The question of which of the living organisms can be considered the most primitive is not so simple. At least two bidders present their rights to this honorary title.
Classical science suggests that the animals probably descended from so-called hoanoflagellates (collarbone flagellates). And the very first animals were supposed to be something resembling a ball (or cake). "Ball" consisted of two types of cells: the outer layer, which was carried by flagella and served for movement, and internal - similar to amoebae and performing the function of digestion. Larvae of lower animals - sponges and coelenterates - are arranged approximately in this way. For this reason, it is believed that it is these creatures - especially the sponges - that are the most primitive animals for today, because they have changed so little since ancient times!
However, the title of the most primitive animal pretends to be a strange organism called trichoplax. It is a flat, slowly crawling blot that has no axes of symmetry, no musculature, no front, no rear ends - and, of course, no digestive, nervous, blood and excretory systems. Trichoplax resembles the larvae of coelenterates, it was long considered a larva of jellyfish. Later, however, it turned out that the creature is a separate animal that forms sex cells and reproduces sexually. True, it has not been possible to find out how the development of a fertilized egg is going on. In aquariums, "blots" reproduce asexually, simply dividing into two parts or budding their small parts.
Trichoplax / ©Depositphotos
In 2006, scientists from the US and Germany read a small, but very informative part of the trichoplax genome - the mitochondrial chromosome. It was found that the mitochondrial genome of the animal occupies an intermediate position between the hoanoflagellates (the closest relatives of animals) and all other animals (including sponges and coelenterates) in their structure.
This, on the one hand, has confirmed the conjecture of many scientists that trihoplax is the most primitive animal of all that exists today. He certainly is not a simplified descendant of sponges or coelenterates, whose mitochondrial genomes have preserved far fewer primitive traits. Evolutionary biology today: unexpected discoveries and new questions. Simplicity of trichoplax organization is primary. It is explained by the fact that of all the numerous and diverse descendants of the "common ancestor of all animals,"trichoplax has changed less than all."
Trichoplax / ©Depositphotos
Nevertheless, it is too early to draw any final conclusions. A little later, the nuclear (main) trichoplax genome was also read. And the results of this study made us doubt the conclusions made on the basis of the mitochondrial genome of this strange creature, since its nuclear genome had fewer primitive features than sponges. Thus, sponges once again claim the role of the primitive animal of our time, in spite of the fact that the most primitive mitochondria are still preserved in the trichoplax.
Sponges / ©Depositphotos
Sponges do not have real tissues, there are no germinal leaves (layers of the embryo of multicellular animals that give rise to different organs and tissues), there are no nervous, muscular, digestive systems and, of course, their own skeleton. The latter is represented by various protein and mineral structures. Reproduction - both sexual and asexual. The shape of the body of the sponge is goblet, cup-shaped and woody. To date, approximately 8,000 species of these animals have been described. Fossil forms are known from Precambrian (650-800 million years ago).
Sponges / ©Depositphotos
It is the sponges that are opposed to all other animals, as the latter are called, "a real multicellular animal" (Eumetazoa).