One of evolutionary biology’s most fascinating mysteries is the origin of the anus. New genetic evidence suggests that this intriguing structure might have evolved from an opening that was used to release sperm. Studies conducted on sea worms, specifically, Xenoturbella bocki and its developmental genetics suggest that this reproductive opening eventually merged with the digestive gut. These findings come as a welcomed surprise to researchers, exhibiting examples of evolutionary biology repurposing structures.
The Evolutionary Puzzle of the Anus
Andreas Hejnol, an evolutionary developmental biologist at the University of Bergen, explains that once an orifice exists, evolution can repurpose it. A structure initially used for sperm release might later become critical for digestion. Early animals like jellyfish still have this evolutionary biology and survive with a single mouth-gut system. This means jellyfish must regurgitate their previous meals’ waste before consuming again.
For decades, scientists debated whether the anus formed from a split mouth or evolved independently. In 2008, Hejnol’s team discovered that genes controlling mouth development differed radically from those shaping the hindgut, hinting at separate origins. This genetic divergence pulled into question initial theories of early animals’ mouth-splitting evolutionary biology. It also hints at the possibility that these evolutionary biological developments occurred independently of each other.
A Living Fossil’s Clue
The Xenoturbella bocki, a pink, 4-centimeter-long sea worm, found in Scandinavian seabeds, lacks an anus but possesses a male gonopore. The male gonopore is a dedicated sperm-release opening. Females lack an opening and their eggs instead exit through its mouth. Hejnol’s team also discovered several of the key genes responsible for the development of the hindgut were also linked to the gonopore. Hejnol’s team proposes that Xenoturbella represents an intermediate stage between ancient jellyfish-like organisms and animals with full-through guts.
In their 2023 study (which has not yet been peer-reviewed), the researchers detail how X. Bocki’s gonopore shares evolutionary biology with the hindgut of other complex animals. “The expression of ‘hindgut’ markers around the male gonopore of acoelomorphs provides molecular evidence for the homology of these structures.” This genetic overlap hints at evolutionary biology repurposing the reproductive opening into a digestive gut.
Read More: Grandmothers Played a Crucial Role in Human Evolution
Genetic Evidence Links Gonopore and Hindgut
Researchers have discovered key insights from analyzing gene expression patterns. They found that BMP signaling pathways, a key component for hindgut formation in animals like humans, also guide gonopore development in Xenoturbella. This genetic overlap implies a deep evolutionary connection. Hejnol explains that the gonopore and anus are built using the dame genetic toolkit and not only just similar. His team hypothesizes that proximity between the gonopore and gut in early animals allowed these systems to fuse, creating a dual-purpose opening that later evolved to become the anus.
Intermediate Step or Evolutionary Reversal?
Not all researchers wholeheartedly agree with Hejnol’s gonopore-fusion hypothesis. Max Telford, a evolutionary biologist at University College London, praises the data but argues Xenoturbella’s ancestors might have lost their anus and their gonopore could be a relic of the gut-reproductive system connection. In other words, the group of animals with this evolutionary biology only appeared after the evolution of the anus. “This group could represent a later simplification, not a primordial intermediate,” he says.
Fossil evidence remains elusive, as early soft-bodied worms left few to no traces. Yet, molecular clocks estimate the through-gut emerged circa 550 million years ago, coinciding with the Cambrian explosion’s body-plan diversification. Hejnol counters that gene expression patterns support his fusion theory. “Why would a lost anus leave behind gonopore-specific genes? The simplest explanation is that the gonopore came first.”
The Blastopore Debate
While Hejnol’s gonopore-fusion theory gains traction, alternative hypotheses persist. A 2025 study of embryonic development proposed that the blastopore, a transient opening in early embryos, elongates to form both mouth and anus simultaneously in some bilaterians. This contrasts with earlier “mouth-first” or “anus-first” hypotheses. However, Xenoturbella’s direct development (lacking larval stages) and lack of a blastopore-derived anus cloud this idea. Its simplicity may represent an earlier evolutionary stage before blastopore specialization became widespread.
The Through Gut’s Legacy in Animal Diversity
Regardless of its origin, the through gut’s emergence reshaped life on Earth. By enabling sequential feeding and specialized digestive regions, it allowed explosive diversification. Over 99% of modern animal species, from insects to mammals, rely on this system.
Hejnol’s team continues to study Xenoturbella to refine their model and to gain a better understanding of the evolutionary relationships of animals. As previous preconceptions about evolution are unraveled, future work may resolve whether the anus’s origin story is a tale of innovative fusion or evolutionary reversal, but either way, evolutionary biology gains new insights into how anatomical breakthroughs arise.
While research is still inconclusive about which came first, the theory that the anus and gut developed from one distinct evolutionary biological channel is becoming prevalent. Evolution and evolutionary biology continue to reveal more about the orifices which make up our gut and reproductive holes as well.
Read More: Scientists Believe Dogs Are Entering a New Phase In Their Evolution