Written in Stone: The Fossil Record of Early Eukaryotes

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Eukaryotes represent one of the three major domains of living organisms. They diverged in the Paleoproterozoic and possibly Neoarchean Era, as indicated by morphologically complex microfossils and somewhat controversial molecular fossils. The early evolutionary history of the eukaryotes, as revealed by the Neoarchean–Proterozoic fossil record, can be described in three stages. Stage I (Neoarchean–Mesoproterozoic) is characterized by a relatively low diversity of eukaryotes, whose exact phylogenetic position within the eukaryotic phylogenetic tree is uncertain. In Stage II (late Mesoproterozoic–early Neoproterozoic), numerous phylogenetically constrained eukaryotic lineages evolved. These include the red algae, green algae, chromophyte algae, euglyphid amoebae, lobose amoebae, and possibly fungi. Their phylogenetic positions in the eukaryotic tree indicate that eukaryotes must have undergone some degree of phylogenetic divergence to give rise to the major clades such as plants (green and red algae), chromalveolates, rhizarians, amoebozoans, and possibly opisthokonts. Stage III (Ediacaran Period) is characterized by a significant increase in the diversity of eukaryotes, particularly multicellular eukaryotes including animals. Animals may have diverged in the early Ediacaran Period or earlier, but macroscopic bilaterian animals and biomineralizing animals did not appear in the fossil record until the late Ediacaran Period. This sequence of eukaryotic evolution, as recorded in the rock record, is broadly consistent with predictions from the molecular phylogeny of eukaryotes.