Evolution
The very first horses evolved on the North American grasslands over 55 million years ago. Then, they deserted North America and migrated across the Bering land bridge into what is now Siberia. From there, they spread west across Asia into Europe and south to the Middle East and Northern Africa.
Our understanding of the horse evolution timeline has evolved as well. Paleontologist Kathleen Hunt outlines this evolution on her Talk Origins Archive site.
In the 1870s, the paleontologist O.C. Marsh published a description of newly discovered North American horse fossils. At the time, very few transitional fossils were known. The sequence of horse fossils that Marsh described (and that T.H. Huxley popularized) was a striking example of evolution taking place in a single lineage. He thought that through a series of clear intermediates, one could see the fossil species, Eohippus, transform into an almost totally different-looking (and very familiar) descendent, Equus.
But horse evolution was not smooth and gradual. Different traits evolved at different rates, didn't always evolve together, and even occasionally reversed "direction". Also, horse species did not always come into being by gradual transformation (anagenesis) of their ancestors; instead, sometimes new species split off from ancestors (cladogenesis) and then coexisted with those ancestors for some time. Some species arose gradually, others suddenly. Overall, the horse family demonstrates the diversity of evolutionary mechanisms. The most modern equids (descendants of Parahippus) are called equines. Strictly speaking, only the very modern genus, Equus, contains what we know as "horses".
Then, about 8000 BCE, succumbing to climate change and human hunters, horses completely vanished from North America. This migration map is a visual representation of the migration patterns of the horse and their eventual reintroduction to North American soils.
Wild Horses Migration Map
Meanwhile, across the sea, horses were becoming a fixture of many ancient civilizations. In 1000 BCE, the first horses were domesticated and used for transportation for both humans and cargo. 500 years later, Persian officials began using mounted couriers for message-relaying. Horses had become an integral part of human life.
It wasn't until the 16th century, when the Spanish came to conquer the New World, that the horse was reintroduced to North America. These small, sturdy mounts once again spread quickly throughout the Americas.
Dr. Michael Voorhies: The Evolution of the Horse
Dr. Voorhies, an equine specialist, is Curator of Vertebrate Paleontology at the University of Nebraska Museum and is in charge of their fossil bone collection. His research interests are paleontology and geologic history of Nebraska and the Great Plains, especially the history of warm-blooded mammals who roamed the area.
Evolution Timeline
EOHIPPUS
The first equid was Hyracotherium, a small forest animal of the early Eocene. It looked nothing at all like a horse (10 – 20” hight). It resembled a dog with an arched back, short neck, short snout, short legs, and long tail. It browsed on fruit and soft foliage and probably would have had mannerisms more like that of a deer (timid, flighty, etc.). This famous little equid was once known as Eohippus or “Dawn Horse”.
MESOHIPPUS
The species Mesohippus celer appears suddenly in the late Eocene, approximately 40 million years ago. This animal was slightly larger than Eohippus, standing 24” at the shoulder. It no longer looked like as much like a dog. The back was less arched, the legs and neck a bit longer, and the stout and face distinctively longer. It had a shallow, facial fossa, and a depression on the skull. Mesohippus had three toes on all its feet and was still pad-footed. The fourth front toe was reduced to a vestigial stub.
MIOHIPPUS
Soon after Mesohippus celer and its very close relative Mesohippus westoni appeared, a similar animal called Miohippus assiniboiensis arose, approximately 36 million years ago. A typical Miohippus was distinctly larger and had a slightly larger skull than a typical Mesohippus. The facial fossa was deeper and more expanded. In addition, the ankle joint had changed subtly. Miohippus also began to show a variable crest on its upper cheek teeth. In later horse species, this crest became a characteristic feature.
MERYCHIPPUS
Seventeen million years ago, Merychippus joined the equine line and was about 10 hands or 40” tall, the tallest yet. The muzzle became elongated, the jaw deeper, and the eyes moved farther back to accommodate the large teeth roots. The brain was notably larger, with a fissured neocortex and a larger cerebellum, making Merychippus smarter and more agile than earlier horses. Overall, this species was distinctly recognizable as a horse with a very “horsey” head.
PLIOHIPPUS
About 15 million years ago in the middle Miocene epoch arose Pilohippus, a three-toed horse. Gradual loss of the side toes is seen in this species through three successive strata. Pliohippus was very similar to and, until recently, thought to be the direct ancestor of Equus, except for two significant differences. First, Pliohippus’ skull has deep facial fossa whereas Equus has no facial fossae at all. Second, Pliohippus’ teeth are strongly curved while Equus’ teeth are very straight. Though Pliohippus is obviously related to Equus, the former probably did not give rise to the latter.
EQUUS
The genus of all modern equines, the first Equus was about 13.2 hands tall, pony size, with a classic “horsey” body – rigid spine, long neck, legs, and nose, and fused leg bones with no rotation. The brain was a bit larger than in early Dinohippus. Like Dinohippus, Equus was (and still is) one-toed, with side ligaments that prevent twisting of the hoof. This species has high-crowned, straight, grazing teeth with strong crests lined with cement.
Equus Family Tree
There are those, including paleontologist Michael Voorhies, 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.
Ashfall: Life and Death at a Nebraska Waterhole Ten Million Years Ago
By Dr. Michael Voorhies, Curator of Vertebrate Paleontology
Hundreds of skeletons of prehistoric animals have been found in a volcanic ash bed buried beneath the rolling farmlands of northeastern Nebraska. Some of the best-preserved fossil horses, rhinos, camels, and birds known anywhere have been, and are being, excavated by museum crews working in the Ashfall Fossil Beds in northern Antelope County. Unlike most fossil deposits, which consist of scattered bones accumulated over extended periods of time, the ash bed contains mostly articulated remains with bones still joined together in the proper order.Quick burial in volcanic ash accounts for the three-dimensional preservation of the skeletons of species that became extinct millions of years before they could have been seen by humans. These remarkably lifelike skeletons, some of which contain unborn young and stomach contents, give paleontologists an opportunity to reconstruct the life appearance and habits of these ancient species with an accuracy never before attainable.
The ash bed also contains abundant additional clues to the vegetation and climate of the landscape in which the rhinos and other animals lived and died. It is truly a 'time capsule' presenting us with a picture of a vanished world unrivaled in detail and clarity. This article describes some of the highlights of the first two decades of exploration of the site. It also invites you, the reader, to experience a sense of discovery of Nebraska's deep past by visiting the locality, which is now open to the public five months each year.
A New Park
Nebraska's newest state park, Ashfall Fossil Beds State Historical Park, opened its gates on June 1, 1991. Located 6 miles north of U.S. Highway 20 between Royal and Orchard, the park is a joint project of the University of Nebraska State Museum and the Nebraska Game and Parks Commission. Two buildings have been constructed at the site: a Visitor Center featuring interpretive displays and a working fossil preparation laboratory and a Rhino Barn covering a portion of the fossil-bearing ash bed. Each summer paleontologists working in the Rhino Barn will continue to expose skeletons buried in the ash. The newly uncovered fossils are being left exactly as they are found. Specially constructed walkways afford visitors an unobstructed, close-up view of the paleontologists at work. When the entire 2000-square-foot area within the Rhino Barn has been excavated, plans call for extending the building to cover more of the fossil bed, only a small fraction of which is currently protected by a roof.
Discovery
The first indication that a fossil bed of major significance might lie buried on Melvin Colson's farm came to light during the summer of 1971 when I noticed the skull of a baby rhinoceros eroding from the wall of a ravine at the edge of a cornfield on Mr. Colson's property. What made the find so unusual was that the skull and lower jaws were in perfect articulation and that the fossil was completely embedded in soft, distinctly layered volcanic ash.
I had originally visited the Colson farm in 1969 as part of a long term study of the geology and paleontology of the Verdigre valley. Verdigre Creek and its many tributaries have carved nearly 500 square miles of Knox, Antelope, and Holt counties into a network of rugged hills and ravines. My wife, Jane, and I located and mapped more than 200 fossil-producing sites in this area between 1968 and 1975. What drew us to the Colson farm was a cliff nearly 100 feet high with a layer of very hard sandstone at the top. Like previous generations of fossil hunters we had learned to expect bones in this particular sandstone, known to geologists as the Cap Rock.
Jane and I found a few fragmentary fossils on our first visit--nothing exciting but enough to prompt return trips in succeeding years. Our attention eventually shifted away from the high cliff to a series of less spectacular outcrops that trickled through Mr. Colson's pasture. It was here, on a deeply gullied hillside recently swept free of soil and debris by torrential rains, that a routine afternoon of prospecting turned into a once-in-a-lifetime adventure.
Excitedly, I brushed the ash away from the little skull, first from the oversized teeth, then farther back, looking for evidence that the rest of the skeleton might be there. It was. Just as the old song has it:
"The head bone connected to the neck bone,
the neck bone connected to the back bone,
the back bone connected to the hip bone..."
Not only did this first rhino turn out to be intact but other equally good skeletons seemed to be extending back into the hill, covered by ten to twenty feet of ash and sandstone. Although it was difficult to resist the impulse to dig straight back into the ash bed and see what else was there, past experience had taught me that this would not only have endangered the fossils (and maybe the digger!) but also would have destroyed important evidence about the origin of the deposit.
Initial Excavations 1977-1979
Because of the unusual nature of the site, special care had to be taken in exploring it. Before any extensive digging could be done, it was necessary to learn as much as possible about the geologic relationships of the fossil-bearing bed. A series of shallow pits excavated with a trowel revealed far too large an area to be worked by one person. In order to obtain funding for a large-scale excavation I needed to document the extent and significance of the deposit so in June 1977 a small crew from the Museum helped me shovel off the overburden from 20 square meters of the fossil bed and collect several skeletons.
The National Geographic Society agreed to support a larger-scale excavation of the site when I sent them photographs and descriptions of the 1977 test excavation which showed that skeletons of horses as well as rhinos (one containing a fetus) were present in the deposit. With funding from the Society I was able to hire a crew of eight students and spend the summers of 1978 and 1979 excavating the ash bed. With Mr. Colson's permission we had a bulldozer gradually remove the sandstone and upper part of the ash bed over an area of about 600 square meters adjacent to our 1977 test excavation. (First, of course, we probed these upper beds to make sure they contained no fossils!)
After gridding the bulldozed area into a series of 3 by 3 meter squares we began to carefully remove the remaining (lower) portion of the ash bed where the skeletons, we believed, should lie. The results exceeded even our most optimistic expectations. Not only did we find dozens of additional rhinoceros and horse skeletons but also remains of camels, birds, turtles, and small saber-toothed deer. It became clear that a major disaster, claiming hundreds of victims, had occurred at the site.
What Happened?
As excavation proceeded through the summer months of 1978 and again in 1979 the museum team collected any and all kinds of information that might help us understand the origin of this deposit, apparently the only one of its kind in the world. Like detectives at the scene of a crime we paid special attention to the position of the bodies--photographing and sketching each one before encasing it in a plaster cast and removing it to the laboratory for further analysis.
It became clear early on that there was a definite pattern to the arrangement of the skeletons in the ash bed. Digging down from the top we always found rhinoceroses first, then, at deeper levels, smaller hoofed animals such as horses and camels, and finally, birds and turtles. The latter were always at the very bottom of the ash bed, in a layer containing numerous footprints of rhinos and other hoofed animals. It seemed evident that the small creatures died first, then the middle-sized ones, and finally, the rhinos. The animals definitely did not die all at once; they were not (with the possible exception of the birds and turtles) buried alive.
The larger animals clearly died more slowly, over a period of a few days to a few weeks. Proof that they were not instantaneously killed and buried can be seen on many skeletons, especially those of horses and camels, which often show bite marks attributed to large scavengers that must have had access to the carcasses before they were completely buried. Every fossil mammal so far discovered at the site has abnormal patches of highly porous superficial bone on various parts of its skeleton, especially on the lower jaw and the shafts of the major limb bones and ribs. Veterinarians have reported very similar growths on animals that have died of lung failure. Inhalation of large amounts of volcanic ash almost certainly caused the deaths of the Ashfall victims.
Further Reading
Articles on the Ashfall site can be found in National Geographic (January, 1981) and Nebraskaland (June, 1990) magazines. More technical data are presented in Science (vol. 206, pp. 331-333) and National Geographic Research Reports (vol. 19, pp. 671-688). General information on ancient mammals is available in Mammal Evolution, an Illustrated Guide by R.J.G. Savage and M.R. Long (Crown Publications, 1986).
