34. The Geologic Time Scale Related to the Calendar Year Dec. 31 (0.05 seconds before midnight) 18 Your birth Dec. 31, 6 P.M. 3 my Early hominids Dec. 31, 5 A.M. 10 my Grand Canyon downcutting Dec. 18 155 my First mammals Dec. 13 220 my First reptiles Dec. 3 340 my First land plants Nov. 10 625 my First vertebrates Oct. 12 1.0 by First cell with nucleus mid-July 2.0 by Oxygen in atmosphere mid-April 3.2 by First single-celled organism Jan. 1 4.6 by Earth formed Time in Year Age Event