Multicellular organism
Multicellular organisms are organisms consisting of more than one cell, and having differentiated cells that perform specialized functions in the organism. Most life that can be seen with the naked eye is multicellular, as are all members of the kingdoms Plantae and Animalia (except for specialized organisms such as Myxozoa in the case of the latter).
Hypotheses for origin
There are various mechanisms which are disputed as being the first responsible for the emergence of multicellularity, but it is difficult to say which is correct. This is because all the suggested mechanisms are viable, but establishing which was responsible for the first multicellular life requires mostly speculation.[1]
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. 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 and specialised cells in one organism (this mechanism is observable in Drosophila). A third theory is that, as a unicellular organism divided, the daughter cells failed to separate, thereby resulting in a conglomeration of identical cells in one organism which could each then specialize.