Scientists trace the human anus’ origins back 550 million years, revealing its fascinating evolutionary journey.
Researchers suggest the anus evolved from an ancient reproductive opening that later fused with the gut.
A team led by Andreas Hejnol at the University of Bergen made this discovery by studying the primitive worm Xenoturbella bocki. Their research suggests this simple creature holds clues to the anus’s evolutionary origins.
These mouth-bearing worms lack an anus but possess a “male gonopore”—a tiny sperm-releasing hole that may hold evolutionary clues.
Genetic analysis revealed that the same genes guiding the worm’s sperm-release pore also shape the anus in higher animals. This suggests a deep evolutionary link between reproductive and digestive anatomy.
These anus-forming genes appear in nearly all animals—from insects to mollusks to humans—revealing their ancient evolutionary roots. The discovery highlights a shared biological blueprint across species.
The study suggests our earliest anus-bearing ancestor may have resembled X. bocki—just a mouth, gut, and sperm-release pore. This simple body plan eventually evolved into the two-opening digestive system most animals share today.
Through millennia of evolution, the gut and gonopore—once separate—gradually merged. This fusion created the first “through gut,” linking mouth and anus via a continuous digestive tract. A simple sperm-release hole thus became the exit point for an entirely new bodily system.
Scientists have traced the human anus’s evolutionary origins back 550 million years.
The vast majority of animals have digestive systems with both an entrance and an exit: the mouth and the anus.
Mouths and guts emerged first in evolutionary history, with anuses developing much later. This staggered timeline reveals how complex digestion evolved step by step in early animals.
Some animals still have that rudimentary body plan, like X. bocki, a worm-like organism that lives at the bottom of the ocean.
These worms eat and excrete through a single opening—effectively making their mouth their anus. This primitive design reveals how separate digestive exits evolved later.
Hejnol proposes these worms may bridge an evolutionary gap—between simple jellyfish and the first anus-bearing creatures. Their anatomy could represent a transitional stage in digestive system development.
‘Once a hole is there, you can use it for other things,’ he told New Scientist.
While awaiting peer review, the study is currently accessible as a preprint on bioRxiv. This allows the scientific community to evaluate these findings before formal publication.
Previously, scientists thought that the anus may have evolved from the mouth splitting into two separate holes.
In 2008, Hejnol disproved the mouth-anus link by showing their genes develop differently—sparking his quest for the anus’s true origin.
This evolutionary milestone holds the key to how 90% of animals gained their body plan.
“The through gut’s invention likely enabled nearly all modern animal diversity,” said Max Telford (UCL), unaffiliated with the study.
By comparing Xenoturbella’s single-hole anatomy to higher species, the team traced how a sperm-release pore could evolve into an anus.
Shared genetic pathways suggest ancient worms’ reproductive holes were repurposed—transforming digestion forever.
The breakthrough, still in preprint, redefines our understanding of animal evolution’s pivotal turning point.
As Telford notes: “Without this innovation, complex life as we know it might not exist.
He said Hejnol’s data are ‘beautiful and very convincing.’
After years studying Xenoturbella, it’s astounding we missed its gonopore,” admitted Telford—but he disputes its ancestral role.
He proposes X. bocki descended from earlier anus-bearing worms that later regressed, losing their digestive exit.
This would make them evolutionary “dropouts,” not the missing link between jellyfish and complex guts.
Hejnol, however, stands by his theory that the worm’s anatomy mirrors our first two-opening ancestors.
The clash centers on whether Xenoturbella represents a primitive blueprint or a simplified offshoot.
If Telford’s right, the true anus pioneer remains hidden in deeper evolutionary time.
Yet both agree: this pore-to-anus genetic toolkit reshaped animal body plans forever.
The debate fuels fresh scrutiny of how—and how often—nature “invents” key anatomical features.
As Hejnol puts it: “Every hole tells a story. We’re just decoding which chapter this worm writes.
