Shocking evolution of the human anus REVEALED

Scientists have discovered exactly how the human anus may have evolved around 550 million years ago.

They believe a hole originally used to release sperm later connected to the gut to form the open end of the digestive tract.

Researchers led by Andreas Hejnol, professor of comparative developmental biology at the University of Bergen, came to this conclusion by studying a worm-like organism called Xenoturbella bocki.

These worms have a mouth, no anus, and a small hole to expel sperm called a 'male gonopore.' 

By analyzing the worms' DNA, the researchers found genes that control the development of the male gonopore also help form the anus in other animals.  

The genes — and thus the anus — are found in the vast majority of animals, from insects to mollusks to humans.

Based on these findings, the researchers believe some ancient common ancestor of animals with anuses might have looked similar to X. bocki, with a mouth, gut and a gonopore.

But over many generations of adaptation, the proximity between the gut and the gonopore allowed them to fuse together, creating a 'through gut' — a digestive system with a mouth, gut and anus that are all connected. 

The vast majority of animals have digestive systems with both an entrance and an exit: the mouth and the anus. 

But these two critical body parts probably didn't evolve at the same time. In fact, many scientists believe that early animals developed mouths and guts long before they had anuses. 

Some animals still have that rudimentary body plan, like X. bocki, a worm-like organism that lives at the bottom of the ocean. 

When these worms feed, they take food in through the same hole they use to expel waste. Basically, their mouths are also their anuses. 

These worms may be a living representation of an evolutionary intermediate between early jellyfish and the first animals with anuses, Hejnol suggests. 

'Once a hole is there, you can use it for other things,' he told New Scientist.

His study still needs to be peer-reviewed before it can be accepted for publication by a scientific journal, but the report is currently available on the preprint server bioRxiv.

Previously, scientists thought that the anus may have evolved from the mouth splitting into two separate holes. 

But in 2008, Hejnol's research debunked this theory by showing that the genes that control the development of the mouth are very different from those that control development of the anus. 

This inspired him to hunt for the true origin of the anus — a critical piece of the puzzle when it comes to understanding how more than 90 percent of animal species achieved the physical structure they have today. 

'The existence of almost all animals we see around us might have something to do with the invention of a through gut,' Max Telford, a molecular biologist at University College London who was not involved in the study, told New Scientist. 

He said Hejnol's data are 'beautiful and very convincing.'

'I've worked on Xenoturbella for a long time, and the fact that we've never noticed it having a gonopore is extraordinary,' Telford added.

But although X. bocki has revealed interesting new clues about how the anus evolved, Telford does not think this species is actually related to the common ancestor Hejnol is looking for.

He suggests that ancient relatives of X. bocki actually had an anus with a connected gonopore, but later lost the anus, resulting in the body plan we see in these worms today. 

This would mean that X. bocki is not an intermediate species between early jellyfish and the first anus-equipped animals, but rather a species that evolved after the anus had already existed. 

Hejnol said he is sticking with his own interpretation of the findings, sparking a new debate in the ongoing search for the origin of the anus.