Yes, the earliest known ancestor of the horse was indeed dog-sized.
The Dawn of Horse Evolution: Hyracotherium
The fascinating lineage of the modern horse began approximately 55 million years ago with the emergence of Hyracotherium, also historically known as Eohippus. This ancient mammal, which scampered through the dense forests that covered North America, was remarkably small—roughly the size of a dog. This contrasts sharply with the large, powerful horses we are familiar with today.
Hyracotherium was primarily a forest browser, meaning its diet consisted mainly of leaves and soft plant material, perfectly suited for its wooded habitat. For more than half of their evolutionary history, most members of the horse family remained relatively small and adapted to browsing in forests. The dramatic shift towards larger sizes, single hooves, and a grazing lifestyle evolved much later, driven by changes in global climates that led to the expansion of open grasslands.
Key Characteristics of Early vs. Modern Horses
The table below highlights some of the significant evolutionary changes from the early horse ancestor to the modern horse:
| Feature | Early Ancestor (Hyracotherium) | Modern Horse (Equus caballus) |
|---|---|---|
| Size | Dog-sized (approx. 20 inches tall at the shoulder) | Large (approx. 56-68 inches tall at the shoulder) |
| Feet | Four toes on the front feet, three on the back feet (each with a small hoof) | Single hooved toe (digit III) on each foot |
| Diet | Forest browser (leaves, soft plants, fruits) | Grass grazer (tough, abrasive grasses) |
| Habitat | Dense forests | Open grasslands, plains, steppes |
This remarkable transformation from a small, multi-toed forest dweller to the powerful, single-hoofed grassland runner demonstrates the incredible journey of adaptation and diversification within the horse family. To delve deeper into this extensive evolutionary path, you can explore resources on the history of equids here.
