The first organisms that existed were undoubtedly unicellular. How organisms then became multicellular is a big step in evolutionary terms and is under much debate. 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.

The evolution of multicellular organisms could have happened in three main ways:

=1. 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 of this theory is that it is still not known how each organism's DNA could be incorparated into one genome, therefore making a replication of the whole organism impossible. Although such symbiosis is known to have occurred (e.g. mitochondria and chloroplasts in plant and animal cells - endosymbiosis) it only happened extemely rarely. 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.

=2. 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 amoeba can have several nuclei and so this could be a valid theory. However, this has never been known to occur in any organism and so is generally disbelieved 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.

=3. 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 symbiosis theory, which suggests the symbiosis of different species)led to a multicellular organism. This is most likely as 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 therefore believe that is by the Colonial theory that Multicellular organisms evolved.

Evolution