More than 50 million years ago, a small fox-sized animal crept
through the forests of North America, browsing on fruit and leaves. Its
arched-back body was only about a foot high at the shoulder, and a long tail
and short-snouted head probably gave it a distinctly dog-like look. In fact,
its feet sported pads like a dog’s, except each toe ended in a tiny hoof
instead of a claw. Interestingly, in modern horses, one toe has become the
hoof, and the others remain as vestigial bumps higher up the leg.
When fossil hunters first discovered the bones of this creature
a century ago, they named it Eohippus — “the dawn horse” — and believed it was
the first link in an evolutionary chain that led directly to today’s horse.
Indeed, many museums and textbooks still have displays and pictures showing
this neat, predictable progression, with horses gradually getting larger,
shifting from many toes to modern hooves, and gaining longer teeth able to
grind down tough prairie grasses.
These
days, however, researchers have a far more complex picture of horse evolution —
and they have given the dawn horse a much less colorful name. While they agree
that today’s horse probably arose from that smaller ancestor, the path was by
no means direct. Instead, paleontologists have uncovered fossils that show that
horse ancestors varied in size: some large early horses gave way later to
smaller ones. They also discovered that some lines of horse-like animals
alternated between many and few toes over time. In addition, some proto-horses
once thought to be direct forefathers of the modern animals were revealed to be
distantly related cousins — just one dead-end branch on a bushy family tree.
Horses' anatomy enables them to
make use of speed to escape predators and they have a well-developed sense of balance and a
strong fight-or-flight
response. Related to this need to flee from predators in the wild is
an unusual trait: horses are able to sleep both standing up and lying down.
Female horses, called mares,
carry their young for approximately 11 months, and a young horse, called a
foal, can stand and run
shortly following birth. Most domesticated horses begin training under saddle or in harness between the ages
of two and four. They reach full adult development by age five, and have an
average lifespan of between 25 and 30 years.
