We know that snakes originally had four limbs; they were basically just long lizards.
The fossil record was already strong as to when and how it happened (at least physically), and we’ve even known for a long time that the front limbs became vestigial and disappeared long before the hind limbs.
But now a new study seems to show that the Sonic Hedgehog Gene (SHH), which also influences eye, brain, and central midline splitting in vertebrates, is the gene responsible for reducing the size of snake limbs. It’s a fascinating gene, and is what’s responsible for most incidences of cyclopia (one-eyedness) in mammals.
While SHH is actually not mutated in snakes, an enhancer gene that turns it “on” and “off” during development has three separate mutations. Whereas a limbed vertebrate has the trigger gene keeping SHH active throughout the embryonic and fetal development process, in snakes, it flickers on and almost immediately shuts off. No limbs if it doesn’t stay on!
While this might not have been the very first step in making snakes legless, it’s a huge clue as to how they evolved.
I wonder what gene mutations are found in legless lizards? Get on it, Science!
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