▁evidence ▁for ▁life ▁on ▁Earth ▁dates ▁to ▁ 3 . 8   b ill ion ▁years ▁ago — al most ▁immediately ▁after ▁the ▁end ▁of ▁the ▁Late ▁He avy ▁Bomb ard ment . ▁ ▁Imp act s ▁are ▁thought ▁to ▁be ▁a ▁regular ▁( if ▁currently ▁inf re quent ) ▁part ▁of ▁the ▁evolution ▁of ▁the ▁Sol ar ▁System . ▁That ▁they ▁continue ▁to ▁happen ▁is ▁ev iden ced ▁by ▁the ▁collision ▁of ▁Com et ▁Sho em aker – Le vy ▁ 9 ▁with ▁Jup iter ▁in ▁ 1 9 9 4 , ▁the ▁ 2 0 0 9 ▁Jup iter ▁impact ▁event , ▁the ▁T ung us ka ▁event , ▁the ▁Ch ely ab in sk ▁meteor ▁and ▁the ▁impact ▁that ▁created ▁Meteor ▁Cr ater ▁in ▁Arizona . ▁The ▁process ▁of ▁acc ret ion , ▁therefore , ▁is ▁not ▁complete , ▁and ▁may ▁still ▁pose ▁a ▁threat ▁to ▁life ▁on ▁Earth . ▁ ▁Over ▁the ▁course ▁of ▁the ▁Sol ar ▁System ' s ▁evolution , ▁com ets ▁were ▁e ject ed ▁out ▁of ▁the ▁inner ▁Sol ar ▁System ▁by ▁the ▁gravity ▁of ▁the ▁giant ▁plan ets , ▁and ▁sent ▁thousands ▁of ▁A U ▁out ward ▁to ▁form ▁the ▁O ort ▁cloud , ▁a ▁spher ical ▁outer ▁sw arm ▁of ▁com et ary ▁nuc lei ▁at ▁the ▁far th est ▁extent ▁of ▁the ▁Sun ' s ▁gravit ational ▁pull . ▁Event ually , ▁after ▁about ▁ 8 0 0 ▁million ▁years , ▁the ▁gravit ational ▁dis ruption ▁caused ▁by ▁gal actic ▁t ides