Monitor lizards – commonly kept as pets – and iguanas produce venom, according to surprising new research that is rewriting the story of lizard and snake evolution.

Until now, nasty swellings and excessive bleeding as a result of a lizard bite were blamed on infection from the bacteria in the creatures’ mouths. Venom had been considered the preserve of advanced snakes and just two species of lizard – the gila monster and the Mexican beaded lizard. And scientists had thought these lizards evolved venom production independent of snakes.

But research Bryan Fry’s team at the University of Melbourne, Australia, now suggests that venomous lizards are much more widespread than anyone realised. Furthermore venomous lizards and snakes are in fact descended from a common ancestor that lived about 200 million years ago.

In a related paper published in the journal CR Biologies this week, two of Fry’s co-authors, Nicolas Vidal and Blair Hedges of Pennsylvania State University, US, christen this new toxic taxonomic clade Toxicofera. They also suggest a complete overhaul of the conventional classification of lizards and snakes, based on new DNA analysis.


“These are very exciting papers,” says Harry Greene, a herpetologist at Cornell University, US. “They threaten to radically change our concepts of lizard and snake evolution, and particularly of venom evolution.”

Fossil record

DNA analysis by Fry’s team suggests that the closest relatives of snakes are iguanians, of which there are about 1440 species, and anguimorphs, a group that includes the two lizard species already known to be venomous, plus monitor lizards. The team found toxin-producing oral glands in species belonging to these two lineages, but not in other lineages.

The genetic work, which agrees with the fossil record, suggests venomous snakes and lizards shared a common ancestor 200 million years ago, pushing the evolution of venom systems 100 million years further into the past. The evolution of venom coincides with the rapid spread of small mammals.

Analysis of the differing toxins produced by these lizards showed that nine types are shared with snakes. “To find the classic rattlesnake toxins in the bearded dragon – a hugely popular pet – was a huge surprise,” Fry says. But some of the lizard toxins are novel, and are now prime candidates for investigation for potential new drugs.

Red herring

But how could venom production in these lizards have previously been overlooked? Fry suggests that blaming bacteria had become dogma. The Komodo dragon – a monitor lizard – will eat carrion, and their mouths are teeming with bacteria. “It was the classic red herring,” Fry says. Also, while the toxins produced by these lizards might kill their usual prey, they have a less potent effect on people.

In the CR Biologies paper, Vidal and Hedges expand on the genetic analysis. They compared the sequences of nine nuclear protein-coding genes from 19 groups representing all the main lineages of lizards and snakes.

The results reposition iguanas and their close relatives from the lowest, most recent branches of the evolutionary tree to the uppermost branches, close to snakes. But the paper also fundamentally re-classifies lizards and snakes, with a number of new taxonomic groups.

The old system that has been used for 80 years was based primarily on the texture of the tongue. “But we have shown this to be a bad character. Instead, we identified other characters that agree with the molecular phylogeny, such as venom and egg teeth,” Hedges says.

Journal reference: Nature (DOI: 10.1038/nature04328)