Chapter

Evolutionary Transitions to Multicellular Life

Volume 2 of the series Advances in Marine Genomics pp 3-29

Date:

Timing the Origins of Multicellular Eukaryotes Through Phylogenomics and Relaxed Molecular Clock Analyses

  • Susan C. Sharpe*Affiliated withCentre for Comparative Genomics and Evolutionary Bioinformatics, Department of Biochemistry and Molecular Biology, Dalhousie University
  • , Laura Eme*Affiliated withCentre for Comparative Genomics and Evolutionary Bioinformatics, Department of Biochemistry and Molecular Biology, Dalhousie University
  • , Matthew W. BrownAffiliated withCentre for Comparative Genomics and Evolutionary Bioinformatics, Department of Biochemistry and Molecular Biology, Dalhousie UniversityDepartment of Biological Sciences, Mississippi State University
  • , Andrew J. RogerAffiliated withCentre for Comparative Genomics and Evolutionary Bioinformatics, Department of Biochemistry and Molecular Biology, Dalhousie University Email author 

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Abstract

Multicellularity has evolved many times during eukaryote evolution. Deciphering the evolutionary transitions to multicellularity requires a robust deep phylogeny of eukaryotes to clarify the relationships amongst multicellular groups and determine their closest unicellular relatives. Here we review progress in understanding the phylogenetic relationships amongst multicellular and unicellular eukaryotes, as well as estimates of the ages of multicellular groups based on relaxed molecular clock (RMC) analyses. In addition, we present an RMC analysis of a large phylogenomic dataset to estimate the divergence dates of select major eukaryotic multicellular groups. Our analyses (and other recent studies) tentatively suggest that multicellular eukaryotes such as Metazoa, Fungi and two of the major multicellular red algal taxa first emerged in the mid-Neoproterozoic, whereas the dictyostelids arose in the Paleozoic. We also hypothesize that the first multicellular organisms emerged within 300–600 Myr after the Last Eukaryotic Common Ancestor. The age of land plants is less clear and is highly dependent on methodology, the genes analyzed, and the nature of fossil constraints. In general, there is great variability in all these age estimates, and their credible intervals frequently span hundreds of millions of years. These estimates are highly sensitive to both the models and methods of RMC analysis, as well as the manner in which fossil calibrations are treated in these analyses. As paleontological investigations continue to fill out the Proterozoic fossil record, genomic data is gathered from a greater diversity of eukaryotes and RMC methodology improves, we may converge on more precise estimates of the ages of multicellular eukaryotes that can be correlated with Earth’s ancient geochemical record.

Keywords

Multicellularity Relaxed molecular clock Origins of complex multicellularity Bayesian framework Microfossils Eukaryote phylogeny