The anus is an important part of the body, the appearance of which in the early stages of multicellularity gave a powerful impetus to the development of animals. Official science claims that in evolution, the anal opening first appeared in roundworms. Recent research has cast doubt on this claim. One thing is certain: the emergence of a digestive digestive system significantly improved the absorption of nutrients, accelerated metabolism, and served as an impetus for the evolution of organisms.
The beginning of time
The digestive organs of the simplest organisms are cellular vacuoles. This is the so-called intracellular type of digestion, which is phylogenetically the oldest. In the intestinal cavity, a body cavity and extracellular digestion appear. At the embryonic stage of gastrula, they have a blastopore (a hole through which the embryo communicates with the medium, the primary mouth) from which the mouth hole develops. It leads into the gastric cavity, which ends blindly. These organisms are forced to defecate food debris through the same hole through which it entered.
Digestion in Worms
The stages of the evolution of the digestive system are clearly visible in worms. In flat and ciliated worms, the digestive tube begins with the mouth opening and ends blindly. Leftover food is removed through the mouth. But for the first time in evolution, the anus appeared in roundworms and annelids (roundworm and earthworm). Their digestive tube is clearly divided into three sections: the front and back are formed from the ectoderm, and the middle from the endoderm of the embryo. The anus in them is formed due to the extrusion of the outer layer of the embryo (ectoderm) and the connection with the intestinal cavity. And now the system "mouth-throat-esophagus-stomach-middle intestine-hindgut-anus" can be traced in the evolution of annelids.
Further complication
In arthropods, as in mollusks, oral appendages appear for grinding food, and processes in the intestine appear to increase the absorption surface. In addition, glands of the digestive tract appear, which then become the liver. An example of the evolution of the digestive system is its improvement in chordates: differentiation of the intestinal cavity and the formation of the large and small intestines, glandular and muscular stomach, and additional organs of the oral apparatus. The liver appears in cyclostomes, and the pancreas in fish.
Amazing combers
One of the most ancient animals - ctenophores - brought confusion to the ranks of biologists. In 2016, at the University of Miami in the United States of America, biologist Brown William fed ctenophores Mnemiopsis leidyi and Pleurobrachia bachei with gene-modified crustaceans daphnia, into which fluorescent proteins were introduced. For three hours, red-glowing food cruised through their transparent body, and then the remnants came out through openings in the back of the primitive animal, equipped with a circular sphincter. The conclusion is staggering - in evolution, for the first time, an anal opening appeared in ctenophores, animals standing at the base of the phylogenetic tree of life on the planet.
Mysterious anus
In the same year, the Journal of Comparative Zoology (Zoologischer Anzeiger - A Journal of Comparative Zoology) published the work of molecular biologists from Norway Andreas Heinol and Hem Martan-Duran on the effect of two genes on the formation of the anus in different groups of animals. All animals with anus have these genes, and those without anus do not. Molecular analysis showed absolute similarity in the development of the digestive system in embryos of marine worms, fish, amphibians and humans. They found that some species lost their anus during evolution. The conclusion made by biologists: the results of evolution in relation to the anus changed, and it appeared in the phylogenetic development of organisms several times. And the scientists pushed back the origin of our stomach 500 million years ago.
Poor scorpions
Scorpion Ananteris balzani is able to separate the tail when attacked by predators. Saving his life, the scorpion, along with its tail, loses its anus. A new tail does not grow, and the wound heals. Scorpio cannot defecate; food inflates it and it dies. The scorpion example only proves the importance of a digestive digestive system for highly organized animals.
People should be grateful to nature for a gift in the form of an anus. After all, if it was not there, then we would have to wait until 9 hours before the next meal and allocate undigested residues through the mouth. The human digestive system with eight sections and digestive glands allowed us to become omnivores, and this also anticipated cephalization (brain development). And it does not matter, for the first time in evolution, the anus appeared in worms or ctenophores. The main thing is that it appeared. So the anus is not only older than the oral cavity, but also more important in evolutionary terms.