Multicellular organisms are organisms that consist of more than one cell, in contrast to single-celled organisms. To form a multicellular organism, these cells need to identify and attach to the other cells.
Only a dozen or so unicellular species have cells that can be seen individually with the naked eye. The rest of the nearly two million visible species are multicellular. In particular all species of animals, land plants and filamentous fungi are multicellular, as are many algae. Some organisms are partially uni- and multicellular, like Dictyostelium.
Multicellular organisms —like plants, animals and brown algae— arise from a single cell and generate a many-celled organism. Pluricellular organisms are the result of many-celled individuals joining together trough colony formation, filament formation or aggregation. Pluricellularity has evolved independently in Volvox and some flagellated green algae.
Evolutionary history 
Multicellularity has evolved independently dozens of times in the history of Earth, for example once for plants, once for animals, once for brown algae, but perhaps several times for fungi, slime molds, and red algae. Multicellularity exists in both prokaryotes and eukaryotes, and first appeared several billion years ago in cyanobacteria. In order to reproduce, true multicellular organisms must solve the problem of regenerating a whole organism from germ cells (i.e. sperm and egg cells), an issue that is studied in developmental biology. Therefore, the development of sexual reproduction in unicellular organisms during the Mesoproterozoic is thought to have precipitated the development and rise of multicellular life.
Multicellular organisms, especially long-living animals, also face the challenge of cancer, which occurs when cells fail to regulate their growth within the normal program of development. Changes in tissue morphology can be observed during this process.
Hypotheses for origin 
There are various mechanisms by which multicellularity could have evolved.
One hypothesis 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 syncytium. 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. One exception may be the demosponge, which may have left a chemical signature in ancient rocks. 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.
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 500 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. 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 three ways, and of which the latter, the colonial theory, is most credited by the scientific community:
The symbiotic theory 
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 into one multicellular organism of their genome. 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 Riterri sea anemones. In these cases, it is extremely doubtful whether 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 theorized to have occurred (e.g., mitochondria and chloroplasts in animal and plant cells – endosymbiosis), it has happened only 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.
The cellularization (syncytial) theory 
This theory states that a single unicellular organism, with multiple nuclei, could have developed internal membrane partitions around each of its nuclei Many protists such as the ciliates or slime molds can have several nuclei, lending support to this hypothesis. 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, 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 proposed by Haeckel in 1874. This 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, it is presumed land-evolved, multicellularity occurs by cells separating and then rejoining (e.g., cellular slime molds) whereas for the majority of multicellular types (those that evolved within aquatic environments), multicellularity occurs as a consequence of cells failing to separate following division. The mechanism of this latter colony formation can be as simple as incomplete cytokinesis, though multicellularity is also typically considered to involve cellular differentiation.
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 500–50,000 cells (depending on the species), only a fraction of which reproduce. For example, in one species 25–35 cells reproduce, 8 asexually and around 15–25 sexually. 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". This problem plagues most hypotheses of how multicellularisation could have occurred.
Multicellularity allows an organism to exceed the size limits normally imposed by diffusion, conferring the competitive advantages of an increase in size. It also permits increasing complexity by allowing the differentiation of numerous cellular lineages within an organism.
See also 
- Becker et al, Wayne M. (2009). The world of the cell. Pearson Benjamin Cummings. p. 480. ISBN 978-0-321-55418-5.
- Brian Keith Hall, Benedikt Hallgrímsson, Monroe W. Strickberger (2008). Strickberger's evolution: the integration of genes, organisms and populations (4th ed.). Hall/Hallgrímsson. p. 149. ISBN 978-0-7637-0066-9.
- Adl, Sina, M; Simpson, Alastair G. B.; Farmer, Mark A.; Andersen, Robert A.; Anderson, O. Roger; Barta, John R.; Bowser, Samuel S.; Brugerolle,Guy; Fensome, Robert A.; Fredericq,Suzanne; James, Timothy Y.; Karpov, Sergei; Kugrens, Paul; Krug, John; Lane, Christopher E.; Lewis,Louise A.; Lodge,Jean; Lynn, Denis H.; Mann,David G.; Mccourt,Richard M.; Mendoza,Leonel; Moestrup,Øjvind; Mozley-Standridge,Sharon E.; Nerad,Thomas A.; Shearer, Carol A.; Smirnov,Alexey V.; Spiegel, Frederick W.;Taylor, Max F.J.R. (October 2005). "The New Higher Level Classification of Eukaryotes with Emphasis on the Taxonomy of Protists". J. Eukaryot. Microbiol. 52. doi:10.1111/j.1550-7408.2005.00053.x/abstract. Retrieved 19 March 2013.
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- Multicellular development in a choanoflagellate; Stephen R. Fairclough, Mark J. Dayel and Nicole King
- In a Single-Cell Predator, Clues to the Animal Kingdom’s Birth
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- AlgaeBase. Volvox Linnaeus, 1758: 820.