Wild Horses: Equus Family Tree

Origins of the Horse

EQUUS FAMILY TREE

 

There are those, including paleontologist Michael Voorhies (watch this video), who characterize the evolution of horses as more like a bush than a tree, with starts and stops and jumps in the development of genetic traits. This development took place over the course of at least 55 million years and covered at least five sub-periods of geologic time. Paleontologist Kathleen Hunt has suggested the following evolutionary tree, with the major development path in copper lines and the geologic periods highlighted.

EOCENE, 55 to 38 million years ago

Fifty-five million years is a long time, but it is nothing compared to the scope of geologic time.  It is estimated that the Earth settled down to its present size between 4 and 5 billion (that’s Billion with a B) years ago!  Earliest life forms developed about 2 billion years ago.  The earliest fossilized fishes date about half a billion (or 500 million) years ago.

So, horse ancestors were relative latecomers, appearing around 55 million years ago.  This time period was known as the Eocene epoch of the Tertiary period.  In the Western Hemisphere, current mountain ranges, like the Rockies, were just finishing being thrust up.  The climate in most of the lowlands was subtropical, moist with palm trees, with alligators living as far north as Nebraska and the Dakotas.

Magnolias and fig trees flourished in Alaska.  Forests covered much of the land.  Ancestral forms of the horse, rhinoceros, camel, and other mammals appear in the fossil records from this period.  Only very primitive “protohumans” were alive at this time.  The animals that flourished during this period had to be able to eat fruits and soft foliage from trees.

OLIGOCENE, 38 to 24 million years ago

By this period, mammals were the dominant form of terrestrial life.  A gradual, long-term, cooling trend began in North America about 38 million years ago and would last through the Pleistocene Ice Ages.  The climate in North America gradually became drier, and vast grasslands replaced the forests.

This change in climate was a critical element in the evolution of horse species, because they were the first animals to take advantage of the new habitat and source of food.  The new species began to develop tougher teeth to grind up the grasses.  They developed longer legs as it became more important for animals to run and escape predators. Horses began to grow larger for more strength.

MIOCENE, 24 to 5 million years ago

During the Miocene epoch, the evolution of the horse was accelerated and split into various branches.  Global cooling continued.  The ice sheet covering Antarctica formed.  In North America, horses shared grassy prairies with rhinoceros, camels, cats, mastodons, and raccoons.

As this third line of Miocene horses began to specialize in eating grasses, several changes occurred.  First, the teeth became better suited for chewing harsh, abrasive grass.  Small crests on the teeth enlarged and connected to become a series of ridges for grinding.  There was a gradual increase in the height of the tooth crowns, so that the teeth could grow out of the gum continuously as the tops were worn down (Hypsodont teeth).  And, in addition, the tooth crowns became harder due to the development of a cement layer on the teeth.

Second, these horses started to become specialized runners.  There was a simultaneous increase in body size, leg length, and face length.  The leg bones began to fuse together, and along with their musculature, became specialized for efficient forward-and-backward strides, with flexible leg rotation eliminated.  Most significantly, the horses began to stand permanently on tiptoe (another adaptation for speed). Instead of walking on dog-like pads, their weight was supported by springy ligaments that ran under the fetlock to the big central toe.  All of these changes occurred rapidly, and we are lucky to have a fairly good fossil record during this time, one of the most interesting in horse evolution.

Throughout the evolution of these Merychippine descendants, the facial fossa became deeper and more elaborate.  With so many equine species overlapping at once, these facial fossae may have housed species-specific glands of some sort, similar to the scent-marking glands of modern antelopes and deer.

PLIOCENE, 5  to 1.6 million years ago

The Pliocene period saw the development of the first species that could be considered true equines as well as the first primate species that could be considered direct ancestors of Homo sapiens or humans.  Mammals had long since established themselves as the dominant terrestrial life form, and the cooling trend continued.  It was also during this period that horses migrated from North America to the other continents of the globe.

In general, the horse species got larger and developed its ability to exploit grass as its primary diet.  Most still had small side toes, but at least two separate groups of horses lost those.  Instead , they developed side ligaments around the fetlock to help stabilize the central toe during running.  The earliest know Equus species entailed a set of three simple Equus species.  They still had some primitive traits from Dinohippus, including a slight facial fossa.  They had zebra-like bodies (stocky with straight shoulders and thick necks) and had short, narrow skulls like donkeys.  They probably had stiff, upright manes, ropy tails, medium-sized ears, striped legs, and at least some striping on the back.  These are all traits shared with modern equines.  They quickly diversified into at least 12 new species that coexisted with other one-toed horses that were evolving on their own paths.

Late in the period (about 2.6 million years ago), glaciers began to descend on North America, and the horses began to migrate to other continents.  They probably crossed over the Bering Land Bridge between what is now Alaska and Russian Siberia.  Some spread as desert-shaped onageers and asses across Asia, the Mideast, and North Africa. Others entered Africa and diversified into modern zebras.  Other Equus species spread into South America while the true horse, Equus caballus, spread across Asia, the Mideast, and Europe.

PLEISTOCENE, 1.6 million years ago to 10,000 years ago

This is the period popularly known as the Ice Age, although the name is somewhat misleading.  We know that huge sheets of ice covered much of the earth during the Pleistocene epoch.  But modern geologists believe that glaciers occupied only small areas of the earth at any one time.  They would form in different places at different times, advance, and then gradually melt and recede.  During the glacial stages, temperatures would drop on average 5 to 7 degrees Celcius (9 to 13 degrees Fahrenheit).  In between, the “interglacial stages” had temperatures similar to or slightly above our current climates.

But even these slight, average differences had huge impacts on the animals living through the period.  The ice changed the surface of the earth and helped provide a suitable climate for huge Ice Age mammals.  The mastodon and saber-tooth tiger thrived.  Equus, the “modern horse” species, developed.  And the first true humans appeared during the Pleistocene epoch.  In fact, early humans have left us drawings and cave paintings of their neighbors: mastodons, woolly mammoths, bears, rhinoceros, tigers, and horses.  Toward the end of the epoch, the glaciers receded, the climate changed again, and the huge mammals disappeared from the fossil record.  Man continued to evolve and adapt, and recent theories suggest that, along with long reproduction cycles, humans hunting these mammals contributed to their extinction.

It was also during this period that horses died out on the North American continent.  The vast grasslands that nurtured early horse species had been covered with ice, new predators (including humans) challenged them, and Equus disappeared from North America for thousands of years.

RECENT, 10,000 years ago to Present

The modern day species of Equus (horses, zebras, and asses) have been around for about 2 million years.  They are very different from the earliest known horse, Hyracotherium, otherwise knows as Oehippus, or “Dawn Horse”.  This ancient horse was a small, dog-sized creature that lived from 55 to 45 million years ago.  In recent years, new fossil skeletons have been discovered, challenging previously held theories of the evolution of the horse.


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