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Few unicellular species 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, fungi, animals and brown algae— arise from a single cell and generate a multi-celled organism. By contrast, colonial, or pluricellular, organisms are the result of many-celled individuals joining together through colony formation, filament formation or aggregation. Pluricellularity has evolved independently in Volvox and some flagellated green algae.
Multicellularity has evolved independently at least 46 times, including in some prokaryotes, like cyanobacteria, myxobacteria, actinomycetes, Magnetoglobus multicellularis or Methanosarcina. However, complex multicellular organisms evolved only in six eukaryotic groups: animals, fungi, brown algae, red algae, green algae, and land plants. It evolved repeatedly for Chloroplastida (green algae and land plants), once or twice for animals, once for brown algae, three times in the fungi (chytrids, ascomycetes and basidiomycetes) and perhaps several times for slime molds, and red algae.
The first evidence of multicellularity is from cyanobacteria-like organisms that lived between 3 and 3.5 billion years ago. 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.
Loss of multicellularity occurred in some groups, like in some red algae (e.g., Porphyridium, but it is possible that they are primitivelly unicellular), and probably in some green algae (e.g., Chlorella vulgaris and some Ulvophyceae). Fungi are predominately multicellular, though early diverging lineages are largely unicellular (e.g., Microsporidia) and there have been numerous reversions to unicellularity across fungi (e.g., Saccharomycotina, Cryptococcus, and other yeasts). In other groups, generally parasites, a reduction of multicellularity occurred, in number or types of cells (e.g., the myxozoans, multicellular organisms, early thought to be unicellular, are probably extremely reduced cnidarians).
Multicellular organisms, especially long-living animals, 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. Cancer in animals (metazoans) has often been described as a loss of multicellularity. There is a discussion about the possibility of existence of cancer in other multicellular organisms. For example, plant galls have been characterized as tumors, however, other authors argue that plants do not get cancer.
In some multicellular groups, which are called Weismannists, a separation between a sterile somatic cell line and a germ cell line evoluted. However, Weismannist development is relatively rare (e.g., vertebrates, arthropods, Volvox), as great part of species have the capacity for somatic embryogenesis (e.g., land plants, most algae, many invertebrates).
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 (Gabonionta). The Doushantuo Formation has yielded 600 million year old microfossils with evidence of multicellular traits.
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:
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. 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.
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 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.
The evolution of multicellularity from unicellular ancestors has been replicated in the laboratory, in evolution experiments using predation as the selective pressure. Similar experiments can demonstrate the facultative induction of multicellularity.
Multicellularity allows an organism to exceed the size limits normally imposed by diffusion: single cells with increased size have a decreased surface-to-volume ratio and have difficulty in absorbing sufficient nutrients and transporting them throughout the cell. This confers multicellular organisms with the competitive advantages of an increase in size. It also permits increasing complexity by allowing the differentiation of numerous cellular lineages within an organism.