( Scientists ( ( have ( ( identified ( ( ( at least ) five ) ( mass extinctions ) ) ) ( since ( life ( began ( ( ( nearly ( 4 billion ) ) years ) ago ) ) ) ) ) ) . ) )
(ROOT (S (NP (NNS Scientists)) (VP (VBP have) (VP (VBN identified) (NP (QP (IN at) (JJS least) (CD five)) (JJ mass) (NNS extinctions)) (SBAR (IN since) (S (NP (NN life)) (VP (VBD began) (ADVP (NP (QP (RB nearly) (CD 4) (CD billion)) (NNS years)) (RB ago))))))) (. .)))
Scientists have identified at least five mass extinctions since life began nearly 4 billion years ago.
(ROOT (S (NP (EX There)) (VP (VBP have) (VP (VBN been) (NP (CD five) (JJ major) (JJ massive) (NNS extinctions)) (SBAR (IN since) (S (NP (NN life)) (VP (VBD began) (PP (IN on) (NP (NN earth)))))))) (. .)))