The Evolution of Multicellularity

Abstract

Because of surface-to-volume constraints, larger size entailed multicellularity and ultimately internal transport systems. This in turn required that some cells of the multicellular organism be dedicated to specialized, nonreproductive functions. Thus, in multicellular organisms, there is usually a distinction between somatic cells (specialized cells, which are often nondividing, and which carry out tasks like circulation) and germinal cells (cells which are capable of unlimited division; these cells produce new multicellular individuals). Herein lies the major peril of multicellularity: genetically variant cells, which do not contribute to somatic duties, but rather monopolize reproduction functions. Multicellular organisms have devised a number of mechanisms to limit such selfish cellular replication. Some of these mechanisms (e.g., a unicellular stage of the life cycle) limit the variation at the lower level and align the evolutionary interests of the cell and the organism via kin selection. Other mechanisms (e.g., programmed cell death) coerce or punish defecting cells and thus also limit the variation at the lower level. Still other mechanisms (e.g., sexual reproduction) increase the variation of the higher-level units. Together, these and other mechanisms align selection at the cell and organism level to the extent that the “modern” evolutionary synthesis more-or-less successfully ignored the former in favor of the latter.

Comparative anatomy is largely the story of the struggle to increase surface in proportion to volume.

J. B. S. Haldane [1]