A recent study has found that the familiar coat colors of today's horses are likely the product of the 5,000-year long relationship between humans and horses.
Scientists believe that up until about 5,000 years ago, when humans first began domesticating wild horses, horses' coats were probably striped and looked more like zebras' coats.
The last truly wild horses remaining today are Przewalski's horses. They're dun-colored horses with stripes on the legs and a striking dorsal stripe running along the back. The distinctive dun color is the result of a dominant gene. So, even if a horse has only one copy of the gene, its coat will be dun-colored.
The frequency of dun coloration among Przewalski's horses is quite a contrast from the coats of most domestic horses, the majority of which have monochromatic coats in colors other than dun, and no stripes.
In order to examine how, if at all, domestication affected horses' coat colors, researchers analyzed the DNA of two ancient horses. One of the horses died 4,400 years ago, and the other died 42,700 years ago. Once the DNA had been analyzed, the researchers compared the genes of both specimens to living zebras, wild horses, and domestic horses.
All of the zebras and the Przewalski's horses in the study had two copies of the dun gene. However, most of the domestic horses used in the study lacked any trace of it. Likewise, the 4,400-year old horse showed no sign of the dun gene. However, the 42,700-year old horse did; it had one copy, and was likely dun colored.
Although this study yielded only two data points, the findings suggest that both dun-colored coats and uniform coat colors did exist prior to domestication, but that, because of the dominance of the dun gene, dun coats were probably more frequently occurring than other coat colors among ancient wild horses.
Moreover, the researchers believe that uniform coat colors began popping up with greater frequency around the time humans began domesticating horses for farm use. They speculate that because dun provides a natural camouflage, humans probably bred selectively for uniform coat colors because such colors would have been much easier to find in the fields.
Want to learn more about the evolution of horses' coat colors? You can access the study, published in "Nature Genetics," here.