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Exercises · 6.10

Q.Using various resources such as your school Library or the internet and discussions with your teacher, trace the evolutionary stages of any one animal, say horse.

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The evolution of the horse, traced through fossil evidence, shows a clear trend from a small, multi-toed forest-dwelling ancestor to the large, single-toed grassland grazer we know today — a classic example of adaptive radiation over 50 million years.

The story of the horse’s evolution is one of the most complete and well-documented fossil sequences in palaeontology. It is not a straight line of progress, but a branching bush of experiments, with only one lineage surviving to the present. The NCERT textbook uses the horse as a prime example to illustrate how environmental change drives anatomical adaptation over deep time.

The earliest known ancestor of the modern horse is Eohippus (also called Hyracotherium), which lived about 55 million years ago during the Eocene epoch. This animal was no larger than a fox, with a short neck, a curved back, and a long tail. Its legs were short and ended in four toes on the front feet and three on the hind feet — each toe tipped with a small hoof. Eohippus lived in warm, swampy forests, browsing on soft leaves and fruits. Its teeth were low-crowned (brachydont), suited for chewing tender vegetation, not tough grasses.

Note

The name Eohippus means “dawn horse,” reflecting its status as the earliest known horse ancestor. The NCERT textbook refers to it as the starting point of the horse evolutionary series.

As the climate gradually cooled and dried over millions of years, forests gave way to vast grasslands. This environmental shift placed new selective pressures on horse ancestors. The next major stage, Mesohippus (about 40–30 million years ago), was larger — about the size of a sheep — and had three toes on each foot. The middle toe was larger and bore most of the weight, while the side toes were smaller and often did not touch the ground. This change improved running speed on open plains. The teeth of Mesohippus also began to show higher crowns (hypsodonty), an adaptation for grinding gritty, silica-rich grasses.

Important

The shift from browsing to grazing was the single most important driver of horse evolution. Grasses contain abrasive silica particles that wear down teeth, so higher-crowned teeth became essential for survival.

By the Miocene epoch (about 20–10 million years ago), horses like Merychippus appeared. These were about the size of a small pony, with longer legs and a single functional hoof on each foot, though side toes still existed as small splints. The skull became longer, and the eyes moved farther back, giving a wider field of vision — crucial for spotting predators on open plains. The teeth were now fully high-crowned and covered in cementum, an extra layer of protection against wear.

The final stage before the modern horse, Pliohippus (about 5–2 million years ago), was the first truly one-toed horse. The side toes had completely disappeared, leaving only the enlarged middle toe encased in a single hoof. The legs were long and slender, built for sustained running. The teeth were extremely high-crowned, and the jaw was deep to accommodate them. …

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