ZOOHCC - 602: Evolutionary Biology (Theory) Unit:- 2














    Evolution of horse



    The evolution of horses is a fascinating subject that spans over 50 million
    years. The story starts with Eohippus, a small, fox-sized animal that lived
    in North American forests approximately 50 million years ago. Over time,
    these animals evolved into larger, more horse-like species, adapting to
    changes in their surroundings.



    One of the most notable adaptations during the evolution of horses was
    their shift from living in forests to thriving in grasslands. As grasslands
    became more widespread, horses evolved longer legs and stronger teeth to
    help them graze on tougher grasses. They also developed a single, elongated
    toe on each foot, which allowed them to run faster and more
    efficiently.



    Around 3 million years ago, horses began to resemble their modern-day
    counterparts. They had longer legs, larger body size, and a more complex
    digestive system to cope with their herbivorous diet. Equus, the modern
    horse, emerged approximately 2 million years ago and spread throughout North
    America, Europe, and Asia.



    Horses have played a vital role in human society, serving as
    transportation, agricultural tools, and even in warfare and sport.
    Currently, there are over 400 horse breeds, each with unique characteristics
    and uses. The evolution of horses is an extraordinary example of how species
    can adapt and evolve over time to survive in their changing
    environment.



    The evolution of the horse can be divided into several stages are:-



    The evolution of the horse can be divided into several stages or periods,
    each marked by significant changes in the horse's physical and behavioral
    characteristics. Here are the five main stages of horse evolution:



    Eohippus-



    This was the earliest known horse-like animal that lived about 50 million
    years ago. It was small, about the size of a fox, and had four toes on each
    front foot and three toes on each hind foot.



    The Eohippus was a forest-dwelling animal that lived in North America and
    Europe during the early stages of horse evolution. It was about the size of
    a small fox, with padded feet that allowed it to navigate through the woods
    safely. Its teeth were designed for chewing fruits, buds, and foliage.



    The Eohippus had a relatively simple brain and inflexible jaws. Its front
    hooves had four segments, while its hind hooves had three. At just 20 inches
    tall, it was the smallest known ancestor of the horse.



    Orohippus-



    This horse lived about 35 million years ago and had longer legs than
    Eohippus. It also had three toes on each foot, but the middle toe was more
    dominant.







    Mesohippus-



    This horse lived about 30 million years ago and had longer legs and a
    larger brain than Orohippus. It had three toes on each foot, but the middle
    toe was more dominant and had a hoof.



    The evolution of horses from the Eohippus to all subsequent forms took
    place in North America. Over time, the teeth of these animals became better
    adapted to specialized browsing, with most of them becoming uniform molars
    with crests. This dental adaptation persisted throughout the evolution of
    the horse and remains a defining characteristic of modern horses.



    During the Oligocene epoch, the Mesohippus emerged as a direct descendant
    of the Eohippus. The Mesohippus was slightly larger than its ancestor,
    standing at 24 inches high, and had longer, thinner forelegs. It had one
    less segment on its front hooves and a larger brain. Despite these changes,
    it retained the footpad and dental structure of its ancestors.



    Parahippus-



    The Mesohippus, which emerged during the Oligocene epoch, gave rise to the
    Miohippus, which in turn branched out into various lines. One of these lines
    led to the Parahippus, the first genus in the Equidae family to have a
    dental structure adapted for eating grass. This dental adaptation was
    characterized by a high crown with a large surface area for grinding.



    The Parahippus evolved in the grasslands of North America during the early
    Miocene period. This adaptation allowed it to take advantage of the
    widespread expansion of grasslands, and it became a successful and
    widespread genus in the Equidae family.



    Merychippus-



    This horse lived about 20 million years ago and was the first to have only
    one toe on each foot, which was elongated and supported by a single hoof. It
    was also the first horse to have teeth adapted for grazing.



    During the late Miocene period, the Merychippus emerged and stood at 40
    inches tall. One of the most significant changes in this genus was the
    fusion of the foreleg bones, which allowed for quick running. As the
    Merychippus evolved in grasslands, the footpad was lost, and a large central
    hoof on each leg emerged, connected to the ankle and foreleg by strong
    ligaments.



    This setup provided a spring-like action for galloping, and it has
    persisted in the evolution of horses until the present day. The Merychippus
    branched out into various lines, including the Pliohippus of the Pliocene
    period, which is a direct ancestor of the modern-day horse.



    Equus-



    This is the modern horse and includes all living horse species. The
    earliest Equus species appeared about 2 million years ago and had a larger
    brain and teeth adapted for grazing. Over time, the horse evolved to become
    larger, faster, and better adapted to its environment.








    These five stages represent major milestones in the evolution of the horse,
    and each one contributed to the development of the modern horse we know
    today.