4 - The snake
Published online by Cambridge University Press: 05 June 2014
Summary
How the snake lost its legs
Vestiges are the evolutionary equivalents of fingerprints at a crime scene. They offer useful clues to the unseen past [1723]. Pythons are a case in point. They have tiny remnants of a pelvis and femur under their skin near the cloaca [1009,1827], and fossil snakes have even more complete legs (pelvis, femur, tibia, ankle, and foot [1029,2181]). These relics demonstrate that the ancestors of snakes had legs.
For other types of vestiges, it has proven possible to rescue their potential by artificially replacing the ingredients that have been lost over time [1502,1778]. However, attempts to re-create walking snakes by restoring missing morphogens (FGFs) to python hindlimb buds have thus far been only marginally successful [337,421,450].
Why did snakes lose their legs? The process apparently began when a lizard-like ancestor adopted a burrowing lifestyle [160,642,2395] like the one that is still pursued by primitive snakes today (Fig. 4.1) [2032,2100,2395] (boldface added):
To nearly everybody it seems perfectly natural to assume that snakes have always looked much like the slender, tubular animals they are today, although in fact the members of this most recently evolved of the reptile groups achieved their current form only after an elaborate physical restructuring. For nearly 60 million years their predecessors ran around as four-legged terrestrial animals similar to contemporary monitor lizards, until, early in the Cretaceous Period, they gradually abandoned their limbs in favor of an attenuated physical form suited to a radically different way of life . . .
In the darkness of their subterranean burrows, the [proto-snakes] came to depend mostly on smell and taste – senses combined in the specialized forked tongues they had already evolved . . . Since wriggling worked better than walking in narrow underground tunnels, their limbs shrank into fetuslike stubs, and as their bodies gradually extruded into the serpentine form of modern snakes, little by little they ceased to be lizards at all. [2187]
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- How the Snake Lost its LegsCurious Tales from the Frontier of Evo-Devo, pp. 75 - 94Publisher: Cambridge University PressPrint publication year: 2014