Life

Abstract This the earliest and most controversial portion of the tree of life and few details can be regarded as well established. It involves the relationships and times of origin of the three super kingdoms, Eubacteria, Archaebacteria, and Eukaryota. But the key to understanding how these earliest events unraveled is to know how eukaryotes arose and their relationship to prokaryotes. Despite the availability of hundreds of completely sequenced genomes from prokaryotes and dozens from eukaryotes, the answers are not yet in hand. New models for the origin of eukaryotes appear frequently. For ease of discussion they are classified here as merger models (1–11) and deep-root models (12, 13). The former ascribe the origin of eukaryotes to a merger between two prokaryotes whereas the latter minimize the role of mergers and instead postulate an ancient origin of eukaryotes, at least as old as the earliest divergences among living prokaryotes. The goal of this brief synopsis is to review the evidence bearing on the earliest aspects of the time tree of life. Details concerning differences among the many proposed models, as well as different points of view, can be found elsewhere (2, 14, 15).