130 million years ago, some snakes might have had limbs, even though they didn't use them to move around. According to a new study in the journal Developmental Cell, snakes still have segments of DNA once used for limb development, but those genes now play a role in phallus development for the reptiles. “We've really been thinking of these as limb enhancers,” University of Georgia researcher and study leader Doug Menke told LiveScience. “But more broadly, these genetic components are actually also participating in development and driving gene expression in other body tissues.”

In studying the enhancers (also called cis-regulatory elements) to learn more about their role in evolution, Menke and his team examined the genomes of boa constrictors, Burmese pythons, king cobras, and Anolis lizards, and found that the number of enhancers in snakes was close to the number present in lizard DNA. After 80 million years, the segments of DNA would not still be intact unless they served a different purpose, according to Menke. Mammalian limb enhancers are known to be active in the process of developing external genitalia. The researchers found that, when spliced into mouse DNA, the snake enhancers had no impact on the mouse back legs and could only “activate the genes in the mouse genital tract”—which means that they are somehow now reserved for phallus-growing.

Menke tells LiveScience that the research could lead to important developments for humans. It suggests that some limb and genital birth defects are linked to these non-coding enhancers, as well as to protein-coding genes. The next step is to figure out exactly how things are connected.