BIO/101 – Organism Physiology
Snake origins are still unclear, but what is certain is that the species is still evolving. Snake fossils are very rare and when they are found, they are usually in poor condition due to the fragile skeletal structure of snakes.
Two main hypotheses prevail in the scientific community about the origin of snakes, which are the “Burrowing Lizard Hypothesis” and the “Aquatic Mosasaur Hypothesis.” Whether snakes originate from terrestrial or aquatic environments, fossils show large skeletal changes among snakes over millions of years (Veigas, 2011).
The “Aquatic Mosasaur Hypothesis” crowd subscribes to the belief that the modern snake evolved from the mosasaurs, which are extinct aquatic reptiles from the Crustaceous Period. The land lizard eventually evolves into a snake to adapt to the new aquatic environment, after entering the sea.
It uses the body for faster locomotion in the water, so it no longer uses the legs, which eventually devolve completely. This ultimately leads to sea snakes, and then these sea snakes eventually returned to land once the evolution from lizard to snake is complete (Snake, 2012). These snakes evolved to protect themselves against the elements in their new environment that would harm them.
The eyelids are thought to have fused over, to protect them in their new watery surroundings. Their externals ears were not useful in water, so their ears devolved and eventually disappeared. Snake (2012), “According to this hypothesis, the fused, transparent eyelids of snakes are thought to have evolved to combat marine conditions (corneal water loss through osmosis), and the external ears were lost through disuse in an aquatic environment.”
The “Burrowing Lizard Hypothesis” crowd believes that land lizards than began burrowing, and then eventually evolved to become more streamlined by losing their legs. If something is not useful, it can have a negative impact on the way the animal successfully adapts to changes in the environment and is ultimately able to survive.
Early fossil records show evidence that during the Crustaceous Period, a two legged burrowing snake named Eupodophis descouensi existed. The new environment created changes to the snake’s eyelids over a long period of time, to prevent them from becoming damaged while burrowing. The snake also lost external ears, in order to better cope with the underground conditions.
Snake (2012), “According to this hypothesis, features such as the transparent, fused eyelids (brille) and loss of external ears evolved to cope with fossorial difficulties, such as scratched corneas and dirt in the ears.”
Scientists are using new technology that allows them to render 3D image models of skeletal structures using high beam x-rays that penetrate deep into fossils. Zimmer (1997), “One of the few remaining signs of their limbed heritage is the presence of vestigial hips imprisoned in the rib cage. “ Scientists have looked at the Eupodophis descouensi fossil and can clearly see the 90 million year old snake had hips and ultra tiny .8-inch legs (Veigas, 2011).
Humans have evolved over time, to adapt to new environmental conditions, just like the snake. The main evolutionary changes that occur in both humans and snakes are largely due to changes in locomotion.
The snake evolved to become more efficient at burrowing, the legs were an unused form of locomotion. If the aquatic origin is correct, then the legs were also shed as a form of outdated locomotion.
According to “Human Skeletal Changes” (2012), “The evolution of human bipedalism approximately four million years ago has led to morphological alterations to the human skeleton including changes to the arrangement and size of the bones of the foot, hip size and shape, knee size, leg length, and the shape and orientation of the vertebral column.”
When humans began walking upright, it was an altogether new form of locomotion. The skeletal structure of the foot had to evolve to support the full body weight of a person, and not just to be used to grasp items like the early hominids.
Modern humans have larger hips and knees to also support the increased body weight placed on them from an upright human (Human Skeletal Changes, 2012). Conditions in the environment will continue to force evolutionary changes to both the skeletal structure of humans and snakes, resulting in the most efficient locomotion possible.
Snake. (2012). Retrieved from Snakes: Ecology and Evolutionary Biology by The Blackburn Press
Veigas, J. (2011). How Snakes Lost Their Legs. Retrieved from http://news.discovery.com/animals/snakes-lost-legs-evolution-110207.html
Zimmer, C. (1997). How the Snake Lost Its Legs. Retrieved from http://discovermagazine.com/1997/jul/howthesnakelosti1185#.UMAT44Ybh20
Human Skeletal Changes. (2012). Retrieved from Skeleton Keys: An Introduction to Human Skeletal Morphology, Development, and Analysis by Oxford University Press
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