[Reading Science] When and How Did Snakes Lose Their Legs?

Chinese Research Team Identifies Genetic Cause

There is a saying called "Sasok (蛇足)," which refers to unnecessary words or actions. It is a metaphor derived from the physical characteristics of snakes, which move well despite having no legs. Similarly, snakes are among the very few vertebrates that have no legs and possess slender, elongated bodies. When and how did this happen? Scientists have attracted attention by specifically identifying the genetic causes.

▲ 'Silbaem' inhabiting Korea (Photo by Dr. Sangcheol Lee)

The international academic journal Science reported on the 20th of last month that a research team from the Chengdu Institute of Biology, Chinese Academy of Sciences, published research results that could resolve this question through genetic analysis of snakes. The snake’s body is so slender that it has only one lung and smells with its tongue. Most notably, it has no legs. However, the reason why the snake’s body is so unique had not been clarified. There were only hypotheses such as snakes evolved from marine animals and lost their legs, or that they primarily lived underground as terrestrial animals and lost their legs during the evolutionary process.

To identify the genetic and evolutionary factors behind these physical characteristics of snakes, the research team collected genes from 14 species across 12 families and analyzed the evolutionary process spanning approximately 150 million years. They also referenced previous genetic analyses of 11 species.

As a result, the research team confirmed that all snakes have three deletions in the PTCH1 gene, which controls limb development in vertebrates. While previous studies had only speculated about this, the current study identified the gene directly involved in the degeneration of legs, which remain only as vestigial organs. This was also proven through animal experiments. When the researchers induced the same PTCH1 gene mutation found in snakes in mice, they observed significantly shortened toe bones.

Research findings on other genetic traits have also attracted interest. Scientists had long believed that snakes lost their vision genes because they can only see very close objects and cannot perceive them stereoscopically. However, this study confirmed that snakes still possess these genes. The research team hypothesized that because primitive snakes lived underground, the activity of vision genes was halted early in evolution. They also found that snakes’ excellent ability to hear high-frequency sounds is related to this, and that gene mutations occurred to adjust the bones in their ears to be sensitive to vibrations.

Additionally, the research team discovered that snakes lack two genes, DNAH11 and FOXJ1, which in vertebrates are responsible for symmetrical body development, such as having two lungs during the embryonic stage.

The research team explained, "This study not only reveals the roles of key genes in snake evolution but also advances our understanding of how vertebrate evolution, including that of humans, has progressed."