The evolution of the horse was complex, involving changes in environment from swamp to savanna and the series of adaptations that occurred along the way. The Equidae evolved from browsers of twigs in swamps to grazers of grass on plains. The fossil record shows large numbers of equids that evolved then went extinct. The domestic horse, Equus caballus, arose from one of those many branches.
The modern horse evolved from a large family known as Equidae. Fossil evidence indicates the earliest members of this family, hyracotheres, were small, had four toes on each foot, and adapted to living in forests.
First Horse Ancestors
The first ancestors of today’s horse existed about 55 million years ago.1 These were dog-sized animals. Horse evolution during the next few million years showed many types and sizes of horses lived at the same time. Then about 20 million years ago, climate changed and more grasslands appeared. About 10 million years ago, many species of Equidae still roamed the Earth.
The ancestor of today’s horse evolved into an animal with four toes on the front feet and three toes on the hind feet. Its natural habitat and feeding area during this period was in the swamplands.

As the horse evolved and grew in size, it changed into an animal with three toes on both the front and the hind feet, and it reached an adult height of about 24 inches—approximately the size of the modern-day Miniature Horse.
Equus was the first “true horse.” It appeared on the earth about 4.5 million years ago. Equus had one toe (hoof) and two side bones on each foot. The two side bones are known today as splint bones.
Evolution of Today’s Horse
The evolution of today’s horse usually is traced through these species: Eohippus (55-45 million years ago), Mesohippus (40-30 million years ago), Miohippus (30 million years ago), Merychippus (20 million years ago), Pliohippus (10 million years ago), Equus (2 million years ago).2


The horse and its close relatives are the only single-toed animals in the world today. There are about 10 species in the Equus family, including zebras, donkeys, horses, and asses.
Asses and Zebras
The donkey (also known as the ass) and the zebra are part of the scientific “family” of horses known as Equidae.
Asses
The scientific classification of the “domestic” ass is Equus asinus. As far as can be documented, the donkey was domesticated in the Middle East, most likely in Egypt.

Well-preserved drawings of donkeys appear beginning around 3400 BC, or approximately 5,400 years ago. Donkeys are genetically similar to the horse; however, they have one fewer chromosome pair.
- Horses and ponies: 32 chromosome pairs (64 “diploid” chromosomes).
- Donkeys: “Domestic” donkeys have 31 chromosome pairs (62 “diploid” chromosomes).
- Donkeys have chestnuts on the insides of only the front legs, not on the hind legs.
- Jack: a male ass, donkey, or burro.
- Jennet or Jenny: a female ass, donkey, or burro.
- Mule: The offspring that results from crossing a male donkey (jack) on a female horse (mare). In general, mules are sterile; only a few cases of nonsterile mules have been scientifically documented. Many mules are used as work and pack animals. They are known for their surefootedness.
- Hinny: The offspring that results from crossing a male horse (stallion) on a female donkey (jennet or jenny). Hinnies are sterile.
Zebra
There are three scientific classifications of zebra. The mountain zebra (or common zebra) is Equus zebra. The Grévy’s zebra is Equus grevyi. The plains zebra is Equus quagga.3

Although zebras are close relatives to the horse and the donkey, they are also very different from horses or donkeys. The several types of zebra that exist vary in their genetic makeup. The “common zebra,” known as Grant’s zebra, has 44 diploid chromosomes. Hartman’s mountain zebra has only 32 diploid chromosomes—which is half as many as the horse. Other zebra family members have diploid chromosome numbers of 45 and 46. In scientific classification, these close family members have different names.
Zebras, like donkeys, have chestnuts on the front legs only.
References
- The Evolution of Horses. American Museum of Natural History.
- Origins of the Horse. Nebraska PBS.
- Zebra Fact Sheet. PBS
Further Reading
- Research on Sound-Reducing Equine Ear Gear. MySeniorHorse.com
- Mules and Hinnies: A Scientific Point of View. Dr. Amy McLean. MySeniorHorse.com
- The Basics of Equine Genetics. MySeniorHorse.com
Adapted from “Equine Science: Basic Knowledge for Horse People of All Ages,” By Jean T. Griffiths. Copyright Equine Network
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Editors of My Senior Horse are journalism professionals, most of whom are lifelong horse owners.View all posts