Journal
JOURNAL OF EXPERIMENTAL ZOOLOGY PART B-MOLECULAR AND DEVELOPMENTAL EVOLUTION
Volume 336, Issue 8, Pages 680-686Publisher
WILEY
DOI: 10.1002/jez.b.22985
Keywords
germ line; individuality; life cycles; multicellularity; timescales
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The major evolutionary transitions from unicellular organisms to multicellularity resulted in a profusion of complex life forms. One key aspect of this transition is the development of multicellular reproductive capacity, which operates on a different timescale than that of single cells. The emergence of specialized reproductive cells, such as the germ line, was essential for natural selection to operate over the longer timescale of a multicellular life cycle.
The major evolutionary transitions from unicellular organisms to multicellularity resulted in a profusion of complex life forms. During the transition from single cells to multicellular life, groups of cells acquired the capacity for reproduction as discrete units; however, the selective causes and underlying mechanisms remain debated. One perspective views the evolution of multicellularity as a shift in the timescale at which natural selection primarily operates-from that of individual cells to the timescale of reproducing groups of cells. Therefore, a distinguishing feature of multicellular reproduction, as opposed to simple growth of a multicellular collective, is that the capacity for reproduction must develop over a timescale that is greater than the reproductive timescale of a single cell. Here, I suggest that the emergence of specialized reproductive cells (the germ line) was an essential first stage of the evolutionary transition to multicellularity because it imposed the necessary delay-allowing natural selection to operate over the longer timescale of a multicellular life cycle, ultimately resulting in the evolution of complex multicellular organisms. This perspective highlights the possibility that the ubiquity of a germ-soma distinction among complex multicellular organisms reflects the fact that such life cycles, on first emergence, had the greatest propensity to participate in Darwinian evolution.
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