( Scientists ( ( believe ( there ( have ( ( been ( five ( great ( extinction episodes ) ) ) ) ( since ( life ( first ( ( appeared ( on ( the Earth ) ) ) ( ( ( more ( ( than 3.5 ) billion ) ) years ) ago ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NNS Scientists)) (VP (VBP believe) (SBAR (S (NP (EX there)) (VP (VBP have) (VP (VBN been) (NP (CD five) (JJ great) (NN extinction) (NNS episodes)) (SBAR (IN since) (S (NP (NN life)) (ADVP (RB first)) (VP (VBD appeared) (PP (IN on) (NP (DT the) (NN Earth))) (ADVP (NP (QP (JJR more) (IN than) (CD 3.5) (CD billion)) (NNS years)) (RB ago)))))))))) (. .)))
Scientists believe there have been five great extinction episodes since life first appeared on the Earth more than 3.5 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)))))))) (. .)))