“There is nothing so good for the inside of a man as the outside of a horse!”

Mark Twain

Credit: © Associated Press

For several thousand years, the destinies of horses and humans have been inextricably linked. In transport, nutrition, warfare and sport, the horse has played its part. But how did the relationship begin? Was the horse domesticated just once, or many times, and from how many founders did the modern breeds of horse descend? Some valuable insights into these questions have been provided in a population genetics study by Vilà and colleagues.

The authors assessed the variation in mitochondrial DNA (mtDNA) from modern horse breeds and compared this with mtDNA from the remains of wild horses dating back 12,000–28,000 years. They then used phylogenetic methods to analyse the data, and to make inferences about the evolutionary origins of modern breeds. The genetic data indicate that modern breeds are genetically highly diverse, and can be divided into distinct clades — groups of animals or breeds descended from a single ancestor. By estimating the rate of sequence divergence, the origin of the clades can be dated at over 100,000 years ago.

However, according to archaeological evidence, the horse was domesticated around 6,000 years ago in a broad region of the Eurasian Steppe, where wild horses would have been captured and selectively bred for desirable characteristics. The diversity found in the genetic analysis can be explained if there were many genetically diverse founders of modern horses, implying that domestication occurred independently on multiple occasions at this time.

Using data from microsatellite markers, the authors suggest that females have contributed more to the genetic diversity of horse breeds than males, perhaps because of a bias towards females in trade and breeding. This is consistent with traditional breeding practices in which a single male is bred with multiple females.

The picture that emerges from the combination of genetic and archaeological data is that, around 6,000 years ago, the technical know-how required for the domestication of horses spread rapidly from one region to the next. Founders would have been captured and bred by communities in different regions, thus accounting for the genetic diversity seen in modern horses, and initiating a relationship that has lasted to the present day.