This is why I don’t mind efforts to disprove evolution - IF they are carried out in a scientific manner – it’s the only way science can truly work. Unfortunately, many efforts are not carried out scientifically. They're similar to that old cartoon, where one scientist is charting the mechanism of a process on the board, and one of his steps is, “and then a miracle happens.”
On the other hand, when a radical change comes to us in a scientific manner, then we should accept it is another possible explanation; now we have multiple explanations to try and disprove. We're never be done testing a hypothesis.
And that brings us to the reptiles; the testudinata (turtles and tortoises, from Latin = movable armored shed), the squamata (snakes and lizards, from Latin = having scales), and the crocodilians (from Greek = lizard). Of these, the turtles and tortoises seem to be the oldest, appearing first more than 220 million years ago. Lizards were next, and snakes were said to have developed from lizards, about 100-120 million years ago. This is harder to follow in the fossil record because snake skeletons are delicate and do not fossilize well.
The snakes get all the publicity; I think it's the big fangs and the fact that they can swallow things bigger than their heads! Children can list the most venomous snakes, boys especially like to taunt young girls with writhing snakes; they even make movies about snakes on airliners!
But did you know that only about 20-30% of all known snake species are known to be venomous? Of the non-venomous majority, only the boas and the anacondas seem to get any attention. Their immediate ancestors, the lizards seem to get even less.
His cousin, the Mexican bearded lizard (Heloderma horridum) lives a little south of the gila, and he is considerably bigger, reaching up to a meter in length. The bearded lizard’s venom delivery is similar to the gila’s, with modified salivary glands in the lower jaw producing venom that flows with the saliva.
I said that recent events have changed our ideas about venomous lizards. Brian Fry at the University of Melbourne has published a series of papers since the early 2000’s that have expanded our understanding of venom evolution. Dr. Fry reported in early 2006 that beyond the heloderma lizards, the monitors and the iguanas also use venom in defense or food acquisition. One of the monitor lizards is the Komodo dragon (Varanus komodoensis), the largest living lizard on Earth.
Well, that’s all anyone remembered - the Komodo is venomous?! This really caught the public’s attention. It had been assumed that it was the vile state of the Komodo’s bacteria-infested mouth that was toxic to its prey, the bite transferred horrible organisms to the wound, which replicated, let loose with some bizarre toxin, and eventually killed the prey. Whole documentaries were made about how patient the Komodo was, following the prey around for days until it succumbed to one infection or another.
This is a computer rendition of the MRI of a Komodo
head. The dragon has six venom glands on each side of
the lower jaw, shown here as alternating red and pink.
The yellow glands produce the mucus that
gives the Komodo its famous drool.
Two items were lost from Fry’s 2006 paper. One, the iguanas have venomous members as well. Iguanas live in North America as well as in Madagascar, Fiji, and Tonga. They have many representative species, several of which are common household pets. It turns out that many of them have venom of a mild potency, but strong enough to hold some predators at bay. The monitors and the iguanas have something else in common, the way the produce and deliver venom.
In both groups, the venom glands are located in both the upper and lower jaws, and the ducts that deliver the venom to the saliva are located between the teeth and at several locations in the mouth. This is different from the snakes that have venom glands only in the upper jaw and deliver venom through a single duct, and the gila monster and bearded lizard that have venom glands only in the lower jaw.
The differences between gila/bearded lizard venom delivery and that of snakes had led to the idea that these two systems developed independently of one another. But following the trail backwards through evolution, Fry showed that venom production started in the lizards, about 200 million years ago. The snakes that evolved from some lizards already had the mechanisms on board to produce venom – it came from a single common ancestor that was around before even T. rex!
This was a radical change for those who studied the evolution of the reptiles. Like I said, one should never consider a theory to be absolutely proven. The questions then is - if lizards started the reptile venom idea, why are so few venomous now? Well, maybe they are venomous. We only seem to care if humans are threatened by them, so we haven’t even looked for venom or venom glands in most lizards. Fry estimates that perhaps more than 100 lizard species are venomous, and that perhaps all of them still contain the genes to produce venom.
Two of Fry’s co-authors jumped on the idea of all lizards being venomous, and expanded it to the idea that perhaps all snakes might have been venomous at one time. As such, they suggested a new categorization of all these animals into one clade (evolutionary group with a single common ancestor), called the Toxicofera clade. This idea became know as the Toxicofera Hypothesis… makes sense, I guess.
But don’t misunderstand the hypothesis. It applies only to reptiles, the Toxicofera family would not include venomous mollusks, like the blue-ringed octopus, or biting ants, like the fire ant, or the stinging bees and wasps. There are certainly instances where venom and venom delivery systems evolved independently. The toxicofera hypothesis applies only to the reptiles.
How about the turtles and tortoises? They are reptiles too; could they be venomous? Well…. no, at least not yet. These reptiles diverged from a common ancestor more than 200 million years ago, before the computer programs estimate the first reptile venom appeared. However, there is one sort of exception. The three-toed box turtle of North America isn’t venomous, but it can be poisonous.
In the wild, three-toed box turtles eat just about anything – insects, small dead animals, vegetation, worms, and fungi; it's the fungi that make the difference. They have a taste for the Death Cap Mushroom (Amanita phalloides). This fungus is related to the toadstools that some people use for their hallucinogenic properties, but the Death Angel is aptly named.
This is something we can investigate next week, along with the difference between just having a poison in your tissues because of something you ate or actively storing it for your own use. Along the way, we will meet a salamander with a lethal rib cage. Yeah- you read correctly, ribs that can kill.
Fry, B., Vidal, N., Norman, J., Vonk, F., Scheib, H., Ramjan, S., Kuruppu, S., Fung, K., Blair Hedges, S., Richardson, M., Hodgson, W., Ignjatovic, V., Summerhayes, R., & Kochva, E. (2005). Early evolution of the venom system in lizards and snakes Nature, 439 (7076), 584-588 DOI: 10.1038/nature04328
Fry, B., Wroe, S., Teeuwisse, W., van Osch, M., Moreno, K., Ingle, J., McHenry, C., Ferrara, T., Clausen, P., Scheib, H., Winter, K., Greisman, L., Roelants, K., van der Weerd, L., Clemente, C., Giannakis, E., Hodgson, W., Luz, S., Martelli, P., Krishnasamy, K., Kochva, E., Kwok, H., Scanlon, D., Karas, J., Citron, D., Goldstein, E., Mcnaughtan, J., & Norman, J. (2009). A central role for venom in predation by Varanus komodoensis (Komodo Dragon) and the extinct giant Varanus (Megalania) priscus Proceedings of the National Academy of Sciences, 106 (22), 8969-8974 DOI: 10.1073/pnas.0810883106
Fry, B., Winter, K., Norman, J., Roelants, K., Nabuurs, R., van Osch, M., Teeuwisse, W., van der Weerd, L., Mcnaughtan, J., Kwok, H., Scheib, H., Greisman, L., Kochva, E., Miller, L., Gao, F., Karas, J., Scanlon, D., Lin, F., Kuruppu, S., Shaw, C., Wong, L., & Hodgson, W. (2010). Functional and Structural Diversification of the Anguimorpha Lizard Venom System Molecular & Cellular Proteomics, 9 (11), 2369-2390 DOI: 10.1074/mcp.M110.001370
For more information, see:
Venomous lizards –
Toxicofera hypothesis –
Three-toed box turtle –