Scientists have found a strong correlation between the evolution of genitals and limbs in animals, which explains why some species develop different kinds of genitalia.

The snakes and genitals grow to mimic leg buds, creating two sets of genitals, whereas humans grow to mimic the tail bud, which causes them to end up with just a single set, according to a Washington Post report citing a study published in Nature.

In making the findings, researchers looked at snakes, lizards, birds, and mammals. In amniotes — which are birds and mammals — they noted that there is only a single external genital structure. But snakes and lizards, although they only use one genitalia at a time when mating, developed paired external organs.

The genitals come from the cloaca, and the structure is developed in the embryo, as cells there tell them to turn into genitals. Scientists wondered why genitalia develop differently during embryonic development despite having the same function between the species.

It might have to do wit the location of the cloaca, as the embryonic trigger tells the cells around it to turn into genitlas. In snakes and reptiles, the cloaca is very close to the hind legs. This causes the cloaca to recruit more cells around it than it would in the body of a bird or mammal.

Scientists proved that this was a case by grafting cloaca tissue next to new limbs in chicken embryos. The cells started growing into genitals simply because of their new position.

Researchers hope the discoveries could lead to medical breakthroughs and a better understanding of the human body.