Today, two evolutionary biologists have published genetic evidence in Nature 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.Current theory holds that in early organisms with one hole (a mouth used for both eating and excretion - imagine the aftertaste!) the mouth eventually separated into two orifices, one of which gradually to migrated to the opposite end (anus next to the mouth? - imagine the smell!).
The new study examined developmental gene expression patterns in worms. They found that in both worms with one 'hole' and those with both a mouth and an anus, the same genes were involved in forming the mouth. However anus-forming genes were different, contrary to what would be expected if the anus branched off from the mouth. Similar 'backside' genes were also expressed at the posterior of the no-asshole worm - one of them in the reproductive tract. This led to the suggestion that rather than branching from the mouth, the reproductive organs may have evolved first and then joined with the gut - think cloaca.