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About 55 to 15 million years agoReputable source · 2 sourcesWell documented

Horses trade the forest for the grassland, tooth by tooth

On the timeline · around About 55 to 15 million years ago · The Miocene GrasslandsThe Miocene GrasslandsHorses trade the forest for the grassland, tooth by tooth13 Ma12 Ma11 Ma10 Ma9 Ma8 Ma7 Ma6 Ma

What happened

The earliest known horse, Hyracotherium, also called Eohippus, was a fox-sized forest browser around 55 million years ago, standing barely 50 centimetres tall, with four padded, hoofed toes on each front foot and three behind, built for soft forest ground rather than open plains. As global cooling spread grassland savannas across the continents over the following tens of millions of years, horse lineages adapted alongside it. By around 17 to 11 million years ago, Merychippus had become the first true grazing horse, with high-crowned cheek teeth able to withstand the punishing, silica-laden wear of grass, long legs built for outrunning predators across open ground, and a body plan recognizably horse-shaped for the first time, even while still carrying three toes rather than the single hoof of a modern horse.

Why it matters

A horse's teeth and legs are a direct record of an entire ecosystem changing shape underneath it, forests thinning into grassland over tens of millions of years. Every plains-grazing hoofed mammal alive today, not just horses, is running some version of the same adaptation this transition forced.

How we know

The claim rests on a densely sampled fossil sequence spanning tens of millions of years, in which tooth crown height, leg bone proportions, and toe count can all be tracked generation by generation as grassland-adapted species replace forest-browsing ones in the same rock layers.

Sources

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Part of a timelineThe Age of Mammals5 events · The 60 million years between the dinosaurs and the first humans, when rat-sized survivors grew into whales, horses, and elephants across a world remade by spreading grasslands.View all →