The Evolutionary Biology of the Unicorn.

by David Ng

By DAVID NG

Unicorns are great. Seriously.

And here I’m going to think out loud and think of them in a conventional biology sort of way. You know – have a little fun evolution wise.

In many respects (except for the magical powers bit) I don’t think this is necessarily too hard to do. i.e. you have something that looks like a horse, but hey what’s this – there’s also a horn there.

I guess the question I’m pondering is whether a unicorn could occur from a realistic evolutionary biology point of view – you know, given the right circumstances and the right timescale. And if so, exactly what sort of things, biologically and genetically, would need to happen?

Anyway, here’s a couple options to sift through.

One possible way to get the whole horn thing started on a poor horse is through a condition known as “cutaneous horn” formation. In this situation, you essentially have an abnormal, sometimes cancerous growth, that results in a keratin structure protruding from the skin.

(Here’s a picture of a cutaneous horn – it’s kind of ugly)

“Cutaneous horns usually arise on sun-exposed skin but can occur even in sun-protected areas. The hyperkeratosis that results in horn formation develops over the surface of a hyperproliferative lesion. Most often, this is a benign verruca or seborrheic keratosis; or it could be a premalignant actinic keratosis. A malignancy has been reported at the base of a cutaneous horn in up to 20% of lesions. More than half of all cutaneous horns are benign.Benign lesions associated with cutaneous horns include angiokeratoma, angioma, benign lichenoid keratosis, cutaneous leishmaniasis, dermatofibroma, discoid lupus, infundibular cyst, epidermal nevus, epidermolytic acanthoma, fibroma, granular cell tumor, inverted follicular keratosis, keratotic and micaceous pseudoepitheliomatous balanitis, organoid nevus, prurigo nodularis, pyogenic granuloma, sebaceous adenoma, seborrheic keratosis, trichilemmoma, and verruca vulgaris. Lesions with premalignant or malignant potential that may give rise to cutaneous horns include adenoacanthoma, actinic keratosis, arsenical keratosis, basal cell carcinoma, Bowen’s disease, Kaposi sarcoma, keratoacanthoma, Paget’s disease, renal cell carcinoma, sebaceous carcinoma, solar keratosis, and squamous cell carcinoma.” (From eMedicine)

So you have something producing horn-like features, and whilst not exactly common, is still within the realm of possibility.

However, this possibility of unicorn evolution is kind of weak, because often the base of the “horn” structure is not at all stable, and in fact it looks like these horn structures can often be quite easily removed surgically. As well, this is not really a heritable trait in the usual sense – i.e. normally these structures are formed due to abnormal growth coming from a one cell, possibly mutated from exposure to a some mutagen (the sun is often sited for example). In other words, whilst susceptibility to the abnormal growth is likely genetic, the act of it always occuring on the horses head is less so (or something like that).

In other words, let’s move on.

One thing I can say is that it’s interesting to note that both the horse ((Equus ferus caballus) and the rhino (Rhinocerotidae) both belong to the Perissodactyla order, also often termed the odd-toed ungulates. This suggests that in the grand scheme of things, these two types of creatures are not so far apart. Whilst obviously interbreeding isn’t an option (since the species barrier would presumably be more than sufficient to disallow the formation, as well as the propagation, of hybrids unicorn like rhino-horses), it does present the idea that however the horn formed on the rhino, this could still be in the realm of reality with something like a horse.

Which actually makes all the more sense when you look through a paper published in the Journal of Morphology in 2006, which did CT scanning of rhinoceros’ horns to get a better sense of their anatomy. Here, the suggestion is that the horn of rhinos are markedly different from a horn of, say, a sheep. Specifically:

“The horns of most animals have a bony core covered by a thin sheath of keratin, the same substance as hair and nails. Rhino horns are unique, however, because they are composed entirely of keratin.”

This might fit a little with the “cutaneous horn” angle, but then another observation came about from the CT scans. The lead authors goes on to say that:

“The horns most closely resemble the structure of horses’ hoofs, turtle beaks and cockatoo bills. This might be related to the strength of these materials, although more research is needed in this area.”

And this nugget of information brings up a delicious possibility.

That is – maybe a unicorn could develop initially from a mutation within a hox gene, resulting in a hoof like structure coming out of the animal’s head. And in case, you’re wondering what a hox gene is all about, it’s essentially:

(From wiki) “A group of related genes that specify the anterior-posterior axis and segment identity of metazoan organisms during early embryonic development. These genes are critical for the proper number and placement of embryonic segment structures (such as legs, antennae, and eyes).”

In plain speak, this simply means that the hox family of genes are the grand controllers of body architecture – as in your arms go here, your head here, and this is about right for your big toes.  In other words, for our unicorn, we really just need a mutational event where something meant to be coming out at the limbs is inadvertently coming out of the wrong part of the general body plan.

Classic examples of hox mutants occur in fly embryogenesis, and here are two of many examples of mutations that result in something along the lines of a foot/leg like structure coming out of the head area.

This is an image of a fly with a mutated proboscipedia gene: basically the labium develops into a pair of legs (image link)

Here’s another:

(Image link)

All to say that I’m wondering if the hox idea might actually have some (pardon the pun) legs to it.

Of course, even so, the hox idea would only be part of the story. Natural selection and the whole epic time scale stuff would still need to do its thing. Here, I must admit that I am curious to see what readers would think are the best environmental conditions (serious and funny ones) to produce the right selecting pressures for unicorn morphology. Maybe a few suggestions in the comments section would be cool?

And what about those magical powers? Well, how about we let the Intelligent Design folks debate over that one…

About these ads