The evolutionary history of the horse is one of the most-covered
subjects in modern biology. And no wonder - of all modern
animals, the horse has behind it the most intact and visible
family tree.Our story begins millions of years ago - with
No, not "pterodactyl" - those clawed flying relics of the
dinosaur age as imagined in B-movies and The Flintstones.
The "Perissodactyls" are hoofed animals with an odd number
of toes on each foot (they are also distinguished by their
tooth structure); this group of animals is itself, say scientists,
descended from the same ancestor as the tapir and the rhinoceros
but, unlike these animals, gradually adapted to life on drier
land than the tropical forests preferred, even today, by the
One creature's evolution often influences that of other creatures
in its environment, and this was true of the equids (the horsey
branch of the Perissodactyl family tree), who began eating
grass as this new crop began to flourish. Such a diet favored
the spread of new sorts of equids who had larger teeth.
Likewise, the equids - adoption of a dry, steppe-like habitat,
where predators lived and where the comparative lack of foliage
made it harder to hide, encouraged the survival of those equids
who ran the fastest. Gradually longer-legged equids with a
long third toe (which allowed for greater running efficiency)
began to predominate. The Mesohippus species of 40 million
years ago reflect this trend.
It's a common - but disastrous - mistake to see evolutionary
history as a smooth straight-line progression from early to
middle to modern versions of an animal, with the modern animal
taken as the final copy of the earlier animals' rough draft,
as if we were viewing successive sketches of Michelangelo's
David in a line that ended with the real statue.
In fact, though, most equid species lived their day and died,
without having any influence on today's horse; they existed
in their own right, and we shouldn't think of the modern horse
as the "goal" of all this equine living and dying. Many genealogical
lines simply ran out, while one (leading to our horse) happened
to survive; but it could as well have been any, or all, of
the others, given slight modifications in some habitat a million
years ago or so.
In any case, of the many horselike species whose fossils have
been found, it's thought that Plesippus - a species descended
from the earlier Dinohippus - is the father of the modern
horse. This species responded to falling North American temperatures
by heading, either to South America or across the Bering Strait
from North America to Eurasia, about 2 and a half million
years ago, with a few staying behind in North America.
Somewhere toward the end of the Tertiary period or at the
beginning of the Quaternary - that's scientists' talk for
the beginning of the most recent Ice Age, roughly 1.8 million
years ago - descendants of Plesippus gave rise to offspring
different enough from their sponsors, and like enough to our
modern horses, that scientists have dubbed them Equus stenonis,
the first "true" horse.
They crossed into North America and survived for millions
of years, perhaps giving rise to the other ancient horses
known to have inhabited the area during this period - the
super-sized Equus scottii giganteus, whom the present author
swears he is not making up (they seem to have exceeded modern
horses in size). But all North American horses died out, rather
inexplicably, around 11,000 years ago - at the same time as
many other kinds of animals, and for reasons scientists have
yet to discover. Was it some mega-virus of the ancient world?
Or, a more tantalizing possibility, did humans (arriving on
the North American scene, according to some theories, at about
this time) hunt them to extinction?
In any case, horses had no purchase on this continent until
after European colonization of the Americas began in 1492;
for this long period, then, from 11,000 BCE to 1491, the horse's
development took place in Eurasia instead. (Another tantalizing
thought - after the colonizers had reintroduced horses to
Mexico, the southwestern US, and Peru, some indigenous tribes
told stories about how "the grass remembers" these new animals.
Did these people groups retain some memory, perhaps through
myth and legend, of the long-gone North American horse?)
The outline of horse-history given here is just one sketch,
based on one strand of scientific theorizing. Like those ancient
Perissodactyls giving rise to many species of not-quite-a-horse,
most of which flourished in their time and died without contributing
in any way to the development of modern horses, scientific
speculation as to the origin of any species will include many
interesting, intelligent "dead ends." So who knows.
A popular theory, the "Four Foundations" theory, suggests
that at some point long predating the horse's disappearance
from North America, four basic types of horses developed in
Europe (from those Plesippi, perhaps, who crossed from North
America to Eurasia before the last Ice Age began). Warmblooded,
forest-dwelling horses and draft horses of northern Europe,
plus taller, slimmer Asian horses and pony-sized Tarpans,
are considered, in this theory, to be the "basic" horses from
which all others are descended.
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