Horses have been lending a hoof to humans for thousands of years. But despite extensive archaeological excavations, researchers have not been able to pin down the history of where and when they were domesticated. Some researchers think it occurred just once, some 5000 years ago on the grassland steppes of Eurasia. Now a genetic study of living and fossil horses suggests that they were reined in many times.
To reach that conclusion, evolutionary geneticist Hans Ellegren of Uppsala University in Sweden and his colleague Carles Vilà studied mitochondrial DNA (mtDNA) from 191 pedigree horses, including primitive English and Swedish animals and one breed derived from animals imported to Iceland by the Vikings. They also obtained DNA samples from a Przewalski's horse, a small Mongolian equine thought by some to be a sister species to the original wild horses. They compared these samples with fossil DNA from leg bones of horses that have been preserved in the Alaskan permafrost for more than 12,000 years, and with other samples from 1000- to 2000-year-old archaeological sites in southern Sweden and Estonia.
Similar mtDNA analyses had shown that cattle, sheep, water buffalo, and pigs are much less genetically diverse than their ancient forebears, a sure sign that they descended from a small number of animals domesticated in just a few places. But the mtDNA samples from the modern horses showed almost as much genetic variation as samples from the fossil horses, the authors report in the 19 January Science. This suggests that "in many places over the world, people must have independently started to domesticate their local horses," Ellegren says.
While not all horse experts agree with these findings, they are nonetheless glad the work was done. Solving the riddle of equine domestication has been so difficult, says Sandra Olsen, an archaeologist at the Carnegie Museum of Natural History in Pittsburgh, that "anything that helps nail it down is a big help."