The four foundations theory is one of many theories in the world of horse breeding which is designed to supplement the history of the horse, filling in gaps in knowledge and explaining how horses evolved into their modern, domesticated form. It is important to remember that the four foundations theory is only a theory, and evidence may emerge to contradict it at some point in the future. There are also competing beliefs about the evolution of the modern domestic horse, some of which contradict the four foundations theory.
This theory suggests that all modern domestic horses are descended from one of four foundation stocks, which developed unique physical traits in response to their environments. According to the four foundations theory, modern horses emerged from Europe, Asia, and the Middle East, with the “protohorses” in the four foundations forming the basis of all modern horse breeds, from the Shetland Pony to the Clydesdale.
There is some argument over whether the four foundation breeds were actually separate horse species, or subspecies, meaning that they could have interbred. According to the theory, the four types of protohorses which emerged in Europe were: forest horses, draft horses, oriental horses, and tarpan horses. Each of these horses would have been distinct from the other foundations. Numerous examples which fit into each of these categories can be produced, although people could also argue that the four foundations theory is a back-formation which relies on modern horses to explain ancient horses, rather than the other way around.
Forest horses are also sometimes called warmbloods in discussions of the four foundations theory, with people believing that they adapted to the forested environments of Central Europe. The draft subspecies developed to handle the damp, cold environments of Northern Europe, with a small, stocky body covered in shaggy hair, while the oriental type developed in the dry, arid deserts of the Middle East, becoming tall, slim, and energetic. The tarpan evolved to handle the cold, dry world of Northern Asia, being small, sturdy, and somewhat shaggy.
Proponents of the four foundations theory say that it could explain the incredible diversity of the modern domesticated horse quite neatly, and there is some evidence in remaining wild horse stocks to support it. However, it can be dangerous in biology to make assumptions on the basis of existing modern information alone, as evolution can be extremely complex, and we may be missing a vital key in the puzzle.