We take our own anuses for granted. While they may not be able to fire out our guts as a defense like some animals we could mention, or disappear and reappear when needed like the sea walnut, they are a pretty great solution that evolution has come up with for expelling waste.

But they are not universal. Some animals, including this ancient beastie, can live well enough without one, if you call expelling your waste (poop) out of your mouth day after day “living”. That’s what jellyfish and other animals do, possessing a sack-like gut “with only one opening to the environment functioning both for food uptake and excretion”, the nice scientific way of saying they poop out of their mouths.

Scientists believe that the mouth and gut evolved before the anus, but it has been debated for over a century how the anus evolved, and what from. In new research, titled The xenacoelomorph gonopore is homologous to the bilaterian anus, scientists found evidence that it may have evolved from an orifice to release sperm. Your butthole may have been your spermhole.

The team looked at the marine worm Xenoturbella bocki, which has a gut with only one opening, through which it takes in food and expels waste.

“Interestingly, studies on the molecular patterning of the digestive system in two acoel species (Convolutriloba longifissura and Symsagittifera roscoffensis) showed that caudal and brachyury are demarcating the region around the male genital opening (gonopore), through which the sperm gets released to the exterior,” the team explains in their paper, which has not yet been peer-reviewed.

“A putative evolutionary relationship of the acoel male gonopore and the bilaterian hindgut/ anal opening was therefore postulated, but not yet thoroughly investigated. Since gonopores do not exist in non-bilaterian animals (Cnidaria, Placozoa and Porifera), Xenacoelomorpha are therefore important in understanding whether the presence of gonopores is connected to the emergence of a through gut with an anus in the lineage of Bilateria.”

While these animals do not have an anus, the males do have a gonopore through which they release sperm, while the females release the eggs through their mouths. The hypothesis, which the team attempted to investigate genetically, is that the anus evolved from this opening, as the digestive system and the gonopore fused together.

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

In an earlier study, the team found that genes expressed in the hindgut are distinct from genes expressed in the mouth. In this study, they found that several genes expressed in the development of gonopores are also key genes found in the hindgut of animals in possession of an anus.

“The expression of ‘hindgut’ markers around the male gonopore of acoelomorphs should be […] interpreted as a secondary recruitment of these genes to the posterior ectodermal–endodermal boundary, where the male gonopore forms. However, although an ancestral bilaterian condition of a through gut cannot be excluded, a plesiomorphic blind gut that gained later an anal opening in connection with the gonopore seems more likely,” the team writes. 

“In fact, these genes are never expressed in association with the digestive system in acoelomorphs indicating that the posterior portion of the gut was not evolved yet in this lineage. Our findings provide strong molecular evidence for the homology of the male gonopore of the Xenacoelomorpha and the anus of the Bilateria.”

As Max Telford at University College London told New Scientist, other possibilities are available, such as that Xenoturbella bocki had an anus and a connected gonopore, before losing the anus. More study is needed, but the odds have increased that your butthole used to be a sperm hole that fused with your gut.

The study is posted to pre-print server bioRxiv.