The rich fossil record of Equus over the past 55 million years were punctuated by many episodes of origin,migration,extinction and evolution,which has made them as a textbook example of evolution.The ancestors of Equus originated in Northern America continent in the Eocene period,called Hyracotherium.As the climate changes,the Bering Strait sea level rise and fall.The Equus groups colonized the Old World several times crossing Beringia but they left no descendants there.Equus genus continued to evolve in North America.Until the Pliocene,the most recent common ancestor of present-day Equus emerged,called Pliohippus.Within the following 3 million years,their ancestors rapidly expanded and occupied the ecological niches in different geographical areas,radiating into 3 main subgenus(horse,donkey and zebra).The main reason for facilitating the speciation was the acute chromosomal rearrangements.With the publication of the horse whole-genome sequences and the donkey draft genome sequences,and the progress of biomolecule level,the identification of Equus species karyotype diversity is more profound. More and more scientists believe that the mechanism of which Equus interspecies hybridization can produce offspring is based on the high similarity of genome sequence.And their hybrid offspring infertility is caused by the karyotype diversity which makes the meiotic recombination breakdown.This assumption still needs to be confirmed by experiments.