The Evolutionary Journey of the Human Anus: Unveiling Its Ancient Origins
How the Human Anus Evolved from an Ancient Sperm-Releasing Hole
Scientists uncover a surprising origin story for a crucial anatomical feature.
Around 550 million years ago, the human anus may have evolved from a hole originally used to release sperm, according to a groundbreaking study. Researchers led by Andreas Hejnol at the University of Bergen propose that a small pore for expelling sperm in ancient worm-like creatures eventually merged with the gut, forming the first “through gut” system—a digestive tract with both a mouth and anus.
Key Evidence from a Primitive Worm
The team studied Xenoturbella bocki, a modern worm-like organism found on ocean floors. These creatures lack an anus but possess a mouth and a “male gonopore,” a hole for releasing sperm. Genetic analysis revealed that the genes controlling the gonopore’s development in X. bocki are the same ones responsible for anus formation in other animals, including humans. This suggests a shared evolutionary origin.
[Image: Xenoturbella bocki under a microscope, highlighting its mouth and gonopore.]
From Sperm Pore to Digestive Exit
Most animals today have a through gut, but early life forms like jellyfish digest food through a single opening. Scientists long debated how the anus evolved. One theory proposed the mouth split into two holes, but Hejnol’s 2008 work disproved this by showing mouth and anus genes differ. His new study argues that in a common ancestor resembling X. bocki, the gonopore and gut gradually connected, creating a dedicated exit.
“Once a hole exists, it can be repurposed,” Hejnol told New Scientist. Over generations, this fusion likely improved digestion efficiency, enabling complex life forms to thrive.
Debate and Implications
While the study (pending peer review) offers compelling evidence, some experts like Max Telford of University College London caution that X. bocki may have lost its anus rather than representing an evolutionary intermediate. He suggests its ancestors had a through gut, which later degenerated.
[Image: Evolutionary timeline showing hypothetical transition from gonopore to anus.]
Regardless, the research highlights the anus’s role in animal diversity. “The through gut was a game-changer,” Telford notes, enabling specialized organs and complex body plans.
Why It Matters
Understanding the anus’s origins sheds light on how over 90% of animal species, including humans, evolved. Hejnol’s work challenges old assumptions and opens new questions about how anatomy adapts over millennia. As debates continue, one thing is clear: this humble body part has a remarkably unexpected history.
The study is available on the preprint server bioRxiv.
