The Neoproterozoic Era records the transition from a largely bacterial to a predominantly eukaryotic phototrophic world, creating the foundation for the complex benthic ecosystems that have sustained Metazoa from the Ediacaran Period onward. This study focuses on the evolutionary origins of green seaweeds, which play an important ecological role in the benthos of modern sunlit oceans and likely played a crucial part in the evolution of early animals by structuring benthic habitats and providing novel niches. By applying a phylogenomic approach, we resolve deep relationships of the core Chlorophyta (Ulvophyceae or green seaweeds, and freshwater or terrestrial Chlorophyceae and Trebouxiophyceae) and unveil a rapid radiation of Chlorophyceae and the principal lineages of the Ulvophyceae late in the Neoproterozoic Era. Our time-calibrated tree points to an origin and early diversification of green seaweeds in the late Tonian and Cryogenian periods, an interval marked by two global glaciations with strong consequent changes in the amount of available marine benthic habitat. We hypothesize that unicellular and simple multicellular ancestors of green seaweeds survived these extreme climate events in isolated refugia, and diversified in benthic environments that became increasingly available as ice retreated. An increased supply of nutrients and biotic interactions, such as grazing pressure, likely triggered the independent evolution of macroscopic growth via different strategies, including true multicellularity, and multiple types of giant-celled forms.

Department of Biology Phycology Research Group Ghent University 9000 Ghent Belgium

Department of Botany Faculty of Science Charles University CZ 12800 Prague 2 Czech Republic

Department of Cell Biology and Molecular Genetics University of Maryland College Park MD 20742

Department of Organismic and Evolutionary Biology Harvard University Cambridge MA 02138

Department of Plant Biotechnology and Bioinformatics Ghent University 9052 Zwijnaarde Belgium

Department of Plant Biotechnology and Bioinformatics Ghent University 9052 Zwijnaarde Belgium Vlaams Instituut voor Biotechnologie Center for Plant Systems Biology 9052 Zwijnaarde Belgium

Department of Plant Biotechnology and Bioinformatics Ghent University 9052 Zwijnaarde Belgium Vlaams Instituut voor Biotechnologie Center for Plant Systems Biology 9052 Zwijnaarde Belgium Bioinformatics Institute Ghent Ghent University 9052 Zwijnaarde Belgium

Division of Plant Sciences University of Dundee at the James Hutton Institute Dundee DD2 5DA United Kingdom School of Biological Sciences University of Western Australia WA 6009 Australia Climate Change Cluster University of Technology Ultimo NSW 2006 Australia

Meise Botanic Garden 1860 Meise Belgium

School of Biosciences University of Melbourne Melbourne VIC 3010 Australia

Vlaams Instituut voor Biotechnologie Center for Plant Systems Biology 9052 Zwijnaarde Belgium Bioinformatics Institute Ghent Ghent University 9052 Zwijnaarde Belgium

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