen grazing on grass, much like horses today. But unlike modern horses, a three-toed Hypohippus tiptoes through the forest, nibbling on leaves. A small, three-toed Nannippus, shown here eating shrubs, ate both grass and leaves. In the background are several other large mammals alive at that time, including Procamelus, a camel relative; a herd of Dinohippus horses; Gomphotherium, a distant relative of true elephants; and Teleoceras, a hornless rhinoceros. Illustration of ancient horses A BRIEF HISTORY OF HORSES By 55 million years ago, the first members of the horse family, the dog-sized Hyracotherium, were scampering through the forests that covered North America. For more than half their history, most horses remained small, forest browsers. But changing climate conditions allowed grasslands to expand, and about 20 million years ago, many new species rapidly evolved. Some—but not all—became larger and had the familiar hooves and grazing diets that we associate with horses today. Only these species survived to the present, but in the past, small and large species lived side by side. The Horse @ Utah– July 19, 2014 - 4 Caption accompanying graph of weight over time CHANGING SIZES Horses were once much smaller than they are today. But there was not a steady increase in size over time. Little Nannippus, shown in the diorama at full adult size, was actually smaller than its predecessors. Mesohippus ~45 kg Merychippus ~100 kg Nannippus ~75 kg Equus ~500 kg Brief fact boxes about species in diorama with small illustrations or photos of models for quick identification Include a very small map of North America with Nebraska highlighted ABOUT THIS SCENE North America 10 million years ago In what is now the Great Plains DINOHIPPUS Single hoof Ate grass 11 to 4 million years ago The Dinohippus shown grazing on the left is a close relative of horses today. Like modern-day Equus, Dinohippus had single-toed hooves and ate mostly grass. The other extinct species shown here had three toes and never developed single hooves. Nannippus Three toes Ate grass, leaves, shrubs 13 to 2 million years ago Hypohippus Three toes Ate soft leaves 18 to 9 million years ago The Horse @ Utah– July 19, 2014 - 5 DIORAMA LABEL DECK #2 ON YOUR TOES Horses, humans and all other mammals share a common ancestor—one with five toes. So how did horses end up with single-toed hooves? Over millions of years, many horse species lost most of their side toes. The middle toe evolved into a single large hoof, while the other toes became smaller and ultimately functionless. Only one species in this scene, the grazing Dinohippus, has a single hoof. What’s the connection between hooves and grazing? Hooves and long legs help horses run farther and faster on the open prairie, helping them flee from predators and find fresh grass for grazing. In the forest, where the ground is softer, many horses retained three toes. STRETCH YOUR LEGS Comparing a human leg to a horse leg shows which bones give horses their great speed. Horses that moved onto grasslands have longer legs than their forest-dwelling ancestors. But their leg bones did not all lengthen equally. Mostly it was the bones of the foot that grew longer, with the ankle moving relatively higher up on the leg. Long, sturdy, light- weight legs help a horse run faster—a useful trait on the open prairie, where there’s no place to hide. The Horse @ Utah– July 19, 2014 - 6 ADAPTING TO A CHANGING CLIMATE Two major changes in climate affected the evolution of early horses. First, about 55 million years ago, global temperatures abruptly rose by 5 to 10 °C (9 to 18 °F), turning much of North America into a warm, wet, subtropical forest—much like what you’d see today in the Brazilian Amazonian rainforest. Small, leaf-eating horses thrived. Then, about 35 million years ago, global temperatures dropped, creating a climate more similar to today’s. Thereafter, dry grasslands replaced much of the North American forest, leading to rapid evolution among horses. By about nine million years ago, most forest browsers had disappeared, leaving primarily grass-eating grazers like those alive today. ABOUT THIS SCENE North America 10 million years ago In what is now Nebraska Dinohippus Single hoof Ate grass 11 to 4 million years ago Nannippus Three toes Ate grass, leaves, shrubs 13 to 2 million years ago HYPOHIPPUS Three toes Ate soft leaves 18 to 9 million years ago Long after hoofed, grass-eating grazers evolved and adapted to the American plains, three-toed forest browsers like the Hypohippus in this scene still continued to thrive for millions of years. This three-toed lineage is now extinct, but in the past many diverse horses lived side by side. The Horse @ Utah– July 19, 2014 - 7 Captions for fossil feet in display case HYPOHIPPUS Three-toed forest browser Hypohippini foot Nebraska, United States c. 12 million years old Hoof: American Museum of Natural History 60596 Foot: American Museum of Natural History composite NANNIPPUS Central hoof with two side toes Nannippus foot Kansas, United States 7–4.75 million years old American Museum of Natural History 87206 DINOHIPPUS Single-hoofed grazer Dinohippus leidyanus foot Edson Quarry, Kansas, United States 7–4.75 million years old American Museum of Natural History 116135 The Horse @ Utah– July 19, 2014 - 8 FOSSIL FEET – TOUCHABLE MODELS ON WALL ON TIPTOE THROUGH TIME The earliest horses had three or four functional toes. But over millions of years of evolution, many horses lost their side toes and developed a single hoof. Only horses with single-toed hooves survive today, but the remains of tiny vestigial toes can still be found on the bones above their hoofs. Can you find the tiny side toes on these horse feet? Captions for touchable casts: FOUR TOES Hyracotherium had four functional toes on its front feet and three on its back feet. Hyracotherium 55–49 million years ago Cast of American Museum of Natural History FM4832 THREE TOES Merychippus was three-toed. Some individuals had three full-size toes; on others, the two small side toes only touched the ground when running. Merychippus 18–11 million years ago Cast of American Museum of Natural History FM9383 ONE TOE Pliohippus walked on a large, central toe—the first true hoof. Pliohippus 15–6 million years ago Cast of American Museum of Natural History FM17225 The Horse @ Utah– July 19, 2014 - 9 MAP Map shows three waves of migration, each in a different color WHERE DID HORSES COME FROM? The majority of horse species evolved in North America. From there, they occasionally walked to other continents. This map shows how horses spread around the world at three different times. Color-coded map key: [green arrows] About 20 million years ago, three-toed horses called anchitheres crossed to Asia and continued to Europe and Africa. [yellow arrows] About 11 million years ago, three-toed horses called hipparions spread from North America around the globe. [brown arrows, bigger] About three million years ago, hoofed Equus, the ancestor of living horses, spread to several continents including South America. THE END OF AN ERA About 10,000 years ago, horses became extinct in North America and South America. Equus, the ancestor of all horses today, survived only in Eurasia and Africa. What ended their 55-million-year run in North America? The prime suspects are changes in the environment, disease and overhunting by humans who likely killed them for food.