|Asian wild asses|
Today that genus consists of just seven widely recognised living species. But, even among just this genus, and ignoring all the older, extinct ones, there were once many more species than there are today. But just how many is that? That's a matter of considerable confusion and debate.
In a way, that's hardly surprising, especially when you consider the focus of attention that there has been on horse evolution. Even just looking at the living species, not everyone agrees that 'seven' is the appropriate number, with some authorities arguing that particular subspecies are distinct enough that they should really be species in their own right.
The most common example of that is Przewalski's horse (Equus ferus przewalskii), a wild horse native to Asia that's normally considered to belong to the same species as the domestic animal, but not always. On the one hand, they can crossbreed with domestic animals to produce perfectly fertile offspring, and even some fast-evolving parts of their genome are virtually indistinguishable. On the other hand, they do have two extra chromosomes, and there are physical differences that indicate they must at least be a different subspecies (as opposed to being, say, whatever the domestic horse was originally bred from). Our best guess is that they diverged about 250,000 years ago.
But, when we come to fossil species, it can be even harder to tell what is, and what isn't, a different species. As a result, a plethora of different extinct species have been named, and it's unlikely that they're all real. Even so, it's likely that there are at least a dozen, and probably more - plus the seven living species, of course.
To try and at least put some pattern on this mess, we can divide the genus Equus into smaller groups, placing the most similar species together. To avoid confusion as to what we mean by "horse", we use the term "caballine horses" to refer to those species most closely related to the domestic animal - of which only one survives today. The other two groups are the asses, which include the domestic donkey, and the zebras.
Which is all very well, and fits nicely with our common sense understanding of the living species. But, naturally, it gets a bit more complicated when we try to decide where the extinct species fit.
Back in the 1930s, the remains of a horse were excavated from a cave in southwestern Siberia. They were dated to 40,000 years ago, roughly in the middle of the last Ice Age, and they were clearly too small to be any kind of caballine horse. Instead, they were identified as belonging to a European ass (Equus hydruntinus).
Today, there are three species of wild ass, two in Asia, and one in Africa - the latter being the ancestor of the domestic donkey. The European ass first appeared around 300,000 years ago, at the dawn of the last-but-one Ice Age, and survived until at least the end of the last one. In the 1930s, it wasn't entirely clear that the animal really was an ass, but this is now generally accepted, and we know that the animal lived in a band from Portugal in the west to Iran in the east. It was probably most closely related to the living Asian wild ass or onager.
Being found in southwestern Siberia would clearly place this particular specimen rather further east than we now think European asses lived. In more recent times, this identification has been questioned, and in 2011, it was given its own species name, Equus ovodovi. This translates simply as "Ovodov horse", and is named for Nikolai Ovodov, a Russian palaeontologist. The assumption was that this was another Ice Age horse that lived in Asia, and that probably died out when the world warmed.
Since then, a number of other specimens thought to belong to the same species have been discovered, mostly along or close to the Altai Mountains, which form part of the southern border of Russia with Kazakhstan and Mongolia. But it's just one supposed species of horse amongst a great many. Is it deserving of full species status, and is actually a kind of ass, or something else?
Although originally identified as an ass, by the time it was named as a distinct species, a number of peculiarities in the form of the teeth made this seem less likely. Without complete fossils, though, it wasn't entirely clear just what the animal was, and it was placed in a fourth sub-group of Equus, named by one of the same authors just the previous year.
But members of this group are all long extinct, identified as belonging to it solely by those subtle differences in the teeth. To really resolve what they are, what we'd like is DNA evidence, of the sort that demonstrates that, yes, living asses are a distinct kind of animal, more related to one another than to domestic horses. Which is a bit of a problem when all you have is fossils.
Except... it's not.
It turns out that modern DNA extraction methods are sufficiently effective that it is actually possible to identify the DNA from these fossils. That they died out relatively recently, and in a fairly cold environment is likely helpful here, but even so, it's an impressive feat. In 2013, three species of extinct horse were studied using this method, and one of the three happened to be the Ovodov horse.
Of the three species studied, one, from North America, proved to be an extinct caballine horse (although further studies have since raised doubts about this). The second, an African animal called the quagga, only died out in 1878, and turned out to be, as previously suspected, a subspecies of plains zebra, rather than a full species. The Ovodov horse, however, proved to be something else entirely, clearly not a caballine horse, but with the evidence sufficiently equivocal that it's hard to know exactly where it fits within the family tree.
In 2016, a second specimen was analysed using a similar technique. This one had been recovered from Denisova cave, a location made famous since it is also the source of a fossil that DNA analysis proved belong to a previously unknown subspecies (or perhaps species) of human. In the case of the horse, the study seemed to confirm that this was a separate species, with strong hints that it was more closely related to zebras than to asses.
Some time ago, a farmer working near Zhaodong in north-eastern China dredged up three leg bones from the Kunni River, and handed them in to the local museum. They proved to belong to horses that had died between 12,000 and 40,000 years ago, and were identified as belonging to Equus dalianensis, an extinct species that probably belongs to the caballine group.
More recently, however, it proved possible to perform DNA analysis on the bones, and it turned out that the original classification was wrong, with an almost perfect match for the Ovodov horse, previously only known from Russia. Apart from the fact that it's difficult to identify an exact species of horse when all you have is a bit of the lower leg, this tells us three things.
Firstly, Ovodov horses apparently lived much further east than we previously knew, as far across as Manchuria. While this is a new discovery, it's not hugely surprising, given that Manchuria is at about the same latitude as the Altai Mountains and other fossil species of the same age are found in both locations; the two regions were probably climactically similar at the time, and there are no obviously impassable barriers between them.
Secondly, since one of the bones may be as little as 12,000 years old, the Ovodov horse also survived for longer than we'd previously realised - none of the Russian fossils are much younger than 30,000 years. Assuming that this represents the last gasp of the species, it would have died out when the Ice Ages ended, either because of the change in the climate, or with an expansion of human hunting (or both).
The genetic evidence also makes it clear that the animal does not belong to any living species, or to any other extinct species that has had its DNA sequenced. Technically, this doesn't necessarily mean that this is one species we know for certain is valid - it could be that some other species of extinct horse, named before 2011, is actually the same animal, and has naming priority. But we can say that, whatever it is, it really does belong to a fourth group of Equus horses - neither a caballine, an ass, or a donkey.
Having said which, the study does back up the earlier genetic evidence that, despite living in central and eastern Asia, the animal was more closely related to zebras than to donkeys and asses. It probably diverged from the zebras proper not long after those animals had split from the ancestral asses, and perhaps at around the same time that the African and Asian asses separated from one another.
It's one glimpse into a corner of recent horse evolution that remains, as a whole, still rather murky and confused.
[Photo by "jinterwas" from Wikimedia Commons. Cladogram adapted from Yuan et. al. 2019.]