Multicellular organisms are organisms that consist of more than one cell, and have 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 Planimalia (except for specialized organisms such as Myxozoa).
Early life was most probably single celled and multicellularity has appeared dozens of times in the history of Earth. In order to reproduce, true multicellular organisms must solve the problem of 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 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.
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.
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 specialized 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, resulting in a conglomeration of identical cells in one organism, which could later develop specialized tissues.