( Cambrian ( ( is ( ( the name ) ( of ( ( a Period ) ( that ( marks ( ( ( ( the beginning ) ( of ( the ( Paleozoic Era ) ) ) ) , ) ( currently ( ( estimated ( ( ( ( at 570 ) million ) years ) ago ) ) ( -LRB- ( ( if ( you ( accept ( ( the estimates ) ( of ( ( geologists and ) geochemists ) ) ) ) ) ) -RRB- ) ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NNP Cambrian)) (VP (VBZ is) (NP (NP (DT the) (NN name)) (PP (IN of) (NP (NP (DT a) (NN Period)) (SBAR (WHNP (WDT that)) (S (VP (VBZ marks) (NP (NP (DT the) (NN beginning)) (PP (IN of) (NP (DT the) (JJ Paleozoic) (NN Era))) (, ,) (RRC (ADVP (RB currently)) (VP (VBN estimated) (ADVP (NP (QP (IN at) (CD 570) (CD million)) (NNS years)) (RB ago)) (PRN (-LRB- -LRB-) (SBAR (IN if) (S (NP (PRP you)) (VP (VBP accept) (NP (NP (DT the) (NNS estimates)) (PP (IN of) (NP (NNS geologists) (CC and) (NNS geochemists))))))) (-RRB- -RRB-)))))))))))) (. .)))
Cambrian is the name of a Period that marks the beginning of the Paleozoic Era, currently estimated at 570 million years ago (if you accept the estimates of geologists and geochemists).
(ROOT (S (NP (NP (DT The) (JJ first) (NN period)) (PP (IN of) (NP (DT the) (JJ paleozoic) (NN era)))) (VP (VBD was) (VP (VBN called) (NP (NN cambrian)))) (. .)))