Science Is Still Unclear About the Evolutionary Origin of the Anus

A newly published scientific review attempts to “get to the bottom” of how animals acquired what some might call the most indecent part of the body

Statue Butt
This shot of a statue from the Louvre is one of the least-shocking anus-related image we came up with. Godong/Robert Harding World Imagery/Corbis

Of all the parts of an animal’s biology, the anus is one of the least glamorous. Associated with taboo and, well, feces, its structure and functioning are not really anyone’s favorite subjects to consider. But some say this anal aversion has bled over to the world of science, where anus-related studies are sometimes avoided.

But the anus really doesn’t deserve such a bum rap. For those organisms that have them, their existence often marks an animal’s ability to digest more efficiently and grow larger than would otherwise be possible. And, as Matt Walker writes for BBC Nature, “the story of the origin of the anus is actually a story about how animals evolved, diverged from one another, and became sophisticated creatures.”

Now, a pair of Norwegian scientists are hoping to spur on further studies of the evolutionary, molecular development of the anus with their paper, aptly titled "Getting to the Bottom of Anal Evolution," recently published in the journal Zoologischer Anzeiger.

It isn’t just squeamishness that keeps such research from taking center stage. The study of anal evolution is less straightforward than that of some other organs’ development. As Walker points out, “the appearance of the anus is of course inextricably linked to the evolution if the digestive system,” which means that, simply put, there are a wide array of anuses in the world, just as there are differing types of digestive processes. This makes nailing down the origin of the body part extra complicated.

Some animals with simple digestive tracks only have one opening in their body for both consumption and excretion. Some even simpler organisms, like the tapeworm, have no digestive track and no anus at all. And then there are the creatures that, as one of the paper’s authors highlighted to BBC, have a “transient” anus—that is, an opening that may appear and disappear throughout an animal’s life. Add to all of this the fact that some animal groups have been found to have had anuses and then evolved to exclude them, and you’ve got a bit of an evolutionary conundrum.

But the study’s authors, Dr. Andreas Hejnol and Dr. Chema Martín-Durán, have nailed down two sets of genes they believe are responsible for the development of the anus: the brachyury and ParaHox genes. Animals that have the body part “almost uniformly express these genes in the tissues surrounding the organ,” writes Walker.

The "deep origin" of the organ is still a mystery, though, Hejnol says. However, he and his research partner Martín-Durán do have a hypothesis—that there is an evolutionary link between the development of the anus and the male gonopore, a sex-related structure used by many invertebrates.

The anus’s possible connection to sex organs could make its scientific origin “even more delicate,” Hejnol told BBC Nature. “Maybe our research can contribute to change the discourse about the subject and communicate about it more openly.” 

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