( However ( , ( ( over ( billions ( of years ) ) ) ( ( many ( asteroid objects ) ) ( ( have ( ( ( collided ( with Earth ) ) and ) ( ( left ( their marks ) ) ( with ( craters ( that ( ( can still ) ( be seen ) ) ) ) ) ) ) ) . ) ) ) ) )
(ROOT (S (ADVP (RB However)) (, ,) (PP (IN over) (NP (NP (NNS billions)) (PP (IN of) (NP (NNS years))))) (NP (JJ many) (JJ asteroid) (NNS objects)) (VP (VBP have) (VP (VP (VBN collided) (PP (IN with) (NP (NNP Earth)))) (CC and) (VP (VBN left) (NP (PRP$ their) (NNS marks)) (PP (IN with) (NP (NP (NNS craters)) (SBAR (WHNP (WDT that)) (S (VP (MD can) (ADVP (RB still)) (VP (VB be) (VP (VBN seen))))))))))) (. .)))
However, over billions of years many asteroid objects have collided with Earth and left their marks with craters that can still be seen.
(ROOT (S (PP (IN In) (NP (DT the) (NN past))) (, ,) (NP (NP (NNS asteroids)) (VP (VBG colliding) (PP (IN with) (NP (NN planet) (NN earth))))) (VP (VBZ has) (VP (VBN caused) (NP (JJ many) (NNS organisms)) (S (VP (TO to) (VP (VB die) (PRT (RP off))))))) (. .)))