Could Unicorns Exist in Nature?

With the help of fairy tales and entertainment companies like Disney, many people have some idea what a unicorn is. Merriam-Webster’s online dictionary says that a unicorn generally looks like a horse with a single horn on its forehead.[1] This agrees with popular fictional media, except this popular fiction tends to depict unicorns only as horses with single horns, instead of also allowing (e.g.) cloven hooves. In my definition, I’m siding with popular fiction and including only members of the genus Equus, whose only known living members are horses, zebras, and donkey relatives. None of these happen to have cloven hooves. (A genus is a level of taxonomy, a means of classifying living things. The traditional taxonomy levels – taxons – from top to bottom are: kingdom, phylum, class, order, family, genus, and species.) Let nature genetically put a single horn on the head of any species of the genus Equus, and I would call it a unicorn.

With the right conditions and history, could unicorns theoretically exist in nature today? My question is not whether they do exist today, but whether unicorns have had enough time to evolve as such and breed that way consistently, as opposed to an accident or the result of manipulation outside of nature. So I’m ruling out accidents, technology, and fairy-tale-type magic.

A search on the Web about unicorns can tell you more than a few things:

Unicorns appear in many cultures around the world.
Various people have claimed to have seen them.
There are fossils for animals similar to unicorns but not exactly the same (such as Elasmotherium, an extinct rhino relative).
There are occasional accidents (maybe animals with birth defects) that resemble unicorns, though they occur in non-horse species that already have 2 horns.
Horn buds of a horned species member can be forced to grow into a single horn.

None of these exactly answer my question with a yes. So it looks like more detailed research is needed. Questions to look into:

How long ago did equines (members of Equus) first appear, that we would today recognize as such?
How long might it take for an equine species to evolve a horn? Or alternatively, how long has it taken for horns to evolve as a feature in animals that (seem to) have none?
Have any of the modern equine species had enough time since first appearing, to have grown a horn?

Tackling the first question first, how long ago did a reasonably modern equine evolve? Equines are the only living members of the family Equidae. New World Encyclopedia says that some equids (members of Equidae) were finally becoming very horse-like during the Miocene period,[2] which apparently started a little over 23 million years ago and ended about 5.3 million years ago.[3] Furthermore, Encyclopedia Britannica says that the Equus genus evolved roughly 4-4.5 million years ago.[4] So with recognizably horse-like equids already have appeared over 5 millions years ago, we got real equines showing up maybe 4.5 million years ago. Is that enough time for any species of horses or other equines to have sprouted a horn?

That leads to the second question of how long it might take for an equine species to grow a horn. The closer genetic relatives of horses probably have more to tell us than the more distant relatives. It would probably be easier for an equine species to grow a horn if some equine species already has or had horns. First, are there any equines that currently have horns, at all? Not exactly. There is, however, a horse breed called the Moyle horse [5] that seems to have extra ridges (bosses) on the sides of the head close to the ears. These seem to be bone formations instead of horn material. That’s all I found for the present day about current equines with horns.

I didn’t find anything about actual equine horns existing in the past either. So, going up a level, equines are part of a larger classification, the family Equidae. Researching Equidae members (called equids) seems to indicate the equines are the only living equids. Possibly, the extinct equids are ancestors – or at least close genetic relatives – of equines. Again, I didn’t run across true horn history for past equids either.

Equids do happen to be grouped with creatures called brontotheres in a suborder (presumably a subdivision of an order) called Hippomorpha, according to Wikipedia.[6] From (speculative) appearance, it seems easy to assume brontotheres are rhinos; however, the Hippomorpha suborder puts them closer to horses. What makes them important here is that some of them have horns. They may be the closest relatives to equines that had horns, though I could be wrong.

Hippomorphs appeared maybe 55 million years ago, and the American Museum of Natural History (AMNH) says that brontotheres are limited to the Eocene period,[7] which lasted roughly 17 million years. The earliest brontheres were hornless and lived around the same time as Hyracotherium, one of the earliest horse ancestors. So, the amount of time required for horn development can at least be narrowed down to around 17 million years. While that’s still longer than the target 4.5 million years for equines, at least it beats the 23 million years of horse-like equid development (not reducing that for the equines). I would like to find a a horn development time shorter than 4.5 million years..

Also of note, I have run into various references to an extinct creature whose fossils may be responsible for some unicorn rumors, called Elasmotherium.[8] Based on the speculative illustrations I’ve seen on the Web, I would not consider it a horse. Others seem in agreement with this and tend to consider it closer to a rhino.

Rhinos just happen to be some of the the next closest living relatives to equids, which would be the logical place to check at this point. The equids and rhinos are in the order Perissodactyla, which are odd-toed hoofed mammals. Toes? Yes, the number of toes seems to matter. Rhinos and equids are considered hoofed animals, which are called ungulates. According to Wikipedia,[9] the ungulates used to be a taxonomic order called Ungulata but has been split into multiple orders. Two of these orders apparently are more genetically related than the others and are distinguished from each other by their members having either an even or odd number of toes on their hind legs. As Perissodactyla covers the odd number of toes, Artiodactyla covers the even number of toes.

So, assuming this newer classification of ungulates also indicates lineage, I’m now looking for horns in the more inclusive perissodactyls, the odd-toed ungulates. According to the AMNH, the known living perissodactyls are limited to equids, rhinos, and tapirs.[10] We already ruled out equids, but what of tapirs? I found no references of tapirs ever having horns.

The word rhinoceros from which we get rhino actually comes from two Greek words meaning “nose” and “horn”, according to Merriam-Webster.[11] How long did it take for rhinos to get horns? The American Museum of Natural History indicates rhinos evolved horns several times.[12] How long have rhinos been around? Wikipedia says that rhinos started splitting off from other perissodactyls in the early Eocene period.[13] That would be maybe a little less than 56 million years ago.[14]

Researching rhinos a little, I’m not convinced I can quickly narrow down the time needed to evolve horns to less than 4.5 million years. A document on rhino evolution from the Rhino Resource Center[15] indicates that rhinos acquired horns in the late Eocene, though rhinos did not yet have horns in the early Eocene. That’s a span of less than 22 million years, which is still quite a stretch compared to 4.5 million years.

Other leads seem to point to going from no horns to horns in roughly 15-20 years. I could move on to artiodactyls, but a quick search indicates about the same thing. Without being a paleontologist or biologist, I can’t conclude yet that a relatively modern equine species could theoretically have grown a horn somewhere under the right conditions in less time than that.

It may be helpful to review a few facts.

Very horse-like Equids appeared 23 million years ago.
Perissodactyls evolved horns in rhinos in less than 22 million years.
Equid-related brontotheres developed horns in less than 17 million years.

With the numbers 22 and 17 both being less than 23 (counting millions of years), it seems to me like there has in fact been enough time for horns to have evolved in at least something horse-like if not one of the equine species. Do unicorns exist today? Fossil evidence does not seem to support their existence, though they seem to have had time to evolve. At least they are not biologically impossible. Had things worked out just a little differently, we might have had farms and ranches with unicorns. Who knows? Maybe there are unicorns on some remote unexplored island somewhere.

References

1. https://www.merriam-webster.com/dictionary/unicorn

2. http://www.newworldencyclopedia.org/entry/Equidae

3. http://www.newworldencyclopedia.org/entry/Miocene

4. https://www.britannica.com/animal/horse/Evolution-of-the-horse

5. https://en.wikipedia.org/wiki/Moyle_horse

6. https://en.wikipedia.org/wiki/Brontotheriidae

7. http://digitallibrary.amnh.org/bitstream/handle/2246/2804/v2/dspace/ingest/pdfSource/nov/N3439.pdf?sequence=1

8. https://www.cnn.com/2016/03/29/living/real-unicorn-remains/index.html

9. https://en.wikipedia.org/wiki/Ungulate

10. https://research.amnh.org/paleontology/perissodactyl/

11. https://www.merriam-webster.com/dictionary/rhinoceros