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Encyclopedia > Evolution of multicellularity

The first organisms that existed were undoubtedly unicellular. How organisms then became multicellular is a big step in evolutionary terms and is under much debate. Because the first multicellular organisms would have lacked hard body parts, they are not well preserved in fossil records. Until recently phylogenetic reconstruction has been through anatomical (particularly embryological) similarities. This is very inexact, as current multicellular organisms such as animals and plants are 500 million years removed from their single celled ancestors. In biology and ecology, an organism (in Greek organon = instrument) is a living being. ... A microorganism or microbe is an organism that is so small that it is microscopic (invisible to the naked eye). ... Multicellular organisms are those organisms containing more than one cell, and having differentiated cells that perform specialized functions. ... This article is about biological evolution. ... A phylogeny (or phylogenesis) is the origin and evolution of a set of organisms, usually of a species. ... Greek anatome, from ana-temnein, to cut up), is the branch of biology that deals with the structure and organization of living things; thus there is animal anatomy (zootomy) and plant anatomy (phytonomy). ... Embryology is the branch of developmental biology that studies embryos and their development. ... The word Animals when used alone has several possible meanings in the English language. ... u fuck in ua ...

The evolution of multicellular organisms could have happened in three main ways:

1. Symbiotic Theory

This theory suggests that the first multicellular organisms occurred from symbiosis or cooperation of different species of single celled organisms, each with different tasks. Over time these organisms would become so dependent on each other they wouldn't be able to survive without each other, eventually leading to their genomes being incorporated into one, multicellular, organism with each organism becoming a differentiated cell. The problem of this theory is that it is still not known how each organism's DNA could be incorparated into one genome, therefore making a replication of the whole organism impossible. Although such symbiosis is known to have occurred (e.g. mitochondria and chloroplasts in plant and animal cells - endosymbiosis) it only happened extemely rarely. Even the two or three symbiotic organisms forming the composite lichen have to reproduce separately from each other and then re-form to create one individual organism again. Common Clownfish (Amphiprion ocellaris) in their magnificent sea anemone (Heteractis magnifica) home. ... In cell biology, a mitochondrion is an organelle found in the cells of most eukaryotes. ... Chloroplasts are organelles found in plant cells and eukaryotic algae which conduct photosynthesis. ... An endosymbiont (also known as intracellular symbiont) is any organism that lives within cells of another organism, i. ... Lichenes from Ernst Haeckels Artforms of Nature, 1904 Crustose and foliose lichens on a wall A foliose lichen on basalt. ...

2. Cellularisation (Syncytial) Theory

This theory states that a single unicellular organism could have developed internal membrane partitions around each of its nuclei. Many protists such as the ciliates or amoeba can have several nuclei and so this could be a valid theory. However, this has never been known to occur in any organism and so is generally disbelieved to have happened in the past. However, a multicellular organism would be called multicellular and not a single celled protozoa that has undergone compartmentalisation, making this route hard to prove due to our current definition of unicellular and multicellular organisms. Classes & Subclasses Class Karyorelictea Class Heterotrichea (e. ... Amoeba (also spelled ameba) is a genus of protozoa that moves by means of temporary projections called pseudopods, and is well-known as a representative unicellular organism. ...

3. The Colonial Theory

The third, final, and most convincing explanation of multicellularisation is the Colonial Theory which was proposed by Haeckel in 1874. The theory claims that the symbiosis of many organisms of the same species (unlike the symbiosis theory, which suggests the symbiosis of different species)led to a multicellular organism. This is most likely as it has been seen to occur independently numerous times (in 16 different protoctistan phyla). For instance, Dictyostelium is an amoeba which groups together during times of food shortage, forming a colony that moves as one to a new location. Some of these amoeba then become slightly differentiated from each other. Other examples of colonial organisation in protozoa are Eudorina and Volvox (the latter of which consist around 10,000 cells, only about 25-35 which reproduce - 8 asexually and around 15-25 sexually). It can often be hard to tell, however, what is a colonial protist and what is a multicellular organism in its own right. Ernst Haeckel Ernst Heinrich Philipp August Haeckel (February 16, 1834 - August 8, 1919) was a German biologist and philosopher who popularized Charles Darwins work in Germany. ... 1874 (MDCCCLXXIV) was a common year starting on Thursday (see link for calendar). ... Families & Genera Dictyosteliidae     Dictyostelium     Polysphondylium     Coenonia Actyosteliidae     Acytostelium The dictyostelids are a group of cellular slime moulds. ... Species Volvox aureus Volvox carteri () Volvox globactor Volvox dissipatrix Volvox tertius Volvox is one of the best known genera of green algae also known as Phylum Chlorophyta, and is the culmination of the evolution of spherical colonies. ... Typical phyla Rhodophyta (red algae) Chromista Heterokontophyta (heterokonts) Haptophyta Cryptophyta (cryptomonads) Alveolates Dinoflagellata Apicomplexa Ciliophora (ciliates) Excavates Euglenozoa Percolozoa Metamonada Rhizaria Radiolaria Foraminifera Cercozoa Amoebozoa Choanozoa Many others; classification varies Protists (IPA: ) are a heterogeneous group of living things, comprising those eukaryotes that are neither animals, plants, nor fungi. ...

Most scientists therefore believe that is by the Colonial theory that Multicellular organisms evolved.



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