Why are penises private parts

How two penises became one

When the first vertebrates left their aquatic life behind, they had to fundamentally rebuild their bodies. Lungs and legs, for example, proved to be urgently needed in the new living space - but the genital organs also had to change under the new conditions. Without water, sperm cannot get to the egg cells, and so the first land animals transferred fertilization inside the body.

Out of this necessity the external genital organs arose. However, there is still a research deficit in this area: While it is now quite well known how arms and legs developed in their current forms, very little is known about the vulva and penis. The sexual organs are a hotspot of evolution - there is no way around them when manufacturing the next generation.

This article is contained in Spectrum - Die Woche, week 46, 2014

However, one thing is striking: not all vertebrates have the same number of sexual organs. Some groups, for example the snakes, have two of them. This already suggests that the genitals of the terrestrial vertebrates have a literally eventful history behind them. A team led by Emma Sherratt from Harvard University has now analyzed how this came about - and the researchers have come to the conclusion: Originally, all terrestrial vertebrates had two penises.

How did these two sexual organs become one? Signals from the cloaca, the later excretory opening, are decisive for the development of the penis and vulva. In the scale creeps, to which the snakes belong, the embryonic cloaca lies exactly between the leg buds. Correspondingly, these animals have paired genitals, the so-called hemipenes - they emerge from those parts of the embryonic leg systems that are close to the cloaca.

Sherratt and her colleagues are now using embryos from various vertebrate groups to show that the penis and vulva migrated together with the cloaca in the course of the vertebrate evolution, namely by moving the necessary genetic circuits from the legs to the base of the tail.

Already in the birds the cloaca migrated towards the tail - as Sherratt and her colleagues show by tissue staining, the organ soon recruited new cells for the genitals to develop. In chickens, only the front part of the embryonic sex organs comes from the leg buds, the rear part from the connective tissue cells of the tail.