▁have ▁occurred ▁at ▁least ▁ 3 . 5   b ill ion ▁years ▁ago , ▁during ▁the ▁hade an ▁or ▁ar che an ▁e ons ▁on ▁a ▁prim ord ial ▁Earth ▁that ▁had ▁a ▁substantial ly ▁different ▁environment ▁than ▁is ▁found ▁at ▁present . ▁These ▁life ▁forms ▁possessed ▁the ▁basic ▁tra its ▁of ▁self - rep lication ▁and ▁inher itable ▁tra its . ▁Once ▁life ▁had ▁appeared , ▁the ▁process ▁of ▁evolution ▁by ▁natural ▁selection ▁resulted ▁in ▁the ▁development ▁of ▁ever - more ▁diverse ▁life ▁forms . ▁ ▁Species ▁that ▁were ▁unable ▁to ▁adapt ▁to ▁the ▁changing ▁environment ▁and ▁competition ▁from ▁other ▁life ▁forms ▁became ▁ext inct . ▁However , ▁the ▁foss il ▁record ▁retain s ▁evidence ▁of ▁many ▁of ▁these ▁older ▁species . ▁Current ▁foss il ▁and ▁DNA ▁evidence ▁shows ▁that ▁all ▁existing ▁species ▁can ▁trace ▁a ▁contin ual ▁ancest ry ▁back ▁to ▁the ▁first ▁primitive ▁life ▁forms . ▁ ▁When ▁basic ▁forms ▁of ▁plant ▁life ▁developed ▁the ▁process ▁of ▁photos yn thesis ▁the ▁sun ' s ▁energy ▁could ▁be ▁har v ested ▁to ▁create ▁conditions ▁which ▁allowed ▁for ▁more ▁complex ▁life ▁forms . ▁The ▁result ant ▁o xygen ▁accum ulated ▁in ▁the ▁atmosphere ▁and ▁gave ▁rise ▁to ▁the ▁o zone ▁layer . ▁The ▁incorpor ation ▁of ▁smaller ▁cells ▁within ▁larger ▁ones ▁resulted ▁in ▁the ▁development ▁of ▁yet ▁more ▁complex ▁cells ▁called ▁e uk ary otes . ▁C ells ▁within ▁colon ies ▁became ▁increasing ly ▁special ized , ▁resulting ▁in ▁true ▁mult ic ell ular ▁organ isms . ▁With ▁the ▁o zone