Evolution of the Horse: From the Swamps to the Sweeps

By Dr. Jenifer Nadeau

Dept. of Animal Science

College of Agriculture and Natural Resources, UConn

Lecture Objectives

• Define evolution• Describe the horse family tree • Describe the horse’s anatomical changes

and why they came to be

What is evolution?

A gradual process in which something changes into a different and usually more complex or better form

Change in the genetic composition of a population during successive generations

3 Mechanisms of Evolution

• Genetic Drift• Mutation• Natural Selection

Horses are a Classic Example of Evolution

Scientific Classification of the Horse

• Kingdom• Phylum• Class• Order• Family• Genus• Species

The earliest ancestors of the horse appeared over 50 million

years ago.

Time Line

Recent: 10,000 years ago to present

Pleistocene: 2.5-0.01 million years ago

Pliocene: 5.3-2.5 million years ago

Miocene: 24-5.3 million years ago

Oligocene: 38-24 million years ago

Eocene: 58-38 million years ago

http://hometown.aol.com/darwinpage/horses.htm

Horse Family Tree

http://www.angelfire.com/mi/dinosaurs/horse.html

Eohippus or Hyracotherium50 million years ago

• About size of a fox, 2 feet long, 8 inches tall at withers

• Long skull with 44 low crowned even teeth without cement

• Originated in

N. America

Eohippus50 million years ago

• Carried its weight on central pad like a dog

• Well adapted to living in forested and swampy environment

Miohippus or Mesohippus35 Million Years Ago

• “Middle horse”• Larger (2 x size of Eohippus)• 3 hoofed toes on the front and hind, side

toes on each leg touching the ground• Splint of 2nd digit

Miohippus or Mesohippus

• Low crowned teeth without cement• Teeth suitable for grazing on the prairie• Greater speed, endurance for finding

forage, water, protection, survival

Merychippus25 million years ago

• Gave way to at least 19 new grazing horse species – “Merychippine Radiation”

• The 3 major groups included: Hipparions, Protohippines, true Equines

• Gave rise to the first burst of diversity in the horse family

Merychippus

• Over 2 feet tall (35 inches)• Long crowned teeth with cement• Eye socket position changes

Merychippus25 Million Years Ago

• 3 hoofed toes on each leg with smaller side toes touching the ground

• Bones of the leg began fusing together

Pliohippus10 Million Years Ago

• Body equipped for savanna type conditions

• Higher crowned molars develop for grinding grasses

• 1st single-hoof equine• No recognizable side

toes• 12 hands tall

Pliohippus

• One toe on front and hind legs• Splints of 2nd and 4th digits• Long crowned teeth with cement• Grandfather of the modern horse

Equus6 Million Years Ago

Develops into modern day horse after 15 million generations

Equus’ Evolutionary Changes

• Increase in body size• Reduction in toe number• Increase in size of cheek teeth• Longer face

Evolutionary Changes in Horse Body Size

http://www.txtwriter.com/Backgrounders/Evolution/EVpage03.html

During the Ice Age, Equus migrated from

North America throughout the world over the Bering Strait

Equus

• One toe on front and hind legs• Splints of 2nd and 4th digits• Long crowned teeth with cement• Found all over Asia, Africa, Europe, and

North America

Extinction in Western Hemisphere

• The appearance of man in the Paleolithic period

• Return to the continent at the Age of Exploration

Why? Where did they all go?• Glacial cold?• Insects?• Disease?• The acts of man?• Starvation?

• Other mammals that became extinct or disappeared from the W. Hemisphere in the Pleistocene epoch include rhinoceros, camels, saber-tooth tigers, elephants, and the mastodon

Survival of Equus

Members of family Equidae that survived:• Equus hemionus• Equus burchelli• Equus grevyi• Equus asinus• Equus caballus

Escaped to Asia by land bridge

Equus Caballus - The First 5• 5 distinct subspecies of Equus caballus

developed west of Siberia and China• Przewalski horse• Tarpan• Proto-Oriental• Proto-Warmblood• Proto-Draft

Proto-Draft

Proto-Warmblood

Proto-Oriental

Przewalski Horse

Tarpan

Lecture Objectives

• Define evolution• Describe the horse family tree • Describe the horse’s anatomical changes

and why they came to be