<P> Multicellular organisms are organisms that consist of more than one cell, in contrast to single - celled organisms . </P> <P> In complex multicellular organisms, cells specialize into different cell types that are adapted to particular functions . In mammals, major cell types include skin cells, muscle cells, neurons, blood cells, fibroblasts, stem cells, and others . Cell types differ both in appearance and function, yet are genetically identical . Cells are able to be of the same genotype but of different cell type due to the differential expression of the genes they contain . </P> <P> Most distinct cell types arise from a single totipotent cell, called a zygote, that differentiates into hundreds of different cell types during the course of development . Differentiation of cells is driven by different environmental cues (such as cell--cell interaction) and intrinsic differences (such as those caused by the uneven distribution of molecules during division). </P> <P> Multicellularity has evolved independently at least 25 times, including in some prokaryotes, like cyanobacteria, myxobacteria, actinomycetes, Magnetoglobus multicellularis or Methanosarcina . However, complex multicellular organisms evolved only in six eukaryotic groups: animals, fungi, brown algae, red algae, green algae, and plants . It evolved repeatedly for plants (Chloroplastida), once or twice for animals, once for brown algae, and perhaps several times for fungi, slime molds, and red algae . Multicellularity may have evolved from colonies of interdependent organisms, from cellularization, or from organisms in symbiotic relationships . </P>

Where do most of the cells life processes occur