THE "STAR BURST" HYPOTHESIS FOR THE DIVERSIFICATION OF EUKARYOTES AND THE GEOLOGIC RECORD INDICATE VERY EARLY ORIGINS FOR ALL MAJOR LINEAGES

Molecular phylogenies based on small subunit ribosomal RNA produce trees for eukaryotes that show the group spread out with early branching amitochondriate lineages (Giardia, trichomonads, microsporidians) most basal followed by slime molds, foraminifera, euglenoids, polycystines, leading to a so-called crown group of the remaining eukaryotes, including dinoflagellates, ciliates, animals and plants. This phylogeny has been used to relate geologic events and times with the evolutionary developments depicted thereon. Recent molecular evidence using other genes shows that this tree is compromised by long-branch attraction artifacts. When these artifacts are removed, the eukaryotes are reduced to a single burst of lineage diversification that gave rise to all the later forms of eukaryotes known. All the lower branching lineages become associated with the higher branching and crown group lineages in a single "star-burst" rather than a "tree". No known geologic age or event can be correlated with this burst of eukaryote diversification. The geologic record of eukaryote fossils begins about 1.6 ga but eukaryote biomarkers are known from rocks about 2.7 ga. This geologic evidence combined with the new molecular phylogenies indicates that all major eukaryote lineages arose much earlier in the history of life than previously inferred, although not necessarily as the organisms we see in the later fossil record or alive today. As far as we can tell from the known geologic record, skeletonization in eukaryotes, hence a fossil record, follows billions of years later than the "big burst" of eukaryote diversification. Continued searching for cellular fossils and biomarkers of eukaryotes in yet older rocks may provide additional evidence about the structure and timing of early eukaryote evolution.