One of the other discoveries we made was the fact that reptile venom is actually older than the snakes themselves. The venom glands go back to about 200 million years ago, right at the dawn of the dinosaur era, and long before Tyrannosaurus rex roamed the planet. The best guess by paleontologists is that snakes are around 100 - 120 million years old. However, venom glands originated much earlier in ancient late Triassic lizards.
For a long time it was assumed that only two lizards, the gilamonster (Heloderma suspectum) and the Mexican beaded lizard (Heloderma horridum), have venom glands. These glands are located in the lower jaws, whereas snake venom glands are located in their upper jaws.
Because of this difference it was always thought that the venom glands in snakes and Heloderma species' evolved independently. Although this is a reasonable assumption, it is probably not correct! In our study, published in 2006 in Nature, we showed the presence of venom glands in additional lizard lineages, and we could provide evidence that these lizard and snake venom glands all evolved from a common ancestor. Bites from varanids (including the Komodo dragon, Varanus komodoensis) can cause symptoms, but these were traditionally attributed to bacteria that thrive inside the lizard's mouth. Although the bacteria do thrive there and can cause symptoms, many symptoms occur within minutes to hours after a bite, which is too fast to be caused by the bacteria. Bacteria need time to multiply and secrete their toxins. Even if you are in the tropical rainforest, and a wound gets infected, bacteria can not cause symptoms within minutes after a bite. And so the symptoms of varanid bites (including swelling that appears within minutes, hypotension, difficulty in breathing, tachycardia, dizziness, localized disruption of blood clotting and pain) are too acue to be explained by bacterial infections. It is more plausible to assume that a fast-acting bioactive secretion is involved.
We found quite large venom glands in the lower jaws of monitor lizards, and small, primitive 'incipient venom glands' in both upper and lower jaws of an Iguania lizard. Through molecular phylogenetic analyses of the venom components, and a new phylogenetic tree based on non-venom protein coding genes, we were able to identify many toxin types shared between lizards and snakes. We could trace their origin back 200 million years. This is the era when the radiation of the small mammals began. This timing coincidence could be interpreted in terms of a causal link between mammal radiation and venom evolution.
One possible interpretation of these data is that the reptile venom glands originated in ancient lizards that had small 'incipient venom glands' in both their upper and lower jaws. Millions of years later, snakes entered the scene (they evolved from lizards) and the glands diverged. Snakes lost the gland in their lower jaw, and evolved the gland in the upper jaw into a large and sophisticated venom gland. Gilamonsters (Helodermatidae) and varanids (Varanidae) lost the gland in their upper jaws, and evolved the gland in their lower jaw into a large and sophisticated venom gland.
A lot of people wondered why this scenario had not been considered before. People think that venomous lizards would have been noticed long ago. However venomous lizards are not as dangerous as some snakes, and so their venomous nature may not have been obvious to naturalists. Venom alone does not mean that lizards would have been obviously dangerous. Factors other than venom contribute to making an animal dangerous, and thes include: (i) the potency of the venom (ii) venom yield (iii) a venom delivery mechanism such as modified teeth (fangs). Some snakes are equiped with truly horrific fangs that work like hypodermic needles and are capable of delivering large amounts of venom in a single, rapid bite. Lizards do not have sophisticated fangs, but instead posess, at best, grooved teeth that are not capable of injecting large amounts of venom quickly. In addition, the fact that varanid bites cause symptoms by bacterial infection has been a major dogma in the history of herpetology, which is why nobody seriously considered the possibility of venomous lizards.