A new passage has been added to the theory of anal evolution.

Insert joke here.

Anuses are believed to have developed during the murky transition from radial to bilateral symmetry – i.e., from jellyfish and starfish to animals with fronts, backs and two sides. There was, scientists surmise, an animal with a nutrient-absorbing slit in its body: the slit closed in the middle, leaving both ends open. Over time, one end became specialized for consumption, and the other for ejection.

But University of Hawaii evolutionary biologists Andreas Hejnol and Mark Martindale think otherwise. They compared the developmental gene expression of Convolutriloba longifissura – a primitive flatworm with a mouth and no anus – to more complicated, anus-possessing flatworms.

Not only did *C. longifissura *and its relatives express the same genes as their mouths developed; they displayed similar patterns of gene expression in their posterior ends. In C. longifissura, this process stopped short of producing an anus, but it suggested to Hejnol and Martindale that anuses evolved separately.

They suspect that the anus is a fusion of the gut with reproductive organs – a suspicion borne out by the anuses of amphibians, reptiles and birds, which do double duty as channels of reproduction.

But Detlev Arendt, a leading proponent of the traditional explanation, found the new theory difficult to digest. "I'm not convinced at all," he said to Nature. "The issue remains open."

Acoel development indicates the independent evolution of the bilaterian mouth and anus [Nature]

Image: Andreas Hejnol and Mark Martindale, University of Hawaii

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