Updated: 26 October 2021
Horses have existed for over 50 million years. However, their size, appearance, species diversity, and feeding habits have changed greatly during the long course of their evolution.
In this review, we trace the fossil history of these extraordinary animals, from their beginnings as miniature broncos the size of housecats and small dogs, to their peak radiation in the Miocene of at least a dozen contemporaneous genera of many different sizes and food habits, and their subsequent decline to one surviving genus today.
Most of this evolution took place in North America which, for millions of years, was an island continent isolated from the rest of the world by seas, much as Australia is today. Here, for long epochs of time, a great diversity of horses inhabited ancient landscapes, interacted with prehistoric fauna and flora, and numerically dominated ungulate communities.
We begin this review by quickly summarising some basic facts about modern-day horse species and their relatives. Then we commence the story of horse evolution with a look at the miniature broncos of the Eocene.
Unless specifically stated, all examples of horse evolution cited here are from North America (Mexico, USA, Canada). While most research has been conducted in USA and Canada, an increasing number of studies are now being made in Mexico, and these are helping to make our understanding of horse evolution more complete (e.g. Priego-Vargas et al. 2016; Bravo-Cuevas et al. 2019).
Extant Horses and their Relatives
All surviving members of the horse family (Equidae) are classified into one genus: Equus. There are four main groups within the genus Equus: zebras, Asiatic wild asses, African wild asses, caballine horses (Oakenfull et al. 2000).
– Grevy’s Zebra (Equus grevyi)
– Plains Zebra (Equus quagga)
– Mountain Zebra (Equus zebra)
Asiatic Wild Asses
– Kulan and Onager (Equus hemionus)
– Tibetan Wild Ass or Kiang (Equus kiang)
African Wild Asses
– African Wild Ass (Equus africanus)
Caballine (True) Horses
– The wild (and domestic) horse (Equus ferus caballus)
Origin of domesticated forms:
(1) The ancestors of today’s domestic horses came from many different populations of wild caballine horses (Vilà et al. 2001; Bendrey 2012).
(2) The wild ancestor of the donkey is the African wild ass.
(3) A mule is the hybrid offspring of a male donkey and a female caballine horse. It is usually sterile. A hinny is the hybrid offspring of a female donkey and a male caballine horse. It is also usually sterile.
The horse family (Equidae) is one of three families in the mammalian order Perissodactyla. The other two families are tapirs (Tapiridae) and rhinoceroses (Rhinocerotidae). Species of all three families are swift runners, herbivores (browsers, grazers, frugivores) and excellent dispersers of seeds (which pass undigested through their gastrointestinal systems).
The Perissodactyla is an order of mammals in decline. All of its surviving families were formerly more abundant, diverse and widespread than they are today.
For example, at a Late Miocene site in Florida dated 9 million years ago, fossils of 21 species of terrestrial ungulates were reported by MacFadden and Hulbert (1990). Of these, 12 were Perissodactyla (9 horses, 1 tapir, 2 rhinos), 8 Artiodactyla and 1 Proboscidea. Perissodactyla comprised 74% of all individual ungulates (horses: 58.6%, tapirs 4.2%, rhinos 11.2%).
Today, the Perissodactyla is no longer a dominant ungulate order in the world. That place has been taken by the order Artiodactyla. The Artiodactyla include deer, wild pigs, peccaries, bison, wildebeests, mountain sheep, goats, pronghorns, antelopes, giraffes, camels, hippopotamuses and many other taxa.
The first horses appeared during the early Eocene of North America, over 56 million years ago. They were miniature broncos the size of house cats and small dogs, and were diversified into many genera. These early horses did not have the hoofs of modern horses. Instead, they had 3 toes on their front feet, and 4 toes on their back feet.
Eocene horses fed mainly on woody and herbaceous vegetation and fruit, rather than on grass. It is believed that they lived lifestyles similar to those of the duikers of modern-day Africa (Janis 1982; MacFadden 1992). Duikers (Cephalophus spp., Sylvicapra spp.) are tiny, browsing ungulates of the family Bovidae (Artiodactyla). They inhabit forests and savannas.
One of the best-known early Eocene horses was Eohippus angustidens, whose name means “dawn horse.” Fossils of this species were first found during the 19th century in North America. For many years, dawn horse was believed to be the first horse, but now fossils of earlier horses have been discovered.
Currently, the earliest known fossil horse is Sifrhippus sandae. Its scientific name means “zero horse” (Froehlich 2002). Zero Horse was the size of a housecat and is also the smallest known horse (Gingerich 1989, 1991; Froehlich 2002).
Fossils of Zero Horse have been found in the Clark’s Fork Basin of Wyoming and the Bighorn Basin of Montana and Wyoming. They date from a period between 55 and 56 million years ago, which places them near the beginning of the Eocene (Gingerich 1989, 1991; Froehlich 2002; Secord et al. 2012).
However, Zero Horse may be much older than these finds because fossils of another, more derived, horse species were found with it at the two sites mentioned in the previous paragraph. This second horse species, Arenahippus grangeri (sand horse), was larger than zero horse and had more advanced molars (Froehlich 2002).
The Eocene ended approximately 34 million years ago. After it came the Oligocene, which lasted roughly 10 million years. During this epoch, all horses continued to be browsers. However, some such as Mesohippus, added significant amounts of grass to their diets (Solounias and Semprebon 2002).
The two dominant horse genera of the Oligocene were Mesohippus and Miohippus, which weighed 40 to 55 kilograms. Both were about 50% larger than most Eocene horses, but still much smaller than the 500 kilograms of many horses today (MacFadden 1992).
Mesohippus appeared in the late Eocene and died out in the mid-Oligocene, 11 million years later (MacFadden 2005). It was the first horse that had 3 toes instead of 4 toes on its front legs. Like Eocene horses, it also had 3 toes on its hind legs.
Miohippus evolved from Mesohippus and the two genera were contemporaneous for about 8 million years (MacFadden 2005). Unlike Mesohippus, Miohippus survived the Oligocene, existing for at least 18 million years, from the late Eocene to the mid-Miocene (MacFadden 2005). During the next epoch, the Miocene, it radiated into many different horse clades.
The Miocene epoch began 24 million years ago and ended about 5 million years ago. For horses, it was a time of great ecological and evolutionary change. During this epoch, body sizes, diets and niches diversified, and significant changes in locomotion and anatomy occurred.
During this period, horses also reached the peak of their biodiversity. For example, the horse subfamily Equinae underwent a major radiation, diversifying from one species, Parahippus leonensis, to 70 species (Maguire and Stigall 2008, 2009).
Diversification of Diets
When the Miocene started, all horses had low-crowned (brachydont) teeth and were mostly browsers, feeding mainly on leaves, twigs and fruits of dicotyledonous plants. When the Miocene ended, all horses had high-crowned (hypsodont) teeth, and their diets were diverse (Mihlbachler et al. 2011).
For the first time, some horses became grazers, i.e. feeding mainly on grasses and sedges (monocotyledonous plants). Examples were Protohippus, Calippus, Cormohipparion and Nannippus (MacFadden 2005). Other horses, such as Parahippus and Merchychippus, ate a mixed diet of both browse and grass (Solounias and Semprebon 2002, MacFadden et al. 1999). Toward the end of the Miocene, a few hypsodont horses derived from grazers, such as Dinohippus mexicanus and Astrohippus stocki, returned to complete browsing (MacFadden et al. 1999). However, neither mixed feeding not the secondary adaptation to browsing endured.
The general shift over time from browsing to mixed diets and grazing, was a significant niche change that has endured up until the present (MacFadden 2005). Modern horses today are mainly grazers. They add browse to their diet in the winter, but eat mostly monocotyledonous plants (>90%) during the rest of the year (Duncan 1992).
Diversification of Genera
During the Miocene, the climate and vegetation of North America changed. Grasslands expanded and forests decreased in size. The advent of extensive grasslands provided a new opportunity for horses to diversify. Consequently, they colonized this new biome and radiated into many new genera, embracing a grazer diet.
This adaptive radiation led to an increase in horse genera and the peak of horse diversity in the early Clarendonian Age of the mid-Miocene (11 to 9.5 million years ago). At that time, there were at least a dozen contemporaneous horse genera co-existing on the central Great Plains of North America (Janis et al. 1998). The two dominant groups were horses of the tribes Hipparionini and Equini (MacFadden 1992, 2005). Like other horse groups, both the Hipparions and Equines would later decrease in diversity. Yet as we shall see, both taxa were contemporaneous in North America up until the Ice Age (Pleistocene).
Horse diversity declined in the late Miocene. The main reason for this decline was that the guild of browsing horses died out (Janis et al. 2000; 2002). Why these steeds disappeared is unknown. However, it appears that ungulate browser diversity decreased on other continents as well during the late Miocene, even among non-horse taxa (Janis et al. 2000). During this period, there was also an associated increase in the average size of the remaining browsers (Janis et al. 2000).
Body Size Changes
For the first 35 million years of horse evolution, horses were generally small, from 10 to 55 kilograms (MacFadden 1992). During the Miocene, however, the size of horses diversified: some horses became larger, others remained similar in size to the early horses, and still others became smaller than their ancestors (MacFadden 1992). The horses that survive today are those whose ancestors generally increased in size. The radiation of horses into a diversity of sizes during the Miocene was undoubtedly related to their diversification into different habitats and diets.
As horses spread out of the forests and onto the grasslands during the Miocene, they also became more cursorial. They evolved a new springing locomotion which made them faster. This increased speed probably enabled them to better escape predators and exploit distant food patches (McNaughton et al. 1985; Benton 1990; MacFadden 1992).
In hypsodont Miocene horses, the lower legs became elongated, “the interosseous muscle was reduced, and the interosseous tendon was elongated (MacFadden 1992)” These changes made the horse foot elastic and like a pogo stick (Camp and Smith 1942). Other morphological changes in muscles and bones, streamlined horses and enabled them to take more efficient, powerful strides (MacFadden 1992).
At the beginning of the Miocene, all living horses were tridactyl, i.e. had three toes on each foot. Later in the Miocene, however, some horses became monodactyl, with one toe on each foot, like modern-day horses. The horses that became monodactyl all belonged to the tribe Equini and included the genera Pliohippus, Astrohippus and Dinohippus.
Early species of these genera were tridactyl, while later species were monodactyl (MacFadden 1992). A Late Miocene population of one primitive Dinohippus species found at Ashfall Fossil Beds State Historical Park, Nebraska, had a mixed population with some individuals being tridactyl and others being monodactyl (Voorhies 1981).
The modern horse genus Equus, which first appeared in the North American Pliocene, evolved out of this clade of horses and was completely monodactyl. The Hipparions did not become monodactyl but evolved a similar adaption: their three toes came to function as one digit (MacFadden 1992).
The Pliocene occurred from 4.5 to 1.8 million years ago. During these years, which led up to the Ice Age, horse diversity in North America declined from 5 genera to one genus (Equus), and the Caballine horses split off from the Wild Asses and Zebras (Steiner and Ryder 2011)
Two genera died out during the early Pliocene: Neohipparion and Dinohippus. Neohipparion was a three-toed horse, while Dinohippus in its final stages was one-toed and the nearest outgroup to the genus Equus. Interestingly, at least some Dinohippus species were principally browsers, even though they possessed high-crowned teeth (MacFadden et al. 1999). However, it is believed that their immediate ancestors were grazers, and that they secondarily acquired the browsing habit (MacFadden et al 1999).
Two other Hipparion horse genera, Ninnippus and Cormohipparion, died out at the end of the Pliocene. Both were three-toed horses and their occurrence throughout this epoch means that three-toed horses lived in North America right up until the beginning of the Ice Age, a fact usually not generally known. Nannippus was widespread across the continent, but Cormohipparion was restricted to the southeast (MacFadden 1992).
Nannippus, was called the gazelle-horse. This genus was composed of small to tiny three-toed grazing horses with slender, elongated limbs (Kurtén and Anderson 1980). The largest ones reached only the height of Shetland ponies, but “were much lighter in build, with long, graceful legs” (Kurtén 1988).
Equus first appeared in the North American Pliocene and spread across the entire continent (MacFadden 1992, 2005). The earliest horse of this genus was Equus simplicidens, and it is believed to be the primitive Equus from which all other Equus evolved.
For those of our readers who would like to know what this first Equus horse looked like, there is a horse living today that resembles it very much: The endangered Grevy’s Zebra of the Horn of Africa is a primitive horse that preserves many of the characters of Equus simplicidens, such as a long skull with prominent occipital region, slender limbs, small hoofs and large body size (Skinner and Hibbard 1972; Kurtén and Anderson 1980; but see also Forsten and Eisenmann 1995 for minor differences). Tragically, widespread lawlessness, poaching and habitat destruction by humans in the small range of Grevy’s Zebra have greatly reduced its numbers and it is today an endangered species.
Pleistocene Horses of North America
The Pleistocene epoch occurred from 1.8 million years ago until approximately 10,000 years ago. It is often called the “Ice Age” because several different glaciations occurred during its time, each separated by warmer “interglacial periods.” The last glaciation ended about 10,000 years ago and is known as the Wisconsinian Glaciation.
By the beginning of the Pleistocene, there were only two genera of horses still remaining in North America. Although horse generic diversity was now low, horses were still very abundant animals and continued to numerically dominate ungulate communities in North America (Guthrie 2003).
Fossil deposits from the mid- and late-Pleistocene of North America usually contain remains of two kinds of horses: a caballine horse (Equus ) and a stilt-legged horse (Haringtonhippus) (Weinstock et al. 2005, Heintzman et al. 2017).
The now-extinct stilt-legged horses had long, graceful legs similar to the extant Asiatic wild asses (e.g. onager). The resemblance, however, was apparently the result of parallel evolution, caused by adaptation to similar arid environments. DNA studies show that the stilt-legged lineage was genetically separate from Asiatic wild asses and was endemic to North America (Weinstock et al. 2005). Some Pleistocene fossils from North America have been assigned to various species of extinct and extant Asiatic wild asses (Winans 1989; Eisenmann 1992; Azzaroli 1998). However, Weinstock et al. (2005) suggest that some or all of these fossils are really stilt-legged equines that have been misidentified. Clearly, more study is needed to determine if Asiatic wild asses ever inhabited North America.
Another important question is how many species of Equus and Haringtonhippus lived in North America during the Pleistocene. In the nineteenth and twentieth centuries, over 50 different species were described by paleontologists, based mainly on variation in the sizes of fossil bones. However, modern caballine horses are extremely variable in size, so there is a consensus among scientists today that the number of Pleistocene species should be reduced (Winans 1989; Azzaroli 1998). Some, such as Weinstock et al. (2005), have even gone so far as to suggest that there were really only two horse species present in North America during the mid and late Pleistocene: a caballine horse (Equus) and a stilt-legged horse (Haringtonhippus francisci). Both species, they assert, showed great geographic variation in size because of adaptation to different environments.
A recent study of caballine fossils in the northern hemisphere reveals that those of the late Pleistocene times belonged to two clades: (1) an endemic North American group, and (2) a Holarctic group found in both North America and Eurasia (Vilá et al. 2001). The familiar domestic horse of today comes from the second clade (see below).
Extinction of Horses in North America
After over 55 million years of evolution and residence in North America, horses became extinct there. This extinction occurred during the late Pleistocene and early Holocene. (The Holocene is the period of time we live in now. It began after the Wisconsonian glaciers melted, approximately 10,000 years ago.)
The extinction of North America’s horses occurred during a time period when many other large mammals in North America and other parts of the world also became extinct.
There is now general agreement about terminal dates. Kurtén & Anderson (1980) dated the latest-known North American fossil horse bones (collected in Alberta) as being approximately 8,000 years old, while Wang et al. (2021) used environmental DNA from Alaska and the Yukon to establish that the last original North American horses lived until approximately 7,900 years ago.
In Alaska, stilt-legged horses became extinct about 31,000 years ago, while caballine horses became extinct 7,900 years ago (Guthrie 2003; Wang et al. 2021). Remarkably, Alaskan caballine horses showed a precipitous decline in body size before extinction (Guthrie 2003).
Various theories have been proposed to explain the extinction of horses and other large mammals from North America. The three most popular hypotheses are (1) overhunting by humans for food (the overkill hypothesis), (2) climate change, and (3) the Clovis Comet hypothesis. These hypotheses are not mutually exclusive so it is also possible that a combination of two or more of them caused the horse’s demise in North America.
The Overkill Hypothesis asserts that the earliest human immigrants to North America found an animal fauna there that had no previous experience with human hunters. Consequently, many of these animals, including horses, were easily killed by early humans who ruthlessly slaughtered them without limit and eventually caused their extinction (Martin 1973).
When this hypothesis was first proposed, it was widely believed that North America had been inhabited by humans for only about 14,000 years. Thus, the rough correlation of large mammal extinction with the earliest-known human settlement led many scientists to accept the overkill hypothesis as the most reasonable explanation for the extinctions. However, archaeological science has advanced much since the overkill hypothesis was first proposed. Today, there is good evidence showing that humans have been living in North America for at least 130,000 years (Bordes et al. 2020).
Also, we now know that many bird species that were not hunted by early humans, and which were not known to be associated with the large mammals that disappeared, also became extinct during the same time period, suggesting that some other cause than overkill was responsible (Grayson 1977).
The Climate Change Hypothesis asserts that changing climates directly and/or indirectly caused the extinctions. Because climate change often causes alterations in the abundance of many other organisms, such as food plants, disease vectors, predators and competitors, extinction scenarios involving climate change can be diverse and involve many different mechanisms.
A recent study (Stewart et al. 2021) found that large mammal declines in the late Pleistocene were correlated with climate change rather than human population growth, showing that the climate change hypothesis better explains North American horse extinction than the overkill hypothesis.
The Clovis Comet Hypothesis, also known as the “Younger Dryas Impact Hypothesis” asserts that many pieces of a fragmenting comet struck the earth 12,800 to 11,700 years ago. The hardest hit was North America, where the comet fragments caused massive environmental devastation and climate change that directly and indirectly resulted in the extinction of many species of wildlife including horses, and some folk groups such as the famous Clovis Culture (Firestone et al. 2007; Firestone 2009; Woolbach et al. 2020).
In contrast to these hypotheses, some researchers have proposed that North American caballine horses never became completely extinct, and instead survived and then interbred with caballines brought by European colonists to North America (Clutton-Brock 1981). This hypothesis is intriguing and one that we wish were true. However, it is not generally accepted because: (1) No horse bones or horse environmental DNA from the late pre-Columbian era have been found to support the idea, and (2) no indisputable images of horses have been found in late pre-Colombian American Indian artifacts.
Furthermore, when the Spanish arrived with their horses to Mexico in the sixteenth century, the Aztecs and other highly educated peoples of that region did not initially understand what horses were. For these reasons, all horses found today in North America are assumed to be descended from horses brought to the New World from the Old World since 1492 A.D.
Domestication of Caballine Horses
When horses became extinct in the New World, some species of Equus still survived in the Old World (e.g. zebras, wild asses and caballines). Their ancestors had dispersed there years earlier via the Bering Land Bridge, which connected Alaska to Siberia during periods when sea levels were lower. Many of these surviving horse species are still extant, however most are now endangered. They were listed at the beginning of this review.
Throughout the Holocene, wild caballine horses continued to range across the grasslands of Europe and Asia. Approximately 5,000 years ago, wild caballines were captured at numerous locations in this vast geographic area and domesticated by diverse peoples, as the knowledge and technology for capturing, taming and riding horses spread (Vilà et al. 2001; Bendrey 2012).
Thus, the domestic horse of today originated not from one local population of wild horses, but from numerous populations spread across Eurasia (Vilà et al. 2001; Bendrey 2012).
Many different species of animals have been domesticated by humans, so it is important to remember that there are varying degrees of animal domestication. Caballine horses today are among the least domesticated of all farmyard animals. Unlike other domestics, most horses are in practice wild, fully capable of living and surviving without any aid from people (Zimov 2005). Yet, amazingly, this remarkable animal has profoundly influenced human history (Vilà et al. 2001; Bendrey 2012).
North America’s Place in Horse History
As we have seen, North America was the birthplace of the horse family and the theater in which almost all its evolution took place. During the 56 million years of horse evolution, a few horse taxa dispersed from North America to other continents, and these emigrations resulted in small radiations.
However, virtually all horse genera, including Equus, evolved in North America. No other continent ever produced such a great diversity of horses as North America did which, during the peak of horse evolution in the Miocene, comprised at least a dozen contemporaneous genera that numerically dominated ungulate communities.
Today, North America is once again inhabited by a diversity of horses. Where various prehistoric horses once lived, the latest versions of Equus now graze, including appaloosas, morgans, quarters, thoroughbreds, and many other breeds. Some of these horses have even established wild herds, reverting to the lifestyle of their ancestors.
Sticklers insist on calling these latest free-ranging North American horses “feral” rather than “wild” because, for a relatively brief period, the ancestors of these horses were bred under the supervision of humans.
However, because the adjective “feral” implies gene selection by humans rather than the environment, many would argue that it is inaccurate since it tells only a small part of the story.
The ancestors of modern-day horses evolved for over 50 million years in North America, and there acquired all of their horse-like characters long before they ever met humans, and indeed long before humans ever existed. Because their most important characteristics are not the result of human-supervised breeding, but rather natural selection by the North American wilderness and selection by other wild horses during mating, the North American horses of today are far more “wild” than “feral”. In addition, as mentioned above, the horse is actually one of the least domesticated of all farmyard animals.
Should North America’s modern wild horses be considered invasive, non-native species? Here again, the answer is not clear because we don’t know for certain yet what caused the extinction of horses in North America thousands of years ago. If humans did extirpate these earlier horses, or if they became extinct because of the collision of comet fragments with the earth, then the presence of wild horses in modern-day North America could be seen as a wholesome restoration of one endemic element that was lost long ago.
Whichever way we decide to view North America’s modern wild horses, it is essential that they be managed in an ecological way. It is unwise and unethical to restore any wild herbivore species to an ecosystem without also restoring the predators that can limit its density and prevent overgrazing and excessive destruction to vegetation and soils. Overpopulation is also bad for the horses themselves because it leads to starvation, stunting and the spread of diseases.
Where it is politically impossible to re-introduce predators, other mechanisms to control wild horse density need to be put in place. The annual Pony Penning Roundup and auction of foals on Chincoteague Island, Virginia, and the use of contraceptive vaccines on nearby Assateague Island are examples of alternative solutions (Henry 1947; Turner and Kirkpatrick 2002).
The modern-day people of North America, like humans everywhere, are passionate horse lovers. The fossil record’s revelation to them that their continent is the ancient motherland of horses has deepened the already strong emotions that they feel toward horses and strengthened the bonds that they have with these extraordinary animals.
One important way that they have found to express their love for horses is to participate in the conservation of endangered species of wild horses. There is a great need for this work because, as we mentioned previously, almost all species of the Equidae are today threatened with extinction. In addition, there is now evidence that restoring populations of wild horses can also help preserve endangered plant species (e.g. Dvorský et al. 2021).
In the wild horse, with its untamed character and independent spirit, many Americans see a soul mate, a natural symbol of their own love for freedom, with deep and ancient roots in their own continent. At the same time, the tragic history of the horse’s prehistoric extinction in North America reminds everyone of the vulnerability of nature and the need to make environmental protection a high priority.
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Information about this Review
The author is: Dr. Paul D. Haemig (Ph.D. in Animal Ecology)
The proper citation for this review is:
Haemig PD 2021 Evolution of Horses. Ecology.Info 33
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