There is a good reason for the diversity of sex organs in animals and it apparently has something to do with the transition of the first animals from living in water to living on land.

Patrick Tschopp, from the Department of Genetics at the Harvard Medical School in Boston, explained that when the first animals crawled into land millions of years ago, they needed to evolve limbs from their fins.

The creatures also had to protect their eggs and ensure that these do not dry out and since their hostile environment prevented them from releasing their eggs and sperms into the water, they evolved to have the capability for internal fertilization which required external genitalia.

In a new study published in the journal Nature on Nov. 5, Tschopp and colleagues offered insights on how these animals developed their sex organs when they transitioned from sea to land.

The researchers discovered that when given the right nudge, they could turn the embryonic limb cells of snakes and lizards into genitals and that they could have the same result when they manipulate the tail bud cells in mice indicating that limbs and genitals have the same evolutionary origins. Tschopp said that genital evolution is an adaptive measure that is similarly crucial to surviving on land as with limbs having evolutionary origins on fish fins.

"The move of vertebrates to a terrestrial lifestyle required major adaptations in their locomotory apparatus and reproductive organs," the researchers wrote. "We show that the developmental origin of external genitalia has shifted through evolution, and in some taxa limbs and genitals share a common primordium."

Crucial to the process was moving the embryonic cloaca, a structure that develops into the urinary and gut tracts. The structure sends a molecular signal that tells nearby cells and tissues to turn into genitals. This means that the location of the cloaca determines which of the tissues receive the signal first.

In squamates (snakes and lizards), the tissue called lateral plate mesoderm, which makes paired limbs is closer to the location of the cloaca and thus receives the signal. The location of the cloaca in mammals on the other hand, is closer to the tail bud.

"It demonstrates that there is a flexibility with what kind of cells can get recruited during development to form genitalia," Tschopp said. "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."

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