The other clade was broadly distributed from North America to central Europe, north and south of Pleistocene ice sheets.
Therefore, the domestic horse today is classified as Equus ferus caballus.
Either way, the most common theories of prototypes from which all modern breeds are thought to have developed suggests than in addition to the so-called Tarpan subtype, there were the following base prototypes: The Tarpan became extinct in the late 19th century and Przewalski's horse is endangered; it became extinct in the wild during the 1960s, but was re-introduced in the late 1980s to two preserves in Mongolia.
Although researchers such as Marija Gimbutas theorized that the horses of the Chalcolithic period were Przewalski's, more recent genetic studies indicate that Przewalski's horse is not an ancestor to modern domesticated horses.
Their analysis placed all equids into a single clade, or group with a single common ancestor, consisting of three genetically divergent species: Hippidion, the New World stilt-legged horse, and the true horse.
How and when horses became domesticated is disputed.
A 2014 study compared DNA from ancient horse bones that predated domestication and compared them to DNA of modern horses, discovering 125 genes that correlated to domestication.
Some were physical, affecting muscle and limb development, cardiac strength and balance.
Other researchers look at broader evidence, including skeletal and dental evidence of working activity; weapons, art, and spiritual artifacts; and lifestyle patterns of human cultures.
There is also evidence that horses were kept as meat animals prior to being trained as working animals.