Here be there no dragons.

February 6, 2012 6:23 AM

Evolutionary biologists: I'm trying to recall the concise explanation a professor gave in an evolution and ecology course about why dragons just plain did not exist.

I should have probably saved my notes from class but... I was trying to explain to someone why dragons, as they are popularly imagined (four legs + two wings), were not evolutionary possible? In terms of that number and arrangement of appendages being very unlikely, given that vertebrates, from bats to elephants to humans, kind of have similarish skeletal body plans? Anyway, I wasn't doing a very good job of explaining, and was forgetting how to use correct terminology ("convergent... uh... synapomorphies that... uh... wait, no...").

Could someone please throw me a bone here and say what I'm trying to say with appropriate evolutionary terms?

posted by hegemone to science & nature (13 answers total)

Something along the lines of: "In the fossil record we see that limbs have evolve to become wings. For whatever reason there aren't any six legged animals in the vertebrate kingdom†, so there are no ancestors from whom‡ a four legged, two winged creature could have evolved." perhaps?

There are other issues involving scaling problems & power to weight ratios which make dragons of the size depicted in myth & legend problematic.

†Is this true?
‡whom or which? My grammar-fu is failing me...
posted by pharm at 6:40 AM on February 6, 2012

I'm afraid I don't know the terms, but I am guessing you are talking about wings vs limbs. In a bat the front legs/hands of its ancestors became its wings. Each wing is like an outstretched hand with webbing between each finger.

Dragons are obviously related to the vertebrate lineage, that includes mammals, reptiles, fish, birds etc. it has four legs AND wings. This would mean that the wings would have had to evolve from something other than a set of limbs. What would that be? A unicorn has a similarly unlikely body structure, as does a cyclops, a Minotaur etc.

Again, the exact terms elude me, but I remember the name of the genes associated with the formation of animal bodies. They are called "Hox" genes, and by messing around with them scientists have been able to swap the functions of body parts, growing legs where eyes should be on a fly for instance. No amount of switching these genes could make a new set of limbs grow out of a fire breathing reptiles spine though ;-)

Great question though. Looking forward to some more precise biological answers.
posted by 0bvious at 6:50 AM on February 6, 2012

Also wouldn't breathing fire be evolutionarily unlikely for an animal in an oxygen-based environment?
posted by Cocodrillo at 6:59 AM on February 6, 2012 [2 favorites]

The term you're looking for is morphologically homologous. The wings of the bat are homologous to the fore-limbs of other mammals (and to that of other tetrapods), the wikipedia article on bat flight has a nice comparative diagram that shows this pretty clearly.

Here is a short article about the differences between bat and bird wings, both of which are homologous to fore-limbs but in different ways.

The wikipedia article on Homology is also worth reading, and has a comparative diagram of bat, bird, and pterosaur wings.

To understand why homologies are so important, you need to understand the role of the development (as an embryo) on evolution. This is also called Evo-devo. A very simplified explanation is that the growth of an embryo can change more easily in some directions than others, it is much easier for evolution to change the proportions of existing structures than to generate new ones ab-initio.

So, the reason that there aren't any animals with four limbs + wings is that there aren't any large animal lineages with six limbs. If there were, then it might have been possible for one of them to evolve flight by "converting" one of its limb pairs into wings.

There are also scaling laws that cause problems for large flying animals, as an animal gets larger its mass scales with the cube of size while surface area scales with the square of size. I'm sure you've noticed that large land animals like rhinos, elephants, and hippos have similar looking legs. Those aren't just scaled up versions of insect legs, they're proportionally much bigger than the legs of smaller animals.

A similar scaling relation affects flight, as the animal gets bigger and bigger it needs more and more gigantic wings relative to its body. As the proportion of the wings grows it becomes more difficult for the relatively small body to metabolically support the giant wings and in fact, the largest existing birds are mostly gliders.
posted by atrazine at 7:04 AM on February 6, 2012 [13 favorites]

On a lighter note, The Flight of Dragons by Peter Dickinson lays out a comprehensive (if tongue-in-cheek) evolutionary explanation for how dragons could have existed. I'm not sure if it's in print, but I still see it in book stores, usually alongside the Gnomes book by Wil Huygen and Rien Poortvliet, which shares its pseudo-scientific framing.
posted by Wretch729 at 7:18 AM on February 6, 2012 [1 favorite]

Again, the exact terms elude me, but I remember the name of the genes associated with the formation of animal bodies. They are called "Hox" genes, and by messing around with them scientists have been able to swap the functions of body parts, growing legs where eyes should be on a fly for instance.

Yeah, I'm not an expert on this either, but the regulation of embryo development is really cool. The reason that limbs, organs, etc. "know" where they're supposed to be is that Hox and other genes produce transcription factors and other proteins that form various gradients throughout the body. So there's a dorsal-ventral concentration gradient, an anterior-posterior concentration gradient, and so forth. The detail in which we understand the development of embryos in the best studied model organisms is really pretty stunning.
posted by atrazine at 7:21 AM on February 6, 2012

Also wouldn't breathing fire be evolutionarily unlikely for an animal in an oxygen-based environment?

The famous bombardier beetle emits a spray of "boiling caustic liquid". It's not breathing fire exactly, but it's not too far from it either.
posted by stebulus at 7:50 AM on February 6, 2012 [2 favorites]

It's worth noting that not all dragons have four legs, though I believe technically the 2-legged variety is called a wyvern.

Sorry -- at work and can't access my Dungeons & Dragons rule books :-)
posted by Flying Saucer at 10:49 AM on February 6, 2012 [1 favorite]

"This would mean that the wings would have had to evolve from something other than a set of limbs. What would that be?"

Ribs?
posted by Pinback at 12:36 PM on February 6, 2012

Also wouldn't breathing fire be evolutionarily unlikely for an animal in an oxygen-based environment?

Depends on how you define "fire". Both the Gila Monster and the Beaded Lizard are poisonous lizards which vaguely resemble what one would expect a dragon to look like, sans wings of course.

It's pure speculation, but I would imagine it's entirely possible that the stories were just a way to inform people to stay away from those "bearded dragons" out in the desert. Of course, the language issue from multiple translations combined with the retelling of the tall tales probably didn't help.
posted by Blue_Villain at 1:52 PM on February 6, 2012

It's pure speculation, but I would imagine it's entirely possible that the stories were just a way to inform people to stay away from those "bearded dragons" out in the desert.

Those are both new-world lizards and as far I know, dragon myths are old world.
posted by atrazine at 2:42 PM on February 6, 2012

And then everyone forgets Vermithrax Pejorative from Dragonslayer, built to be at least potentially possible.
posted by mephron at 3:16 PM on February 6, 2012

Just to offer a counterpoint, there was a very cool special on Discovery called Dragons: A Fantasy Made Real that hypothesized how dragons could have existed.

I believe they had hollow bones (with a "honeycomb structure") and two bladders filled with hydrogen and methane that gave them lift, along with gigantic wings. Those gasses also gave them the ability to breathe fire, though I think they had to swallow swallow platinum to make the gasses combust. Platinum = precious metals = legends of dragons hoarding gold.

Even Asian dragons and Nessie-like sea serpents were part of the dragon "species," according to the show's experts. It was a very clever special.
posted by Flying Saucer at 4:53 PM on February 6, 2012 [1 favorite]