Revised chronology of the 'Tree of Life'. The present is represented by the horizontal line at the top and geological periods are shown on the left with their approximate dates. The Phanerozoic era encompasses the Paleozoic, Mesozoic and Cenozoic (Cen) eras. (a) All organisms; (b) multicellular organisms; (c) amniotes. A variety of important evolutionary events have been estimated using data from fossils (gray horizontal lines) or sequences (black horizontal lines). See the text for discussion of specific divergence times. Where multiple estimates from sequence data have been made, the midpoint of the range is shown.