Scientists usually study fossils, morphology and the DNA of living individuals to determine when a group of animals first evolved. Because snakes all share broadly similar body plans, morphological analyses are challenging. Additionally, snake fossils are uncommon and usually consist solely of vertebrae. Scientists are increasingly relying on DNA and molecular studies to determine when the first snakes appeared. Though scientists debate the details, most agree snakes arose approximately 90 to 100 million years ago.
Snake fossils consist primarily of tiny vertebrae, though scientists occasionally discover ribs or skulls. The oldest fossils known are from the middle Cretaceous, about 90 million years ago. The fossilized snake, Najash rionegrina, is the only known snake with a sacrum -- a bony part of the pelvis. Lizards possess sacrums, but they are lost in modern snakes.
For years, most herpetologists subscribed to one of two different hypotheses regarding the origin of snakes. Some hypothesized that snakes originated from within a group of burrowing lizards, while others suspected that the group originated within marine habitats. However, a 2004 Proceedings of the Royal Society paper, "Molecular Evidence for a Terrestrial Origin of Snakes," by Nicolas Vidal and S. Blair Hedges, may have set the issue to rest. According to the marine-origination hypothesis, snakes and monitor lizards were each others closest living relatives, and as monitor lizards are likely close relatives of Mosasaurs -- an extinct group of marine lizards -- both snakes and monitor lizards evolved in the sea. However, using DNA analysis, the pair showed that monitor lizards and snakes -- though closely related -- are not each other’s closest living relatives, thus refuting the marine hypothesis.
Looking further into the origin of snakes, Vidal and Hedges published another paper, "The Phylogeny of Squamate Reptiles (Lizards, Snakes, and Amphisbaenians) Inferred from Nine Nuclear Protein-coding Genes" in a 2005 issue of Comptes Rendus Biologies. The study compared nine genes found in 19 taxa of lizards and snakes to devise a phylogeny of the group. According to their study, the last common ancestor of all living snakes diverged from a shared ancestor of a group including iguanas, monitor lizards, Gila monsters and a few other clades, about 180 million years ago. The first snakes must have evolved after this divergence.
Standing Up to Scrutiny
Beginning in 2008, R. Alexander Pyron, Frank T. Burbrink and John J. Wiens began creating the most complete phylogeny of lizards and snakes ever created. In 2013, the team completed their work, and published "A Phylogeny and Revised Classification of Squamata, Including 4161 species of Lizards and Snakes" in the journal "BMC Evolutionary Biology." Their results largely support the phylogeny suggested by Vidal and Hedges in 2005.
In 2005, Dr. Bryan G. Fry and 13 colleagues found that some “snake venoms” were surprisingly widespread. The researchers not only found similar toxic proteins and glands in several snake species traditionally considered to be non-venomous, but they also found these same glands and proteins in several closely related lizard groups as well. Publishing their results in a paper called "Early Evolution of the Venom System in Lizards and Snakes" in the journal “Nature,” the authors conclude that because both groups -- snakes and their closest living relatives -- produce nine identical toxins, they are all part of a single clade, subsequently named Toxicofera. Because all of the groups within the clade -- which includes snakes, iguanas, Gila monsters, beaded lizards, glass lizards, alligator lizards and monitors -- possess similar toxins, and snakes arose from within the group, venom must have evolved before snakes did.
- Discover Magazine: Which Came First, the Snake or the Venom?
- Nature: Early Evolution of the Venom System in Lizards and Snakes
- New Scientist: Oldest Snake Fossil Shows a Bit of Leg
- Proceedings of the Royal Society: Molecular Evidence for a Terrestrial Origin of Snakes
- Comptes Rendus Biologies: The Phylogeny of Squamate Reptiles (Lizards, Snakes, and Amphisbaenians) Inferred From Nine Nuclear Protein-Coding Genes
- BMC Evolutionary Biology: A Phylogeny and Revised Classification of Squamata, Including 4161 species of Lizards and Snakes
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