Friday, March 14, 2014

THE HISTORY OF THE HORSE

Once upon a time, horses were really little, like the size of foxes.  This is very hard to believe, but it's true.  Then, over millions and millions and millions of years, they evolved to be bigger.  Their legs got longer, and they started having just one toe instead of 3 or 5, and their teeth got better at eating grass.  And eventually they turned into the animals we know today as horses.  And that's it, the entire history of the horse in one fairly short paragraph.

Artwork by Danielle Byerley

But if you want a few more details, you can go on reading.  However, I am not going to tell you everything there is to know about the horse's history because when I started doing my in-depth research on this topic, I found out it was a lot more complicated than I expected it to be.  Scientists used to think the evolution of the horse was a straight-line sort of thing, and so they were shocked to learn that it was more like a complicated bunch of tree branches.  But the horse's evolutionary story is still more complete than that of any other animal, and I guess there's something to be said for that.

Okay, so horses belong to a group called perissodactyls, which are hoofed mammals (ungulates) with an odd number of toes.  In this same group there are also animals that have a similar tooth structure and mobile upper lips.  Which means that horses are actually more closely related to rhinos and tapirs than they are to deer and cattle.


So anyway, these little perissodactyls lived in the tropical forests, and they browsed on bushes and trees and stuff like that.  The ground in the forests was moist, so it was helpful to have several spread-out toes to walk on it.  But after a while, some new plants called grass appeared, and it grew on drier steppe land.  Grass was harder to chew than foliage, so horses started developing larger, stronger teeth.  And on the open steppes, the horses needed longer legs, so that they could outrun their predators instead of just hiding.  So the horses' legs bones gradually became longer, and the weight of the animal shifted to the third toe, while the other toes lifted off the ground.


The evolution of the horse happened in North America.  Paleontologists all seem to agree that the very earliest ancestor was the Eohippus, or "dawn horse."  Of course, scientists always like to change the names of things, just when you finally learn the original name, so now Eohippus is called Hyracotherium.  This new name means "shrew-like beast," which sounds much less noble than "dawn horse," if you ask me.

Hyracotherium.  Artwork by Heinrich Harder, public domain

Hyracotherium appeared in the early Eocene age, about 52 million years ago.  It weighed less than 50 pounds, and it had 4 toes on the front feet and 3 on the back.  It looked sort of like a little deer, but you could tell it was a perissodactyl because it put most of its weight on a single toe of each foot.  Hyracotherium had a small brain, and it ate soft foliage and fruit.  But after 20 million years or so, it began to develop teeth that were more suited for browsing.

Orohippus.  Artwork by Bob Strauss, public domain

This change in teeth was part of what led Hyracotherium to evolve into Orohippus, about 50 million years ago.  Orohippus means "mountain horse," but I don't know why it was named that because the animal didn't live in the mountains and it wasn't actually a horse.  It was about the same size as Hyracotherium, but had a longer head, slimmer body and forelimbs, and longer hind legs.  Also, Orohippus didn't have the outer toes that Hyracotherium had.

Next came Epihippus, about 47 million years ago.  This animal had teeth with well-formed crests that worked very well for grinding.  Epihippus was about 2 feet tall.

Mesohippus.  Artwork by Heinrich Harder, public domain

By the Oligocene, 32-24 million years ago, the climate of North America had become drier, and grasses and prairies were developing.  In response to all these changes, Mesohippus ("middle horse") developed and became one of the most widespread animals.  It walked on 3 toes, but the middle toe was bigger and stronger.  It had a bigger brain than its ancestors, but was still only about 2 feet tall.


Miohippus, the "lesser horse" emerged about 36 million years ago.  It lived alongside Mesohippus for about 4 million years and then replaced Mesohippus.  After a while, the temperatures got lots colder in North America, and most animals moved south.  But one group called Plesippus crossed the Bering land bridge into Eurasia about 2.5 million years ago.  Plesippus was about as tall as an Arabian horse today, but it had a very stocky build.  This branch of the horse family tree went extinct about 8,000 B.C.

Plesippus

Eventually, all the ancestral horses died out in North America, and horses didn't come back to the continent until the Spaniards brought them in 1493.  The native people did not have a word for "horse" because they had never seen one before.  So they gave them names like "elk-dog" or "big dog" or "seven dogs," (because one horse could pull the same amount of weight as seven dogs).


After a while, some of the Spaniards' horses got loose or lost or stolen or somehow ended up being feral out on the prairies.  And that is how mustangs were invented.  The adoption and use of horses changed the lives of the American Indians in lots of important ways, but that is a story for another day.


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