Life › The age of mammals › Grasslands spread
Horses
from c. 55 million years ago (Eohippus)· Organism or group· Animals
Well establishedStrong fossil, genetic or geological evidence.
A mammal family that evolved over about 55 million years from dog-sized forest browsers into fast, single-hoofed grazers.
See it on the Life map →What happened
Early horses such as Eohippus ('dawn horse') were small forest animals with four toes on the front feet and low teeth for browsing leaves. Over about 50 million years, mostly in North America, horse lineages grew larger, lost their side toes until they stood on a single hoof, and evolved tall-crowned grinding teeth. Modern Equus, which includes horses, zebras and donkeys, appeared about 4 million years ago. The family tree was bushy, not a straight line.
Why it matters
Horses are a textbook record of evolution tracking a changing landscape. Domesticated horses later reshaped human warfare, trade and travel (see the Curious History map).
The evidence (fossils, genes, rocks)
A dense fossil record from North America; Othniel Marsh's 1870s horse series was early evidence for evolution. Ancient DNA traces domestication to the Russian steppe about 4,200 years ago.
Still debated
Why horses died out in the Americas, where they evolved, about 12,000–10,000 years ago is debated: climate change, human hunting or both.
Connections (4)
▸Mammals radiate after the dinosaurs gave rise to HorsesWell established
The connection
Horses come from the odd-toed hoofed mammals, the group that also includes rhinos and tapirs, which appear suddenly around 56 million years ago. The earliest horses were small leaf-eaters in warm forests. From there, the horse family branched repeatedly, mainly in North America, over the next 50 million years.
When and how
North America and Europe, about 56–55 million years ago; Sifrhippus, from Wyoming's Bighorn Basin, is among the oldest known horses.
The evidence
Fossil teeth and limbs from Wyoming and Europe, and DNA and protein family trees placing horses with rhinos and tapirs.
Debates and open questions
Where odd-toed hoofed mammals came from is debated; a 2014 study pointed to India, with the 54.5-million-year-old Cambaytherium as a close relative.
▸Grasslands spread shaped the teeth and legs of HorsesWell established
The connection
Grass is tough and full of silica, and open ground is gritty, both of which wear teeth down. As grasslands spread, horses such as Merychippus, about 17–11 million years ago, evolved tall-crowned teeth that last a lifetime of grinding. Open country also favored speed: longer legs and a single hoof with springy ligaments for outrunning predators.
When and how
North America's Great Plains, about 20–5 million years ago, with later horse spreads into Asia, Europe and Africa across land bridges.
The evidence
Fossil horse teeth change from low to high crowns as grass phytoliths increase in the same rocks; tooth-wear studies show diets shifting to grass.
Debates and open questions
Timing does not always match: in South America, high-crowned teeth appeared before grasslands, so grit and volcanic ash may matter as much as grass itself.
▸Horses returned to the Americas in Columbian exchangeWell established
The connection
Horses evolved mostly in North America and crossed to Asia over the Bering land bridge, but died out in the Americas by about 10,000 years ago. Spanish ships brought them back from 1493. Over the next two centuries horses spread across the Great Plains, where peoples such as the Comanche and Lakota built new horse-based ways of life.
When and how
Columbus's second voyage landed horses on Hispaniola in 1493; Cortés brought them to Mexico in 1519; horses reached the Great Plains and Rockies by the 1600s.
The evidence
Spanish records, Indigenous oral histories and, from 2023, radiocarbon-dated horse bones and DNA showing horses in Plains communities by the early 1600s.
Debates and open questions
How quickly horses spread north, often ahead of Europeans, and how to weigh Indigenous oral histories and Spanish records against the new bone dates.
▸Horses lost their American branch in Ice-age megafauna extinctionsStrong evidence
The connection
Wild horses were common across North and South America through the last ice age. Between about 13,000 and 11,000 years ago they vanished there along with mammoths and camels, while Eurasian horses survived on the steppes and were domesticated about 4,200 years ago. The Americas then had no horses until the Spanish returned them.
When and how
North and South America, about 13,000–11,000 years ago, overlapping with the spread of Clovis-era people and end-of-ice-age climate swings.
How sure are we?
Radiocarbon-dated horse bones from Alaska to Patagonia stop around 11,000 years ago; ancient DNA shows American and Eurasian horses were linked across Beringia.
Debates and open questions
Whether hunting or shrinking grasslands did more to finish them, and whether horse DNA in Yukon soils means some lingered until about 5,700 years ago.