Tuesday, March 13, 2012

Why did snakes evolve to be legless?


For millions of years, animals have enjoyed the many conveniences of transportation which legs provide; yet, there are some which travel without legs or even fins. One representative of these appendage-less creatures is the suborder serpentes, which encompasses the animals more commonly known as snakes (“Phylogeny of Snakes”). Now, why would snakes evolve to be without such a useful adaptation? Some argue that it was God’s punishment for misleading Adam and Eve. However, evolutionary biologists have other ideas.

It is a common consensus amongst evolutionists that snakes evolved from lizards; however, exactly how is still up for debate. The actual genetic reason behind the absence of legs has been found in a mutation in the expression of Hox genes which are involved in the development of different types of vertebrae. This mutation led to the formation of more thoracic vertebrae, which have ribs, than other types of vertebrae which allow for the formation of limbs (Gilbert). The genetic component has been determined. The reason for the push in that evolutionary direction, however, is still disputed.


While some scientists believe this lack of limbs stems from terrestrial origins, others insist it has an aquatic basis. The functional argument behind the terrestrial side is that when burrowing, limbs may hinder movement (Gyekis). However, some argue that the lack of homoplasy amongst other burrowing animals hampers this idea. After all, front limbs would certainly help in the removal of dirt while burrowing. Yet, paleontological evidence shows that the forelimbs were actually the first to go (Viegas). On the aquatic side, scientists argue that the skeletons of snakes closely resemble those of mosasaurs, marine lizards of the Cretaceous era that are related to Komodo dragons. Skeptics of this theory point to genetics. They argue that the snake genome most closely resembles the genome of terrestrial lizards. Additionally, to counter the questioning of the functional argument, terrestrial origin proponents propose that the snake’s ancestors may not have been creating their own burrows, but rather, invading the burrows of small prey. In this case, limbs could be very much in the way when trying to squeeze down a tight burrow to get a tasty morsel of food (Gyekis).

Personally, this author sides with the terrestrial arguments. The functional explanation and genomic evidence are much stronger than skeletal similarity. However, despite this, the consensus is far from unanimous. Thus, the “why did snakes evolve to be leg-less?” debate lives on.

Sources:
Gilbert, SF. “Hox Genes: Descent with Modification.” http://www.ncbi.nlm.nih.gov/books/NBK9978/
Gyekis, Joseph. “When Did the Snake Lose Its Legs?” http://www.rps.psu.edu/probing/snake.html
Viegas, Jennifer. “How Snakes Lost Their Legs” http://news.discovery.com/animals/snakes-lost-legs-evolution-110207.html

1 comment:

  1. I enjoyed reading your post. Just an interesting article that I found online:

    http://www.itsnature.org/ground/reptiles-land/legless-lizard/

    that talks about legless lizards- although they look a lot like snakes, they actually belong to Anguidae family. Some of the notable differences is that legless lizards have movable eyes, as well as detachable tails in case of danger. I wonder if this is another example of parsimonious evolution.

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