Are rib jellyfish the forefathers of all animals?
Genetic tests show that the first animals were not as simple as previously thought. Simple structured sponges were later developed in evolution as the glibberigen beings.
Animals were first in the world? Researchers have been asking themselves this for a long time. They had previously assumed that it should be as simple an organism as possible, one that does not yet have many different tissue types. A primeval variant from which the diversity of life that exists today could develop.
Until now, biologists believed that this extremely simple living creature must have looked like today’s sponges: They only have three different cell types, completely lacking muscle, nerve and sensory cells. They are obviously very basal organisms. The ancestors of today’s sponges fit well into the picture of the evolution of life. Simple structures have given rise to complex cell systems.
But now Joseph F. Ryan of the Genome Technology Branch of the American National Institute of Health presents a different story of early animals in „Science“. Genetic analyses have confirmed that it is the colorfully shimmering rib jellyfish that form the basis of the animal pedigree. There is no fossil evidence of the delicate glimmer creatures, since they consist almost exclusively of liquid. Palaeontology cannot help evolutionary biologists here.
Rib jellyfish are relatives of the normal jellyfish that everyone knows from the beach. Unlike these jellyfish, however, they do not have the typical nettle cells with which, for example, fire jellyfish can spoil bathing fun for holidaymakers.
Geneticists are now trying to help. They compare the genes of different animal strains and search for similarities and differences. From this, they reconstruct the family tree of life.
Until now, it was considered that such complex cell types as muscle or nerve cells only developed later in evolution. A special analysis of the genome of the ribbed jellyfish Mnemiopsis leidyi (sea walnut) provided information on the exact structure of these animals. They have muscle cells, but in their genome there are considerably fewer genes for these cells than in other animals. Genetically, the muscle cells of the rib jellyfish are therefore quite primitive. The researchers even believe that these genetically very simple muscle cells developed earlier and independently of the muscle cells known in other living organisms.
The researchers were also surprised by the genes of the rib jellyfish nervous system. The sea walnut possesses nerve cell genes that have also been found in sponges. Unlike jellyfish, however, the sponges do not have any nerve cells, so they do not use these genes. The new study, which was published in the journal „Science“, shows that ribbed jellyfish already existed before the sponges. Imagine the pedigree of the animals and the branch of the ribbed jellyfish branches off far below. The branch for the sponges bends only a little further above. The ancestor of all animals was thus probably an extremely tender being.
Source: „Die Welt“ vom 15.12.2013
Articel by Pia Heinemann
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