Multicellularity

Abstract

Multicellularity evolved many times independently in both bacteria and eukaryotes, but only in the latter did it attain high levels of complexity. Multicellularity dramatically enhanced niche construction and ecosystem complexity. The key factor behind the success of multicellularity is increased fitness from labour division and cellular specialization. Multicellularity required the evolution of mechanisms of cellular communication and differentiation, with frequent instances of convergence across the taxonomic spectrum. Of 119 major eukaryotic clades currently recognized, 83 are exclusively unicellular and 36 encompass multicellular forms; among the latter, six clades evolved forms with a high level of cellular differentiation. Large-sized multicellular organisms evolved vascular systems for long-distance transport. The animals and land plants added internal extracellular compartments subject to homeostatic control. In closed-form multicellular organisms (e.g. the animals and volvocine algae), the body shape is determined during embryo development, after which stem cells control cellular turnover and isometric growth. Open-form organisms (e.g. land plants and fungi) retain totipotent cells that produce new organs throughout life duration. In weismannist organisms (essentially insects, vertebrates and volvocine algae), a germ line precociously separates from the somatic line. Complex multicellular organisms have three hierarchically interlinked levels of organization and three related levels of death, i.e. systemic, organ and cellular.

Is then the evolutionary transition to multicellularity a difficult one or not? The blunt answer is: not at all, since multicellularity has arisen more than twenty times in evolution. However, there are only three lineages that produced complex organisms: plants, animals, and fungi. Three hits in 3.5 billion years are not that many.

Szathmàry and Wolpert (2003)