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 the Perissodactyls.
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 rhino.
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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