图绘原始马--来自美国科学研究

Hyracotherium This genus of small early horse roamed the early woodlands of Asia, Europe and North America some 55 million to 45 million years ago. It was already larger than Sifrhippus, weighing about 22.7 kilograms. But when Richard Owen first discovered Hyracotherium in 1876, it was so diminutive that he thought it was some unknown hyrax species, a group of extant mammals that live in Africa and the Middle East.

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Mesohippus This early horse relative lived in North America about 40 million to 30 million years ago. Mesohippus (or "middle horse") was about as tall as Hyracotherium but had developed a larger brain that more closely resembled that of a modern horse.

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Hipparion Galloping across four continents for nearly 12 million years (23 million to 781,000 years ago), the Hipparion genus was by all accounts extremely successful. Termed "pony" in Greek, these horses were indeed close to modern-day pony size, weighing between 63 and 119 kilograms and standing about 1.4 meters at the shoulder.

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Hypohippus Strutting through the ancient western states of Colorado, Montana and Nebraska some 17 million to 11 million years ago, Hypohippus was a three-toed early equine. As its name "low horse" indicates, it was also not much bigger than a pony.

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Sifrhippus The Sifrhippus sandrae is one of the earliest-known North American horse relatives. It lived during the Eocene epoch, some 55.6 million years ago. It was also one of the smallest. When it trotted onto the scene, it weighed just 5.6 kilograms. But it was destined to shrink even further--down to house cat-size of about 3.9 kilograms. It likely got smaller to adapt to warmer temperatures during the Paleocene-Eocene Thermal Maximum, some 56 million years ago, according to research published Thursday in Science.

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Orohippus The Orohippus--or "mountain horse" genus also emerged during the Eocene about 50 million years ago. These lean-legged horse ancestors were about the same size as Hyracotherium.

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Hippidion Hippidion, or "little horse," genus was one of the ancient genera that lived into the current Holocene epoch, existing in South America two million to about 10,000 years ago. Like many ancient horses, t stood less than 1.5 meters high and was about the size of a Welsh pony. Genetic analyses have suggested that Hippidion might fit within the extant horse genus Equus.

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