appears to have spread quickly into the Old World, with the similarly aged E. livenzovensis documented from western Europe and Russia. Molecular phylogenies indicate that the most recent common ancestor of all modern equines (members of the genus Equus) lived ~5.6 (3.9-7.8) Mya. Direct paleogenomic sequencing of a 700,000-year-old middle Pleistocene horse metapodial bone from Canada implies a more recent 4.07 Mya for the most recent common ancestor within the range of 4.0 to 4.5 Mya. Mitochondrial evidence supports the division of Equus species into noncaballoid (which includes zebras and asses) and caballoids or "true horses" (which includes E. ferus and E. przewalskii). Of the extant equine species, the lineage of the asses may have diverged first, possibly as soon as Equus reached the Old World. Zebras appear to be monophyletic and differentiated in Africa, where they are endemic. Members of the subgenus Sussemionus were abundant during the Early and Middle Pleistocene of North America and Afro-Eurasia, but only a single species, E. ovodovi survived into the Late Pleistocene and Holocene in south Siberia and China, with the youngest remains from China dating to around 3500 BP (1500 BC), during the Shang dynasty. Genetic data from E. ovodovi has placed the Sussemionus lineage as closer to zebras and asses than to caballine horses. Molecular dating indicates the caballoid lineage diverged from the noncaballoids 4 Mya. Genetic results suggest that all North American fossils of caballine equines, as well as South American fossils traditionally placed in the subgenus E. (Amerhippus), belong to E. ferus. Remains attributed to a variety of species and lumped together as New World stilt-legged horses (including E. francisci, E. tau, and E. quinni) probably all belong to a second species that was endemic to North America. This was confirmed in a genetic study done in 2017, which subsumed all the specimens into the species E. francisci which was placed outside all extant horse species in the new genus Haringtonhippus, although its placement as a separate genus was subsequently questioned. A separate genus of horse, Hippidion existed in South America. The possible causes of the extinction of horses in the Americas (about 12,000 years ago) have been a matter of debate. Hypotheses include climatic change and overexploitation by newly arrived humans. Horses only returned to the American mainland with the arrival of the conquistadores in 1519. Extant species Prehistoric species Many extinct prehistoric species of Equus have been