The transition took place over the course of 50 weeks and was caused simply by the introduction of a predator to the environment. Time-lapse videos are available in the supplementary info.
The transition from unicellular to multicellular life was one of a few major events in the history of life that created new opportunities for more complex biological systems to evolve. Predation is hypothesized as one selective pressure that may have driven the evolution of multicellularity. Here we show that de novo origins of simple multicellularity can evolve in response to predation. We subjected outcrossed populations of the unicellular green alga Chlamydomonas reinhardtii to selection by the filter-feeding predator Paramecium tetraurelia. Two of five experimental populations evolved multicellular structures not observed in unselected control populations within ~750 asexual generations.
De novo origins of multicellularity in response to predation
(Score: 3, Informative) by HiThere on Saturday February 23 2019, @05:58PM
You're making things too linear. Evolution is a massively parallel process. The man-o-war is closely related to jellyfish, as we are closely related to sharks (perhaps closer, I'm not sure, but it's a separate path of evolution).
The question of "Weren't some of the ancestors of these algae as multicellular?" is much more to the point. Certainly stromatolites date back a VERY long way. So this is more likely re-evoking a pre-existing mechanism.
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