Multicellular organism

Multi-cellular organisms are  organisms with more than one cell. This is the case for animals, plants and most fungi. In such organisms, cells are usually specialised. All the cells with the same function work together. Such a group of cells is then called a tissue. 

Multi-cellular organisms have a set of cells that specialize in reproduction. Reproduction in such organisms is usually sexual, but for some, asexual reproduction is usual. The sex cells are either sperm or ovum (also named "egg") cells. If they are sperm cells, the organism is male, if they are egg cells it is female. If both are present, the organism is a hermaphrodite.

 Evolutionary history 
We know, from their cell structure, that multicellularity has evolved independently many times in Earth history, for example in plants and animals.Bonner J.T. 1998. The origins of multicellularity. Integr. Biol. 1, 27–36
The earliest forms of life in the fossil record are cyanobacteria from the Archaean era at 3.5 billion years ago. They grew as single cells but lived in colonies as stromatolites. 

The first multicellular fossils have been found in 2.1 billion years old Palaeoproterozoic rocks from the Gabon in Africa.Donoghue P.C.J. and Antcliffe J.B. 2010. Origins of multicellularity. Nature 466, p41.El Ababi A. et al 2010. Large colonial organisms with coordinated growth in oxygenated environments 2.1 Gyr ago. Nature 466, 100–104. These are eukaryotic cells which live colonially. The fossils are visible to the naked eye, and hence much larger than bacteria. The authors comment that one advantage of  multicellularity is larger size, and that an adequate level of oxygen is required to support this kind of life.

 Consequences of multicellularity 
To reproduce, 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. 

Multicellular organisms, especially long-living animals, also face the challenge of cancer, which occurs when cells fail to regulate their growth within the normal program of development.

 Related pages 
 Life timeline
 Metazoa
 Unicellular organism

 Other websites 

 Multicellular organism -Citizendium

 References 

Category:Evolutionary biology
Category:Cell biology