Evolution of multicellularity
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2008-07-09T00:58:38Z
Chira
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The first [[organisms]] that existed were [[unicellular]]. How organisms then became [[multicellular]] is a huge [[evolutionary]] step and is consequently under much debate. However, unlike some other huge steps in evolution, multicelularity is thought to have evolved many times independently - plants, animals and fungi had independent evolution paths, and several other clades of organisms have evolved multicelularity; multicellularity exists in both [[prokaryote]]s and [[eukaryote]]s, and first appeared several billion years ago in [[cyanobacteria]]. In eukaryotes, it appeared several times starting from around a billion years ago, in algae, kelp, fungi - several independent paths, and in the animal kingdom.
== 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. This allows both [[divergent evolution|divergent]] and [[convergent evolution|convergent]] evolutionary processes a huge amount of time to mimic similarities and differences between groups of modern and ancestral species that don't actually exist. While modern phylogenetics uses more sophisticated techniques such as [[alloenzymes]], [[satellite DNA]] and other molecular markers, they are still rather imprecise over such huge timescales. Nevertheless, it is [[hypothesis|hypothesized]] that the evolution of multicellularity in most, if not all, extant [[clades]] could have happened in one of three distinct ways:
=== Symbiotic Theory ===
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This theory suggests that the first multicellular organisms occurred from [[symbiosis]] (cooperation) of different species of single celled 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 their genomes being incorporated into one, multicellular, organism. Each respective organism would become a separate lineage of differentiated cells within the newly created species.
This kind of severely co-dependent symbiosis can be seen frequently, such as in the relationship between [[clown fish]] and [[Heteractis magnifica|Riterri sea anemone]]s. In these cases it is extremely doubtful if either species would survive very long if the other became extinct. 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 known to have occurred (e.g. [[mitochondria]] and [[chloroplasts]] in animal and plant cells - [[endosymbiosis]]) it has only happened extremely rarely and, even then, the genomes of the endosymbionts have retained an element of distinction, separately replicating their DNA during [[mitosis]] of the host species. For instance, the two or three symbiotic organisms forming the composite [[lichen]], while dependent on each other for survival, have to separately reproduce and then re-form to create one individual organism once more.
=== Cellularisation (Syncytial) Theory ===
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This theory states that a single unicellular organism could have developed [[endomembrane|internal membrane]] partitions around each of its [[cell nucleus|nuclei]]. Many protists such as the [[ciliates]] or [[slime moulds]] can have several nuclei, lending support to this [[hypothesis]]. However, 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 while the [[micronucleus]] is used for sexual-like 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. 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 ===
The third 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|symbiotic theory]], which suggests the symbiosis of different species) led to a multicellular organism. At least some, presumably land-evolved, multicellularity occurs by cells separating and then rejoining (e.g., [[slime mould|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 simple as incomplete [[cytokinesis]], though multicellularity is also typically considered 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. [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=15714559 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 [[Volvocaceae|Eudorina]] and [[Volvox]] (the latter of which consists of around 10,000 cells, only about 25-35 of 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, as the two concepts are usually indistinguishable, and this problem plagues most hypotheses of how multicellularisation could have occurred.
However, most scientists accept that is by the Colonial theory that Multicellular organisms, from all [[phyla]], evolved.
== Notes ==
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==See also==
* [[Nicole King]]
[[Category:Evolution]]
[[Category:Microbial population biology]]
[[Category:Microbiology]]
[[pt:Evolução de organismos multicelulares]]