The genus Equus was first described by Carl Linnaeus in 1758. It is the only recognized extant genus in the family Equidae.[7] The first equids were small, dog-sized mammals (e. g. Eohippus) adapted for browsing on shrubs during the Eocene, around 54 million years ago (mya). These animals had three toes on the hind feet and four on the front feet with small hooves in place of claws but also had soft pads.[8] Equids developed into larger, three-toed animals (e. g. Mesohippus) during the Oligocene and Miocene.[7][8] From there, the tridactyl toes became progressively smaller through the Pleistocene until the emergence of the single-toed Equus.[9]
The genus Equus, which includes all extant equines, is believed to have evolved from Dinohippus, via the intermediate form Plesippus. One of the oldest species is Equus simplicidens, described as zebra-like with a donkey-shaped head. The oldest material to date was found in Idaho, USA. The genus appears to have spread quickly into the Old World, with the similarly aged E. livenzovensis documented from western Europe and Russia.[9] Molecular phylogenies indicate 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 Myr before present date for the most recent common ancestor within the range of 4.0 to 4.5 Mya.[10]
Molecular evidence supports the division of Equus species into stenoids (which includes zebras and assess) and callaboids or "true horses" (includes E. ferus).[6][11] Of the extant equine species, the linage of the asses may have diverged first,[7] possibly as soon as Equus first reached the Old World.[11] Zebras appear to be monophyletic and differentiated in Africa