Evolution of Horses
Note: This online review is updated and revised continuously, as soon as
results of new scientific research become available. It therefore
presents state-of-the-art information on the topic it covers.
Horses have existed for over 50 million years. However, their
size, appearance, species diversity and feeding habits have changed greatly
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 their subsequent decline to one surviving
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, horses inhabited ancient landscapes, interacted
with prehistoric fauna and flora, and numerically dominated ungulate
We begin this review by quickly summarizing
basic facts about modern-day horse
species and their relatives. Then we start the tale of horse
evolution with a look at the miniature broncos of the Eocene.
Unless specifically stated, all examples of horse evolution are from
all living members of the horse family (Equidae) are classified into one genus:
Equus. There are four main groups within the genus (Oakenfull
et al. 2000):
Grevy's Zebra (Equus
grevyi) Photo 1,
Photos 2 and 3
Plains Zebra (Equus quagga)
Photo 4, Photo 6
Mountain Zebra (Equus zebra) Photo 5
Asiatic Wild Asses
- Kulan and Onager (Equus hemionus)
- Tibetan Wild Ass or Kiang (Equus kiang)
African Wild Ass (Equus asinus)
Przewalski's Horse (Equus caballus) (Photo 8,
(1) The ancestors of domestic horses came from many different
populations of wild caballine horses (Vilà et al. 2001; Bendrey 2012). Przewalski's horse
was one of these (see page 2 of this review).
(2) The wild ancestor of the donkey (Photo 10) is the African wild ass.
(3) A mule (Photo 11) 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.
family 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 fast runners, herbivores (browsers and/or grazers) and
excellent dispersers of seeds (which pass undigested through their
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%,
Today, the Perissodactyla
is no longer the 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 in the early
Eocene of North America, 50 to 56 million years ago. They were miniature broncos the size of housecats 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.
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
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
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).
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.
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
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
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
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.
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 living 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).
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).
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
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
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
Nannippus was widespread across the
continent, but Cormohipparion was restricted to the southeast (MacFadden
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).
appeared in the North American Pliocene and spread across the entire continent (MacFadden 1992,
2005). The earliest horse of this genus was Equus simplicidens.
It is believed to be the primitive
Equus from which all other Equus evolved.
The endangered Grevy's
Zebra (Photo 1,
Photos 2 and 3) of modern times is a
primitive zebra 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). Grevy's Zebra inhabits the Horn of Africa,
where widespread lawlessness and poaching have greatly reduced its
numbers and made it an endangered species.
Finish reading this
review on Page 2.
Photograph at top of page by Brenton Nicholls (Australia).