The anus may have evolved from a hole originally used to release sperm

The anus is a wildly successful innovation, but how did it evolve? A genetic analysis suggests it probably began as an opening used to release sperm that later fused with the gut – an example of evolution repurposing structures.

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

It is thought that early animals evolved the mouth and gut before the anus, as some simple creatures such as jellyfish still have this body plan. They have to expel the remains of their last meal out of their mouth before they can eat again, says Hejnol.

One idea for how early animals evolved an anus is that their mouths split in two. However, in 2008, Hejnol showed that the key genes controlling the development of the mouth region are quite different to those for the hindgut, suggesting an independent origin for the anus – and now he thinks he has tracked it down.

Hejnol and his colleagues have been studying animals such as Xenoturbella bocki, a worm-like organism found on the seafloor with a mouth and gut, but no anus, which may be a living representative of an ancient group that was intermediate between the ancestors of jellyfish and the first animals with an anus.

Now, they have discovered that X. bocki has a separate opening for releasing sperm 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 key genes controlling the development of the hindgut in animals with an anus also control the development of the gonopore in animals such as X. bocki, suggesting an evolutionary link.

“What happened is likely that the hole [gonopore] existed, and the digestive system was close by,” he says. “And then they just fused. They connected to each other, and they made a common opening.”

“The data are beautiful and very convincing,” says Max Telford at University College London. “I’ve worked on Xenoturbella for a long time, and the fact that we’ve never noticed it having a gonopore is extraordinary.”

Tracing the origins of the anus is more than just idle curiosity, because it is thought that once animals had a through gut running from mouth to anus, it laid down a body plan still in use today. “The existence of almost all animals we see around us might have something to do with the invention of a through gut,” says Telford.

However, he doesn’t think X. bocki is pointing us in the right direction. He thinks the group of animals to which it belongs once had an anus with a connected gonopore, then lost the anus. In other words, according to Telford, this group appeared only after the evolution of the anus rather than representing the stage immediately preceding it. Hejnol thinks his own interpretation is more likely, but for now there is no way to bring about an end to the debate.