The ancestors of today's snakes once had full-fledged arms and legs, but genetic mutations caused the reptiles to lose all four of their limbs about 150 million years ago, according to two new studies. Studies showed that mutations in a stretch of snake DNA called ZRS (the Zone of Polarizing Activity Regulatory Sequence) were responsible for the limb-altering change. Adult snakes don't have limbs, but extremely young snake embryos do, according to the other study, published online today in the journal Current Biology. Even during that short time, python embryos managed to begin development for leg bones such as a femur, tibia and fibula, the researchers found. But those limb structures degenerate before they fully develop into cartilage, and python hatchlings are left with just a rudimentary femur and a claw.
I found this article interesting because the DNA for arms and limbs in snakes are still there, and maybe over a course of a million years or so they can grow back if needed. Though, I do believe snakes just didn't have the right genes to begin with, so it is a matter of non-development rather than loss of something.