Anus may have evolved from a hole originally used to release sperm

Anus is a wildly successful innovation, but how did it develop? A genetic analysis suggests that it probably began as an opening use to release semen that later merged with the gut – an example of evolution that recycled structures.

“When a hole is there, you can use it for other things,” says Andreas Hejnol at the University of Bergen in Norway.

It is believed that early animals developed the mouth and intestine before the anus, as some simple beings like jellyfish still have this body plan. They have to expel the remains of their last meal out of their mouths before they can eat again, Hejnol says.

An idea of ​​how early animals developed the anus is that their mouths are divided into two. In 2008, however, Hejnol showed that the most important genes that controlled the development of the mouth region are very different from them for the tailgate, suggesting an independent origin of the anus – and now he thinks he has traced it.

Hejnol and his colleagues have studied animals such as. Xenoturbella Bocki, A worm -like organ found on the seabed with a mouth and intestine, but no anus, which can be a living representative of an old group intermediate between the ancestors of jellyfish and the first animals with an anus.

Now they have discovered that X. Bocki Has a separate opening to release semen called a male gonopore. There is no female opening as eggs are instead released through the mouth. The team also found that several of the most important genes that control the development of the tailgate of animals with the anus also control the development of gonopore in animals, such as X. Bockithat suggests an evolutionary link.

“What happened is likely that the hole [gonopore] Other, and the digestive system was shut off, “he says.” And then they just melted together. They connected to each other and they made a common opening. “

“The data is beautiful and very compelling,” says Max Telford at University College London. “I’m working on Xenoturbella For a long time, and the fact that we never notice that it has a gonopore is extraordinary. “

Tracking the origin of the anus is more than just free curiosity because it is believed that ICE animals had one through the gut that ran from the mouth to the anus, it made a body plan still in use today. “The existence of common all animals we see, surround us, can have something to do with the invention of one through the gut,” says Telford.

However, he does not think X. Bocki is Us in the right direction. He believes that the group of animals to which it belongs once had anus with a connected gonopore, then lost the anus. In other words, according to Telford, this group appeared only after the development of the anus rather than representing the scene that is in advance that preceded it. Hejnol thinks his own interpretation is more likely, but for now there is no way to put an end to the debate.