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Loss of multicellularity occurred in some groups.

Animals have evolved a considerable diversity of cell types in a multicellular body (100–150 different cell types), compared with 10–20 in plants and fungi.

To reproduce, true 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 evolutionary developmental biology. The first evidence of multicellular organization, which is when unicellular organisms coordinate behaviors and may be an evolutionary precursor to true multicellularity, is from cyanobacteria-like organisms that lived 3.0–3.5 billion years ago. It evolved repeatedly for Chloroplastida (green algae and land plants), once for animals, once for brown algae, three times in the fungi ( chytrids, ascomycetes, and basidiomycetes) and perhaps several times for slime molds and red algae. However, complex multicellular organisms evolved only in six eukaryotic groups: animals, symbiomycotan fungi, brown algae, red algae, green algae, and land plants. Multicellularity has evolved independently at least 25 times in eukaryotes, and also in some prokaryotes, like cyanobacteria, myxobacteria, actinomycetes, Magnetoglobus multicellularis or Methanosarcina. There are also macroscopic organisms that are multinucleate though technically unicellular, such as the Xenophyophorea that can reach 20 cm.Įvolutionary history Occurrence However, it can often be hard to separate colonial protists from true multicellular organisms, because the two concepts are not distinct colonial protists have been dubbed "pluricellular" rather than "multicellular". Colonial organisms are the result of many identical individuals joining together to form a colony. Multicellular organisms arise in various ways, for example by cell division or by aggregation of many single cells. All species of animals, land plants and most fungi are multicellular, as are many algae, whereas a few organisms are partially uni- and partially multicellular, like slime molds and social amoebae such as the genus Dictyostelium.

The nematode Caenorhabditis elegans stained to highlight the nuclei of its cellsĪ multicellular organism is an organism that consists of more than one cell, in contrast to unicellular organism.