A new genus of extinct horse
Horses, zebras, asses and donkeys are all members of the group, or genus, called “Equus”. Their first ancestors arose millions of years ago. But as recently as a few tens of thousands of years ago there were two anatomically quite different groups of these animals alive side by side on Earth. One group - called the stout-legged horses - are a close genetic match for all of the surviving horses around today. But the other group - called the stilt-legged horses - has since disappeared although it had a similar leg bone structure to certain family members that are still around today. Based on the anatomical similarities, palaeontologists had previously suggested that the extinct horses were related to today’s surviving species. Now a much more comprehensive genetic analysis using fossil DNA from stilt-legged specimens suggests that, instead, these stilt-legged horses were an entirely separate genus, which Peter Heintzman and his colleagues are dubbing Harringtonhippus...
Peter - In North America during the Pleistocene around 225 to 12000 years ago there were two groups of horses. One group that had these, kind of like, broad foot bone, stout foot bones which are very much like living horses and then there was this other group, the stilt-legged horses that had much more slender or thinner foot bones. In fact, these thinner foot bones that stilt-legged horses had were very similar to those of living Asiatic asses. Kiangs, Onagers, and Kulans which are relatives of donkeys and currently live in parts of Asia such as Tibet. However previous ancient DNA had shown that actually, they looked more like these stout-legged horses so like actually living horses. So we had these, kind of, competing ideas as to whether the stilt-legged horses were closely related to the living Asiatic asses or living horses.
Chris - Horses go back a long while though don't they, I mean they've been on Earth for what? More than 50 million years?
Peter - Yes, so the original ancestor of the horse lineage, so that includes living's zebras and donkeys, goes way back to around 55 million years ago to a very small animal called Hyracotherium, which was about the size of a dog. It had three toes instead of one and was a browser rather than a grazer. So it would eat leaves rather than grasses. Throughout the 55 million year history of horses in zebras and their relatives, they got larger, they lost two of their toes so they only have one toe now, and they generally became grazers but they didn't evolve in a linear fashion like that, there were many offshoots and extinct lineages now completely dead.
Chris - So how do you think we ended up in this situation about a hundred thousand years ago or so where you saw this these two groups living side by side with quite different foot structures then?
Peter - What we can do is infer from living horses and these Asiatic asses as to what they were potentially used for. So the Asiatic asses which have these similar foot structures to the stilt-legged horses live in very dry and quite inhospitable conditions today, so like on the Tibetan plateau. So it's likely that these stilt-legged horses based on these but these kind of like bone dimensions were living in quite dry and harsh environments.
Chris - So how did you seek to resolve some of these questions about where these animals came from and where they went?
Peter - We wanted to build on the previous ancient DNA findings that stilt-legged horses were quite closely related to living horses, and what we did is we generated a lot more data than have been generated before so we looked at more individuals and we generated whole mitochondrial genomes. So the mitochondria are these powerhouses that are in cells and they have their own genomes that are independent from the nuclear genome. So the main genome that's found inside living cells. And in addition to these mitochondrial genomes we also generated information from the nuclear genome and in the past people had only used the mitochondria before. And when we took all of that new data together we actually found something very interesting that the stilt-legged horse was actually very very different from living horses and actually from the Asiatic asses.
Chris - So where did it come from?
Peter - What we actually found is that it was so different that it fell completely outside of living horse diversity. So it seems that the lineage leading to the stilt-legged horses and that leading to modern horses, zebras, and donkeys diverged around 4 to 6 million years ago most likely in North America.
Chris - So it's a bit like what we see with early human ancestors where Nature was doing all these experiments in parallel with lots of different ancestors all coexisting and sitting side by side and slowly emerging from that are effectively the fittest and the best specimens. So you're saying you've got evidence you have a completely new genus which is sitting side by side, or running round side by side with what ultimately became modern surviving horses but these ones have gone.
Peter - Yes absolutely. So actually the genus Equus which living horses belong to, they also have a lot of extinct relatives but this new genus which we called Harringtonhippus, it became extinct around 14000 years ago in North America.
Chris - Any idea why it disappeared?
Peter - We don't know for sure, but what is intriguing is that it went extinct around the same time that Sabertooth Cats and Woolly Mammoths were going extinct. So it may be that there was a common cause to that.
Chris - Just thinking about what might have been happening then, we had an end of an Ice Age around that time. We also had a lot of human migration coinciding with that so they could be both climatological, but there could also be a mankind kind of blame.
Peter - Absolutely. These are kind of like, two very competing ideas at the moment. Whether, yeah, a lot of these extinctions were caused by climatic changes or by human overhunting or a mixture of the two. At this point in the case of the stilt-legged horse, it's a little too early to be able to answer that. But from other taxa it seems like human hunting probably exacerbated a problem that was caused by rapid climate warming that was occurring at the end of the last ice age.