Evolution of multicellularity

The first organisms that existed were unicellular. How organisms then became multicellular is a huge evolutionary step and is consequently under much debate.

Hypotheses on origins of multicellularity
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. This allows both divergent and 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 hypothesized that the evolution of multicellularity in most, if not all, extant clades could have happened in one of three distinct ways:

Symbiotic Theory
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 riterri sea anemones. 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
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, 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, 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., 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. 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 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 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.