Published online 17 September 2008 | Nature | doi:10.1038/news.2008.1117


Getting to the bottom of evolution

Genetic study investigates the origin of the anus


It may not be the sort of thing to discuss over dinner, but research is opening a lively debate on the origin of the anus.

Today, two evolutionary biologists have published genetic evidence in Nature1 that they claim refutes the leading theory of anal evolution. Their work suggests that the anus may have evolved multiple times in many different organisms, and they propose that, in some lineages, the anus may have formed through a fusion of the gut with the reproductive organs.

The first organisms to benefit from guts got by with just a mouth, through which food went one way and waste the other. However, as the organisms grew in size and length, having one hole for everything became impractical. "A long gut makes sorting food and waste through a single opening inefficient," says Andreas Hejnol, a researcher at the University of Hawaii in Honolulu and one of the study's authors. "So they needed to evolve an anus."

Orifice politics

"The very simple question is how to get from one opening to two," says Detlev Arendt, a researcher at the European Molecular Biology Laboratory in Heidelberg, Germany. The question may be obvious, but it is not easy to answer. Punching a new hole on the opposite end from the mouth is unlikely from an evolutionary standpoint, says Arendt. So he and others have suggested that over time, the mouth elongated and then separated into a mouth and anus. Once the body included a gut with two ends, the anus could migrate to the far end of animal.

“The issue remains open.”

Detlev Arendt

Hejnol and his co-author Mark Martindale, also at the University of Hawaii, think otherwise. They compared the patterns of gene expression during development at each end of Convolutriloba longifissura, a simple flatworm with a cul-de-sac for a gut, to those seen in more complex worms which have a mouth and an anus.

C. longifissura and the other worms expressed the same genes while building their mouths. But perhaps more importantly, some genes that are expressed in the hindguts of several other species are also expressed at the posterior end of C. longifissura, rather than in the mouth. The findings suggest that the anus did not develop from the mouth, but may instead have evolved in a different way.

Alternative route

Another way for nature to make an anus, Hejnol says, is from reproductive tissue. There is a gene expressed in the anuses of some of the more complex worms that is also expressed in the reproductive tract of C. longifissura. This suggests that the reproductive tract may have evolved first and then joined with the gut. Many more-complex organisms such as birds, reptiles, and amphibians share a single opening for all their non-eating business, and this work may point to its origin.

But Arendt disagrees. C. longifissura are fast-evolving creatures, he points out, and so what is now a reproductive tract may have started out as an anus, rather than the other way around. He would like to see a follow-up study of slower-evolving creatures before he will be won over. "I'm not convinced at all," he says. "The issue remains open."

Claus Nielsen, professor emeritus of zoology at the University of Copenhagen in Denmark, agrees that the question is not yet answered. But he thinks that the new work will stimulate future discussion about the anus and its origins. "It's interesting and it makes you think," he says. 

  • References

    1. Hejnol1, A. & Martindale, M. Q. Nature advance online publication, doi:10.1038/nature07309 (17 September 2008).
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