|MadSci Network: Evolution|
That’s a really good question. In fact, the evolutionary history of snakes is still something of a “hot topic” in vertebrate biology.
You are correct in saying that snakes evolved from lizards. The 2500 species of snakes alive today all share a common ancestor that was a lizard, and probably lived during the time of the dinosaurs (the earliest snake fossils known are from the Cretaceous period). The problem is that it’s not entirely clear what kind of lizard was the ancestor of snakes.
There are several hypotheses for the origin of snakes. One, as you mentioned, is the hypothesis of an aquatic origin. According to this hypothesis, the snake lineage is closely related to mosasaurs, huge marine lizards that lived during the Cretaceous. If this hypothesis is true, the closest living relatives of snakes would be the monitor lizards (such as the Komodo dragon).
Another popular hypothesis is that the origin of snakes was related to a burrowing lifestyle. This hypothesis makes a lot of sense, because many burrowing animals lack limbs. There are a number of species of legless lizards (that are not snakes) that live mostly or entirely underground.
A third hypothesis is that the ancestor of snakes spent most of its time on the surface of the ground, possibly moving among a lot of dense vegetation, where legs would perhaps get caught on things and make it more difficult to move.
Biologists study the relationships between species, and groups of species, by looking for unique evolutionary innovations. Groups of species that share these unique innovations, or synapomorphies, are probably relatives. Innovations that biologists find might be morphological or anatomical features, or they might be changes in a DNA sequence.
So far, many biologists who look at morphological traits think that snakes are related to mosasaurs and had an aquatic origin. However, some data from DNA sequences suggests that snakes are not closely related to mosasaurs, and probably evolved on land. For now, the question remains unresolved.
One of the interesting things about studying the evolutionary relationships between species and groups of species, which means reconstructing the Tree of Life, is that each branch of the tree is unique and irreplaceable. For example, all snakes had a common ancestor that was just one species – and whatever it was, it’s now extinct. Even if it were still around, the origin of snakes was a unique event that it’s unlikely we could duplicate. As I mentioned, there are plenty of legless lizards around today, but they aren’t snakes. They are found on different branches of the tree, and they lack a lot of the special features that diagnose snakes, such as the highly unusual eye structure of snakes, the particular scale patterns of snakes, and the anatomical features that enable snakes to swallow prey larger than themselves (legless lizards are forced to eat only small food items).
What all this means is that, even if we do figure out the evolutionary origins of snakes, we can’t replicate that origin and recreate the same branch of the tree. If snakes all go extinct at some point in the future, they will be gone forever. The time to conserve snakes is now, before that happens.
REFERENCES (for further information)
Holman, A.J. "Fossil Snakes of North America: Origin, Evolution, Distribution, Paleoecology." Indiana University Press.
Pough, F.H., C.M. Janis, and J.B. Heiser. "Vertebrate Life." Prentice Hall.
Radinsky, L.B. "The Evolution of Vertebrate Design." University of Chicago Press.
“Snake Ancestors Lost Limbs on Land, Study Says” – National Geographic News http://news.nationalgeographic.com/news/2004/02/0211_040211_snakeevolution.html
“Just About Mosasaurs” http://www.oceansofkansas.com/about-mo.html
There are quite a few more books and websites out there -- try a web search to find some of them!
Try the links in the MadSci Library for more information on Evolution.