A 2005 study analyzed the mitochondrial DNA (mtDNA) of a worldwide range of equids, from 53,000-year-old fossils to contemporary horses. Their analysis placed all equids into a single clade, or group with a single common ancestor, consisting of three genetically divergent species: "Hippidion", the New World stilt-legged horse, and "equus", the true horse. The true horse included prehistoric horses and the Przewalski's horse, as well as what is now the modern domestic horse, belonged to a single Holarctic species. The true horse migrated from the Americas to Eurasia via Beringia, becoming broadly distributed from North America to central Europe, north and south of Pleistocene ice sheets. It became extinct in Beringia around 14,200 years ago, and in the rest of the Americas around 10,000 years ago. This clade survived in Eurasia, however, and it is from these horses which all domestic horses appear to have descended. These horses showed little phylogeographic structure, probably reflecting their high degree of mobility and adaptability.