Origin of Multicellularity.

http://en.wikipedia.org/wiki/Cell_(biology)

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, fromcellularization, or from organisms in symbiotic relationships.

The first evidence of multicellularity is from cyanobacteria-like organisms that lived between 3 and 3.5 billion years ago.Other early fossils of multicellular organisms include the contested Grypania spiralis and the fossils of the black shales of thePalaeoproterozoic Francevillian Group Fossil B Formation in Gabon.

The evolution of multicellularity from unicellular ancestors has been replicated in the laboratory, in evolution experimentsusing predation as the selective pressure.