( ( For ( thousands ( of years ) ) ) ( ( the ( last mammoths ) ) ( ( have ( remained ( hidden ( from ( ( humans , ) ( who ( believe ( them extinct ) ) ) ) ) ) ) ) . ) ) )
(ROOT (S (PP (IN For) (NP (NP (NNS thousands)) (PP (IN of) (NP (NNS years))))) (NP (DT the) (JJ last) (NNS mammoths)) (VP (VBP have) (VP (VBN remained) (S (VP (VBN hidden) (PP (IN from) (NP (NP (NNS humans)) (, ,) (SBAR (WHNP (WP who)) (S (VP (VBP believe) (S (NP (PRP them)) (ADJP (JJ extinct)))))))))))) (. .)))
For thousands of years the last mammoths have remained hidden from humans, who believe them extinct.
(ROOT (S (NP (JJ Woolly) (NNS mammoths)) (VP (VBD began) (S (VP (TO to) (VP (VB go) (NP (NP (ADJP (JJ extinct)) (NNS thousands)) (PP (IN of) (NP (NNS years)))) (ADVP (RB ago) (PP (IN because) (PP (IN of) (S (VP (VBG hunting) (PP (IN by) (NP (NNS humans)))))))))))) (. .)))
Woolly mammoths began to go extinct thousands of years ago because of hunting by humans.