The development of multicellularity was a pivotal point in eukaryote evolution: different cells in a multicellular organism may become specialized to carry out different tasks, enabling much greater functional complexity. The task of ensuring that the organism is able to reproduce is delegated to specialized germ cells. The remaining somatic cells can be programmed to become diverse types of cell, such as neurons, lymphocytes, and so on, in the case of animals.
Repeated cell division allows the organism to grow from a single fertilized egg cell, and complex cell–cell interactions and cell–environment interactions during development permit progressive development of cell specialization. Cells can then co-operate to build complex tissues and organs.
The development of multicellularity allowed stunning evolutionary advances: com plex fungi, plants, and animals evolved, and one species went on to develop sufficient cognitive powers that it began to radically transform its environment, rather than simply adapt to it. One might expect that the development of multicellularity would have required progressive changes to the genome. Genetic studies in yeast, however, have suggested that single gene mutations that prevent mother cells detaching from daughter cells might have been highly significant (Figure 1).
Fig1. Might multicellularity have evolved by a single gene mutation? Artificial manipulation of the yeast Saccharomyces cerevisiae can produce a mutation inactivating the ACE2 gene (which makes a type of transcription factor). Disrupting the production of just this one transcription factor is enough to prevent mother–daughter cell separation, generating multicellular “snowflake” yeast. The cells in the cluster showed evidence of coordination and a high-broad-sense heritability for multicellular traits. By 60 days, the cells had evolved in concert to be 2.2-fold larger than an early snowflake yeast from the same population (14 days). Scale bars, 50 μm. (From Ratcliff WC et al. [2015] Nat Commun 6:6102; PMID 25600558. With permission from Springer Nature. Copyright © 2015.)
