Wild New Study Suggests Buttholes Once Had a Very Different Purpose
Nature01 April 2025ByTessa Koumoundouros
(AnaSha/iStock/Getty Images Plus)
If the genes of a tiny, bumless invertebrate are anything to go by, our anuses might be repurposed sperm chutes.
A new study suggests instead of releasing waste, the first anal orifice was an exit hatch for male sex cells which was given a new task.
Researchers from the University of Bergen in Norway investigated the genetics of xenacoelomorphs; distant relatives of flatworms that have a cul-de-sac for a gut. Despite this lack of a dedicated poop-hole, xenacoelomorphs use some of the same genes we use to turn our digestive system into a tube, only to create a genital opening known as a gonadopore instead.
"Once a hole is there, you can use it for other things," zoologist Andreas Hejnol told Michael Le Page at New Scientist.
Animal anuses exist in an unexpected variety of forms, from myriad waste-releasing pores on flatworm backs to jellyfish that don't bother with a dedicated channel for rubbish at all.
Like jellyfish, xenacoelomorphs use their mouths to both take in food and expel waste. Yet unlike jellyfish, the males have a separate hole to release their sperm. Xenacoelomorph females, however, use their mouths to release their eggs as well as food intake and waste disposal.
Developmental biologist Carmen Andrikou and team found that when the xenacoelomorph inverts its outer skin to develop a gonadopore, it uses some of the exact same genes other animals use to make their butt holes.
A number of animals today, including birds and platypus, also have a joint hole for both reproductive and digestive functions – a cloaca.
Diagram illustrating how the cloaca develops into two separate tracts in mammals.
(Hyman et al./Wikipedia)
"The presence of cloaca within animals as well as the gonopore-oral fusion witnessed in species of [flatworm], suggests that a connection between the digestive and the reproductive system is either easy to evolve convergently or shares a common ancestry," Andrikou and colleagues write in their paper, which is still awaiting peer review.
This all suggests that our anuses evolved after a male's sperm chute merged with the digestive tract to form a second opening, the researchers explain – implying animals didn't evolve anuses until after our own branch of the family tree parted ways with xenacoelomorphs' ancestors.
Common genes are shared between the sperm hole (male gonopore) of xenacoelomorphs and butt hole (hindgut) in later animals. (Andrikou et al., bioRxiv, 2025)
Other researchers dispute this sequence of events, arguing that xenacoelomorphs' lack of an anus came after these flatworm-like animals developed a butt hole and then later lost it, which could mean they belong in a different position on the animal family tree.
Regardless of how it happened, those of us blessed with an anus that's separate from our mouths – a throughgut – have a more efficient way to process nutrients from our food.
So, the development of the anus allowed animals to grow larger than the buttless, paving the way for our existence.
This research has been uploaded to biorxiv and is still awaiting publication.
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Wild New Study Suggests Buttholes Once Had a Very Different Purpose
If the genes of a tiny, bumless invertebrate are anything to go by, our anuses might be repurposed sperm chutes.
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