For horses, it's been a long and winding road from the steppes of Eurasia 6,000 years ago to the Kentucky Derby today. A new genetic study sheds light on how early equines expanded geographically before and after they were domesticated by humans.
Archaeological evidence, including the first appearances of horse remains amongst human burial sites and traces of mares' milk found on pottery shards, pins the first domestication of horses to the western Eurasian Steppe.
But previous studies examining horse mitochondrial DNA -- which is passed down from mother to offspring -- have revealed several distinct female lineages, which some scientists have taken to mean that separate herds of horses were domesticated in multiple instances. That conflicts with the single-origin theory of domestication supported by the archaeological evidence.
A team led by Vera Warmuth of the University of Cambridge plugged genetic data from more than 300 horses from Eastern Europe and Central Asia into a computer modeling program in an effort to reconstruct history and resolve the supposedly conflicting evidence.
The results were published on Monday in the journal Proceedings of the National Academy of Sciences.
The genetic patterns the team saw suggest that 160,000 years ago, the wild ancestor of horses expanded from eastern Eurasia into the western steppe. It was in this region, comprising parts of modern-day Ukraine, Kazakhstan, and Russia, where the horse's wild ancestor was domesticated about 6,000 years ago.
Then, when horses spread out of this area following domestication, lots of wild horses incorporated into the herds, Warmuth said in a telephone interview.
The authors speculate that early humans may have had trouble breeding domesticated horses, pointing to the difficulties zoos have faced in breeding Przewalski's horse -- the closest wild relative of the domestic horse -- in captivity.
Because stallions are inherently more difficult to handle than mares, the easiest way to maintain or grow herd sizes would have been to restock existing herds with wild females, they wrote.
That would then account for the different female lineages amongst modern horses while preserving a single origin of domestication.
Horse domestication coincided with an increased number of human settlements in the eastern Eurasian Steppe, according to Warmuth.
The region didn't support the large herds of animals that would have sustained hunter-gatherers, nor was it suitable for farming.
But as soon as they had horses and sheep, people were able to exploit the inner Asian steppes, Warmuth says.