Evolution of multicellularity
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The first organisms that existed were unicellular. How organisms then became multicellular is a big step in evolutionary terms and is under much debate.
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[edit] Hypotheses on origins of multicellularity
Because the first multicellular organisms would have lacked hard body parts, they are not well preserved in fossil records<ref>Knoll, A. H. 2003. Life on a Young Planet. Princeton University Press, Princeton, NJ. (this is an excellent book on the early history of life, very accessible to the non-specialist, that includes extensive discussions of early signatures, fossilization, and organization of life) ISBN 0-691-00978-3 (hardcover), ISBN 0-691-12029-3 (paperback)</ref>. 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. Nevertheless, it is hypothesized that the evolution of multicellularity could have happened in one of three distinct ways:
[edit] 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 with this theory is that it is still not known how each organism's DNA could be incorporated into one genome, therefore allowing replication of the whole organism (which would otherwise be impossible). Although such symbiosis is known to have occurred (e.g. mitochondria and chloroplasts in plant and animal cells - endosymbiosis) it only happened extremely rarely, and did not directly lead to multicellularity. 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.
[edit] 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 slime moulds can have several nuclei and so this could be a valid theory.
The problem with this theory is that actual partitioning of cell nuclei by membranes other than nuclear membranes is not known to occur so it is generally not believed 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.
[edit] 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 symbiotic theory, which suggests the symbiosis of different species) led to a multicellular organism. At least some presumptively land-evolved multicellularity occurs by cells separating and then rejoining (i.e., cellular slime molds) whereas for the majority of multicellular types (those which evolved within aquatic environments), multicellularity occurs as a consequence of cells failing to separate following division<ref>Wolpert, L., and E. Szathmary. 2002. Multicellularity: evolution and the egg. Nature 420:745 []</ref>. The mechanism of this latter colony formation can be as simply as incomplete cytokinesis, though multicelluarity typically also is consided to involve cellular differentiation<ref>Kirk, D. L. 2005. A twelve-step program for evolving multicellularity and a division of labor. BioEssays : news and reviews in molecular, cellular and developmental biology 27:299-310. abstract & pay article</ref>
The advantage of the Colonial Theory hypothesis is that 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.
Most scientists accept that is by the Colonial theory that Multicellular organisms evolved.
[edit] Notes
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