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In some multicellular groups, which are called Weismannists , a separation between a sterile somatic cell line and a germ cell line evolved. However, Weismannist development is relatively rare e. One hypothesis for the origin of multicellularity is that a group of function-specific cells aggregated into a slug-like mass called a grex , which moved as a multicellular unit.

This is essentially what slime molds do. Another hypothesis is that a primitive cell underwent nucleus division, thereby becoming a coenocyte. A membrane would then form around each nucleus and the cellular space and organelles occupied in the space , thereby resulting in a group of connected cells in one organism this mechanism is observable in Drosophila. A third hypothesis is that as a unicellular organism divided, the daughter cells failed to separate, resulting in a conglomeration of identical cells in one organism, which could later develop specialized tissues.

This is what plant and animal embryos do as well as colonial choanoflagellates. Because the first multicellular organisms were simple, soft organisms lacking bone, shell or other hard body parts, they are not well preserved in the fossil record. The earliest fossils of multicellular organisms include the contested Grypania spiralis and the fossils of the black shales of the Palaeoproterozoic Francevillian Group Fossil B Formation in Gabon Gabonionta.

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Until recently, phylogenetic reconstruction has been through anatomical particularly embryological similarities. This is inexact, as living multicellular organisms such as animals and plants are more than million years removed from their single-cell ancestors. Such a passage of time allows both divergent and convergent evolution time to mimic similarities and accumulate differences between groups of modern and extinct ancestral species.

2.50 Transport systems in multicellular organisms

Modern phylogenetics uses sophisticated techniques such as alloenzymes , satellite DNA and other molecular markers to describe traits that are shared between distantly related lineages. The evolution of multicellularity could have occurred in a number of different ways, some of which are described below:. This theory suggests that the first multicellular organisms occurred from symbiosis cooperation of different species of single-cell organisms, each with different roles.

Over time these organisms would become so dependent on each other they would not be able to survive independently, eventually leading to the incorporation of their genomes into one multicellular organism.

Multicellular Animals

This kind of severely co-dependent symbiosis can be seen frequently, such as in the relationship between clown fish and Riterri sea anemones. In these cases, it is extremely doubtful whether either species would survive very long if the other became extinct.

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However, the problem with this theory is that it is still not known how each organism's DNA could be incorporated into one single genome to constitute them as a single species. Although such symbiosis is theorized to have occurred e.

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For instance, the two or three symbiotic organisms forming the composite lichen , although dependent on each other for survival, have to separately reproduce and then re-form to create one individual organism once more. This theory states that a single unicellular organism, with multiple nuclei , could have developed internal membrane partitions around each of its nuclei. However, the simple presence of multiple nuclei is not enough to support the theory.

Multiple nuclei of ciliates are dissimilar and have clear differentiated functions. The macronucleus serves the organism's needs, whereas the micronucleus is used for sexual reproduction with exchange of genetic material. Slime molds syncitia form from individual amoeboid cells, like syncitial tissues of some multicellular organisms, not the other way round.

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  • To be deemed valid, this theory needs a demonstrable example and mechanism of generation of a multicellular organism from a pre-existing syncytium. The Colonial Theory of Haeckel , , proposes that the symbiosis of many organisms of the same species unlike the symbiotic theory , which suggests the symbiosis of different species led to a multicellular organism. At least some, it is presumed land-evolved, multicellularity occurs by cells separating and then rejoining e. The advantage of the Colonial Theory hypothesis is that it has been seen to occur independently in 16 different protoctistan phyla.

    For instance, during food shortages the amoeba Dictyostelium groups together in a colony that moves as one to a new location. Some of these amoeba then slightly differentiate from each other. Other examples of colonial organisation in protista are Volvocaceae , such as Eudorina and Volvox , the latter of which consists of up to —50, cells depending on the species , only a fraction of which reproduce. However, it can often be hard to separate colonial protists from true multicellular organisms, as the two concepts are not distinct; colonial protists have been dubbed "pluricellular" rather than "multicellular".

    About million years ago, [37] a minor genetic change in a single molecule called guanylate kinase protein-interaction domain GK-PID may have allowed organisms to go from a single cell organism to one of many cells. Genes borrowed from viruses have recently been identified as playing a crucial role in the differentiation of multicellular tissues and organs and even in sexual reproduction, in the fusion of egg cell and sperm. Such fused cells are also involved in metazoan membranes such as those that prevent chemicals crossing the placenta and the brain body separation.

    Two viral components have been identified. The first is syncytin, which came from a virus. The second identified in is called EFF1, which helps form the skin of Caenorhabditis elegans , part of a whole family of FF proteins.

    Multicellular AnimalsAx, P - NHBS

    Felix Rey, of the Pasteur Institute in Paris has constructed the 3D structure of the EFF1 protein [39] and shown it does the work of linking one cell to another, in viral infections. The fact that all known cell fusion molecules are viral in origin suggests that they have been vitally important to the inter-cellular communication systems that enabled multicellularity.

    Without the ability of cellular fusion, colonies could have formed, but anything even as complex as a sponge would not have been possible.

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    Multicellularity allows an organism to exceed the size limits normally imposed by diffusion: Multicellular organisms thus have the competitive advantages of an increase in size without its limitations. They can have longer lifespans as they can continue living when individual cells die. Multicellularity also permits increasing complexity by allowing differentiation of cell types within one organism. From Wikipedia, the free encyclopedia. When you click on a Sponsored Product ad, you will be taken to an Amazon detail page where you can learn more about the product and purchase it.

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    The order in Nature consists of relationships between organisms. It is the result of an historic process that we call phylogenesis. When working with this order we must clearly differentiate between its identification and the subsequent description of what has been identified. When identifying the order we are placing ourselves in the framework of "hypothetical realism" Vol. Read more Read less. Sponsored products related to this item What's this?

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