|Multicellular yeast cluster. Photo courtesy of William Ratcliff.|
Multicellularity was invented several times during the history of life (Rokas, 2008), but since it happened a long time ago it is difficult to reconstruct the exact sequence of events. Experimentation on today's unicellular organisms, however, allows researchers to test mechanisms (and associated mutations) that could lead to a multicellular lifestyle. Of course, this cannot decide for good how the phenomenon occurred many millions years ago – which is not at all the author's claim – but this can prove that such mechanisms can occur, given that an appropriate selection pressure is present.