The story of how wild horses were tamed and domesticated by humans has never been told before. New genetic research by biologists at UCLA and three Swedish universities now reveals some of this history.
The biologists, writing in the Jan. 19 issue of the journal Science, show that today's domestic horse resulted from the interbreeding of many lines of wild horses in multiple locations.
"The taming of wild horses was extensive and widespread, and not confined to a small area and a single culture," said Robert Wayne, a UCLA professor of organismic biology, ecology and evolution, and a co-author on the study. "The domestic horse has ancient and diverse origins. We found a huge diversity in DNA sequences.
"Because many characteristics of wild animals must be suppressed so they can live alongside humans, many biologists believe animals were domesticated through selective breeding over many generations to produce such traits as docility," Wayne said. "However, we discovered that the theory of small numbers of animals carefully bred over many generations to adapt to humans could not nearly explain the enormous genetic diversity we found in horses.
"If all domestic horses descended from a handful of female horses, we should have seen a limited diversity in mitochondrial DNA sequences, which are inherited from the mother," he said. "Instead, we found many more sequences, and they were much more diverse than one would find in a single wild population. Many other animals were domesticated in one location, but this genetic diversity suggests that the domestication of horses must have been widespread. The theory that a few horses from one population were domesticated by a single clever culture does not fit."
"In many parts of the world, people must have independently started to domesticate wild horses," said Carles Vila, a former UCLA postdoctoral scholar in Wayne's laboratory who is an evolutionary geneticist at Sweden's Uppsala University, and lead author of the study.
Using molecular genetic techniques, the biologists studied DNA sequences from both modern and ancient horses, including more than 190 pedigree horses and a breed derived from animals imported to Iceland by the Vikings. The UCLA biologists extracted and analyzed DNA samples from the leg bones of wild horses that lived in Alaska more than 20,000 years ago and have been preserved in the Alaskan permafrost. Biologists from Sweden also analyzed DNA from horses in southern Sweden and Estonia that are more than 1,000 years old, and from a small Mongolian equine thought by some to be a sister species to the original wild horses.
Archaeological remains show that horses were hunted as a source of food. Wayne and his colleagues infer from their genetic evidence that abundant wild horses were eventually hunted to the point where they became rare, and that scarcity led to the breeding and taming, rather than capturing, of wild horses.
"Our evidence suggests that as the wild ancestors of the domestic horse became rare, techniques to raise and tame them became a great advantage," Wayne said. "Knowledge of these techniques spread rapidly from culture to culture, because having a horse that could be ridden provided a huge advantage. Our findings suggest that as wild horses dwindled due to their exploitation or environmental changes, knowledge of how to tame and raise horses increased.
"When you could capture an animal any time you needed one, what would be the point of domesticating it? Scarcity changed the equation when over-exploitation led to the disappearance of horses in the wild."
Horses are believed to be among the last large mammal to be domesticated, presumably, Wayne said, because of their abundance in the wild. Domestic dogs, cattle, sheep and goats were established several thousand years before the horse was domesticated.
Similar DNA analyses of cattle, sheep, water buffalo and pigs conducted by other scientists indicate that they derived from a small number of animals domesticated in just a few places 8,000 to 10,000 years ago. These modern animals are much less genetically diverse than their ancient forebears.
The UCLA and Swedish scientists analyzed DNA sequences from the control region of the mitochondrial genome -- a region with a high mutation rate.
Additional contact: Harlan Lebo (firstname.lastname@example.org)
The other scientists on the team were Jennifer Leonard, a UCLA graduate student in Wayne's laboratory; Kerstin Liden from Stockholm University's Archeological Research Laboratory; his graduate student Anders Gotherstrom; Stefan Marklund and Kaj Sandberg from Swedish University of Agricultural Sciences; and evolutionary biologist Hans Ellegren from Uppsala University.
The research was funded by the National Science Foundation, the Swedish Research Council for Agriculture and Forestry, the Swedish Royal Academy of Sciences, and the Bank of Sweden Tercentenniary Foundation.