The genus is noted for its members having just one toe on each foot. The story of how this happened, and the number of toes became reduced, is one of the most frequently repeated in mammalian evolution, although the detail may be more complex than is sometimes presented. The story of how the genus evolved since that point, however, is much less so.
There have been a great many fossil species named within Equus, and it's unlikely that they're all genuinely distinct from one another. On top of that, we would also have to question just how close to modern horses a fossil has to be in order to place it within the same genus - we can hardly check to see if they crossbreed, after all.
Like all named ranks within biological classification, the concept of a genus is somewhat arbitrary. But given the fact that we use it as the first part of the scientific name and it's therefore rather important to our ability to communicate exactly what animal we're talking about, there have at least been attempts to come up with a definition of "genus".
One way of resolving the question is to simply admit that named ranks are fundamentally arbitrary, assume that the people who name living species at least know what they're doing, and say that we can place a fossil species within a group if it is descended from the last common ancestor of all living members of that group. This "crown group" definition is the narrowest possible one, although it still adds plenty of extinct species to Equus. Since the living genus includes at least three different kinds of animal - horses, asses, and zebras - so as long as you can say that your fossil is one of those three things, it must be part of the genus. (Of course, this definition doesn't work if the genus is entirely extinct, but that's a matter for another day).
Other definitions are broader, allowing us to acknowledge the similarity of some fossil species to living ones and avoid giving them all new names. One possibility is to say that a genus is a group of very closely related species that all share the same mode of life. In the case of Equus, that would be something like "adapted to living in open arid environments and eating low-quality roughage such as grass". Since the one-toed thing is (probably) an adaptation to running rapidly across open plains that should also come close to our visual concept of what a horse should be.
In practice, some of the earliest known species of Equus probably don't fit that exact definition although they may fit with a proposed guideline of "you can call it a genus (or higher rank) if it's been around for at least five million years or so". Even then, it's difficult to know for certain, because to estimate when two different types of animal diverged we need genetic samples from both of them to see how much they have changed since their common ancestor lived, as well as fossil remains that we can use to calibrate how much time that amount of change is likely to have taken.
Using that technique, we can estimate that the common ancestor of zebras and asses probably diverged from the common ancestor of true horses around 4 million years ago, during the early Pliocene epoch. But with every other genus in the horse family thoroughly extinct, we can't do the same for Equus itself - there's nothing to compare it to.
Using the traditional naming scheme, however, the earliest known member of the horse genus is the Hagerman horse (E. simplicidens). This lived between about 5 and 3.5 million years ago in North America, with the original fossil being uncovered in Hagerman, Idaho in the 1890s. It physically resembled a modern zebra in size and shape, although obviously, we don't know anything about the colour. Some analyses have suggested that it may be the common ancestor of all living members of the horse family and literally the first member of the genus, but others imply that it's an early side branch. In which case, under the stricter definitions of "genus", it has to be given a different name which, in this case, would be Plesippus.
In any event, it's clear that horses, as we know them today, first evolved in North America, and numerous other species are known from the continent over the last few million years. The last of them died out around 10,000 BC, presumably due to hunting by humans; it's unclear whether this was a now-vanished species or a representative of the modern horse. They even crossed into South America; the species E. neogeus lived across much of the continent during the latter part of the Ice Ages, dying out around the same time as its northern counterparts.
The reason that horses exist today at all is because, at some point, they left America by the northwesterly route and ended up in Eurasia, eventually spreading from there to Africa. This is conventionally thought to have happened in a single event 2.58 million years ago, which places it at exactly at the beginning of the Pleistocene epoch. So significant is this that it has a name: the Equus Datum. Essentially, the idea is that if you can find a fossil horse in a Eurasian rock deposit, that deposit must date to the Pleistocene or later. From an ecological point of view, the presence of horses can be said to define the line between the Pliocene and the following epoch.
But how accurate really is that?
Certainly, there are fossil horses known from Eurasia that date right back to that time. That their first crossing can't have been any later than that isn't in dispute. Furthermore, they crossed the continent almost instantly from a geological perspective, since at least one of those very early fossil species, E. major, is known from as far west as Italy. Although it's worth mentioning that "almost instantly from a geological perspective" may still mean thousands of years...
Still, it wasn't necessarily a single event. For one thing, the ancestors of true horses had long since diverged from those of the other members of their genus by this point. So, as a bare minimum, two different species must have made the crossing and they didn't necessarily do so simultaneously. Indeed, we know of around five different species that appeared in China almost immediately after the Datum, none of which are likely to be ancestors of the living sorts, so it's likely that several different kinds of horse took the opportunity to cross over to pastures new in the earliest part of the Pleistocene.
In between Europe and China, the first one-toed horse on the Indian subcontinent is E. sivalensis. This, too, goes back right to the Datum, and it survived until about 500,000 years ago, when it was replaced by E. namadicus, which in turn died out around 13,000 BC, just before the end of the Ice Ages. So similar are these two species that it has been argued that they are really the same animal, and even if they aren't, it's at least plausible that the one evolved directly into the other.
Judging from the remains we have, E. sivalensis was more closely related to asses and zebras than to true horses. Strictly speaking it was probably neither, although it was similar in size. The oldest fossil horses from Africa date to 2.44 million years ago, 140,000 years after the Equus Datum in Eurasia. At least one of these, E. tabeti, appears to have been an early ass, and has been suggested as a direct ancestor of the modern domestic donkey. E. koobiforensis, which dates from 2.1 million years ago in Kenya may be the oldest known African zebra, although it's likely closely related to the slightly older E. stenonis from Greece.
It has been suspected for a while that E. sivalensis may actually predate the Pleistocene and a recent study confirms this, identifying a partial skull from just below the magnetic anomaly in the rock that, geologically speaking, defines the start of the epoch. So it looks as if the most significant event in the history of modern horses (other than their initial origin), the Equus Datum that distinguishes the start of the Pleistocene, does nothing of the sort.
Horses, it seems, snuck into Eurasia earlier than we'd thought.
[Picture by Alex Uchytel, from Wikimedia Commons.]
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