How did snakes lose their legs?

The story of how snakes lost their legs is a fascinating part of evolution. Snakes and lizards both come from a common ancestor and belong to the same group of reptiles. While lizards kept their legs, snakes evolved to be legless over millions of years.

The key to this change is a gene called Sonic hedgehog (SHH). This gene helps in the development of limbs. In lizards, the SHH gene is active and helps form legs. In snakes, however, this gene was deactivated through several genetic changes that happened over 100 million years ago. A 2016 study in Nature Communications found that a crucial part of DNA, called an enhancer, was removed in snakes. This enhancer is needed for the SHH gene to work, and without it, legs did not develop.

Fossil evidence also helps explain this change. A 90-million-year-old snake skull from the species Dinilysia patagonica provided important clues. Scientists used CT scans to look at the inner ear of the fossil. They found that its inner ear was similar to that of modern burrowing snakes. This suggests that early snakes lost their legs as they adapted to living in burrows, not to an aquatic environment as some earlier theories suggested.

Interestingly, some modern snakes still have traces of their legged ancestors. For example, python embryos have small leg bones and foot structures that disappear before birth. This shows that while the genetic instructions for legs are still there, they are no longer used because of the changes to the SHH gene.