To track the trail of domestic horses, Thomas Jansen of Biopsytec Analytik in Rheinbach, Germany, and his colleagues sequenced DNA from 318 horses representing 25 different breeds. Specifically, the team analyzed mitochondrial DNA (mtDNA), which is inherited from the mother, and compared the recent samples to previously published DNA data from 334 other animals. The researchers identified 17 distinct types of mtDNA and calculated that at least 77 different wild mares must have been domesticated in order to account for today's domestic horses. Just how these animals were domesticated remains unclear, however. Because of the necessary diversity of the mares, the team posits that several separate and geographically diverse populations participated in the process. One theory holds that domestication occurred independently at a number of locales. Alternatively, the procedure may have slowly spread from a single starting point. In that case, the authors write, "the knowledge and the initially domesticated horses themselves would have spread, with local mares incorporated en route, forming our regional mtDNA clusters."