The genes of a marine animal with a very primitive body scheme suggest that the anus could have a different function at the beginning.
The anus may have started his mission among the living not as an orifice to bring out waste products, but as Opening conduct for spermatozoa. This, if the genes of a marine worm with a very primitive digestive system do not take us off the road. Once spread, the opening would then merge with the intestine, and would have changed its function. This is what a study awaiting publication posted on the server suggests BIORXIV.
Exit … from the entrance. Before the appearance of the anus, very primitive animals they were equipped only with mouth and intestineand to feed on a new meal they had to expel the remains of the previous one from the mouth. Some very elementary creatures, such as jellyfish, still use this method. For a long time it was considered that the anus had evolved from an ancient bipartition of the mouth into primitive organisms. The hypothesis, however, does not keep, because the genes that govern the development of the oral region they are very different from those responsible for the development of the anus.
As we were. Andreas Hejnol, professor of comparative biology at Friedrich Schiller University in Jena, Germany, now suggests a different origin for the anus. Hejnol studied for a long time Xenoturbella bockia kind of worm that lives in the seabed and that an anus does not have it at all, but is satisfied with a mouth and an intestine. The X. Bocki It is considered a modern representative of a group of common ancestors among the ancestors of jellyfish and the first animals with anus.
Genetic affinities. Studying the vermone, Hejnol has reached two interesting conclusions. The first is that the males of this species have a gonoporothat is, an opening for the release of the spermatozoa; The females, on the other hand, expel the eggs through the mouth. The second is that several genes that control the development of the final stretch of the intestine in animals with an anus also guide the development of the gonoporo in primitive animals such as X. Bocki. There may therefore be between the two openings an evolutionary link.
One thing. Hejnol imagines that the gonoporo existed before the anus and that he was near the end of the digestive system. The two entities would be At one point mergedusing a single common opening. Moreover, many modern animals, such as amphibians, birds, reptiles and monothemes, have a single orifice in which the terminal part of the digestive system, of the reproductive and urinary system (the cloaca).
An important matter. Other scientists believe that it X. Bocki It belongs to a group of animals that once had an anus, with a connected gonoporo, and which later lost the anus during evolution.
In practice they do not think that the animal studied by the German group represent the previous step The appearance of the anus.
However, see it, the question of the origin of the anus is far from trivial, because the appearance of a complete internal intestinal tract, with a mouth and an anus at the ends, would have laid the bases for animals with a complex body scheme, such as vertebrates.
