The hole you sit on was, very possibly, a reproductive organ first.
That’s the evolutionary hypothesis explored in a study published Oct. 24, 2025, by Carmen Andrikou, Andreas Hejnol and colleagues. The researchers, including scientists at Friedrich Schiller University Jena and the University of Bergen, published their findings in Nature Ecology & Evolution. Their argument: the anus in animals like us “evolved from a male gonopore that secondarily connected to the digestive tract.” A gonopore is the opening an animal uses to release sperm or eggs.
What the Panther Worm Study Found
The team studied xenacoelomorphs, little worms about the size of a grain of rice that live on the sea floor. Their family goes back roughly 550 million years. They sit near the base of the family tree for every animal with a left side and a right side, which includes you.
They don’t have an anus. “Although they only have one mouth opening that takes in food and expels the waste,” Hejnol said in a Jena release, the worms do have a gonopore at the back of the body where the males release sperm. “They therefore occupy a very interesting position in the animal family tree.”
“We have now discovered several genes at this opening that are also responsible for the formation of the rectum,” Hejnol said.
The panther worm, Hofstenia miamia, made the case harder to dismiss. Its gonopore sits up front, near the mouth, and still turns on the rectum genes. That combination of gene activity appears around the reproductive opening as it develops. The researchers distinguished it from the activity of those genes elsewhere in younger worms. So it’s the sex organ itself carrying the “hindgut” signature, not just the back end of the body.
How a Gonopore May Have Become the Anus
At some point, according to the researchers, the gut joined up with the sperm duct, and that opening became the anus. For a while it may have handled both jobs, the way the cloaca still does in birds and platypuses. Nobody knows exactly when the switch happened.
Animals without an anus can digest food, too. The advantage Hejnol describes is a through-gut: food passes through specialized regions rather than entering and leaving through the same opening. The genetic parallels support the team’s proposed origin of that exit, but they are not a direct observation of an ancient animal acquiring one.
His shorter version came in an interview with New Scientist, quoted by ScienceAlert when the work was still a preprint: “Once a hole is there, you can use it for other things.”
We’ll just leave that there.
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