A new study has examined the evolutionary changes in genitalia that needed to happen for vertebrates to transition from sea to land.

According to BBC News, authors of a study published in the journal Nature believe the key to the answer, at least for snakes and lizards, is in the limbs. Though snakes do not have limbs, they found that early genital evolution resembled that of a limb.

"It demonstrates that there is a flexibility with what kind of cells can get recruited during development to form genitalia," study lead author Dr. Patrick Tschopp, a genetics research fellow at the Harvard Medical School (HMS), told BBC News. "What we were able to show is that if you ectopically transplant this cloaca into either limb or tail bud cells, these cells respond in a way that reflect their development being redirected to a genital fate.

"In other words, by misplacing a molecular signal you can misguide these cells in their developmental trajectory."

For their study, the multi-institutional team found that lizard's limb cells could be altered slightly to form genitalia and that the same could be done with mice and their tails. They had to go to where limb cells and genital cells originated.

"While mammal and reptile genitalia are not homologous in that they are derived from different tissue, they do share a 'deep homology' in that they are derived from the same genetic program and induced by the same ancestral set of molecular signals," Clifford Tabin, the HMS genetics department chair, said in a press release.

Like how fins evolved into limbs, the genital evolution the researchers observed in their paper was necessary for vertebrates to transition from living in water to living on land.

"This paper dealt with the longstanding unresolved issue of the origin of genitalia. It turns out that the mouse is the odd one out, it was not similar to the snakes or the chicken," Dr. Liang Ma, a researcher at Washington University in St Louis who was not involved in the study, told BBC News.  "This paper provides a new twist to a previous hypothesis that genitals and limbs share a deep homology [shared ancestry], it provides formal evidence of how this co-evolution between the two structures can happen in an organism."