Unicorns are great. Seriously. And here I'm going to sort of 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 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).
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:
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...
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The Moyle horse has "bosses" on the forehead, a hornlike growth covered with hair, ~ 3/4 inch. An entry on the breed can be found in the International Encyclopedia of Horse Breeds at Google Books (or in the library). The article notes that Carthusian Andalusions sometimes have this feature.
Wait...are you saying unicorns aren't real?
As far as selecting pressures go, I believe leprechauns would be part of the environment.
There was a paper, "Population Biology of Unicorns", back in the 60's, about how one could get population biology looking curves out of random numbers.
Wouldn't a better way to get an animal that looks like a unicorn to be to start with an ancestor of antelopes? They've already done the hard work for you, developing horns. From there you just need to move the horns together until they intertwine and grow the animal to the proper size.
Since unicorns are supposed to have goat feet, it works out pretty well.
It can't be as easy as with flies, because flies (and arthropods in general) already have several pairs of appendages on the head and need genes to regulate which ones grow into which type of appendage (antennae, various mouth parts, actual walking legs, genitalia), and this regulation is (part of) what the Hox genes do. That's not how it works in vertebrates.
Ooh! Ooh! Could you do the Loch Ness Monster next? Or maybe the yeti or sasquatch?
That's not how it works in vertebrates
Partly true - in that given the patterning of mammals is far less convenient for such a obvious, neat and tidy transformation.
But the principle is still sound. You could technically have a chance mutation resulting in ectopic expression or altered nuance of one of the mammalian hox genes that results in this sort of change.
It's less convenient in the sense that such a homeotic transformation is likely for sure going to be lethal, but hey you never know! Long shot for sure, but I'd imagine in evolutionary timescales, it would be doable.
An example in the mouse system could be along the lines of a hox alteration that resulted in the skull bone morphology in mice transforming into bone structure more common place in posterior parts of the animal (link to abstract).
This obviously isn't a mouse with horn on its head(!), but it does show that the change in body architecture is in principle possible.
Ooh! Ooh! Could you do the Loch Ness Monster next? Or maybe the yeti or sasquatch?
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