( ( ( The ( first ( life forms ) ) ) ( -LRB- ( ( ( ( about ( 3 billion ) ) years ) ago ) -RRB- ) ) ) ( ( may ( have ( been ( bacteria ( which ( were ( able ( to ( live ( on ( ( methane and ) ammonia ) ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NP (DT The) (JJ first) (NN life) (NNS forms)) (PRN (-LRB- -LRB-) (ADVP (NP (QP (RB about) (CD 3) (CD billion)) (NNS years)) (RB ago)) (-RRB- -RRB-))) (VP (MD may) (VP (VB have) (VP (VBN been) (NP (NP (NNS bacteria)) (SBAR (WHNP (WDT which)) (S (VP (VBD were) (ADJP (JJ able) (S (VP (TO to) (VP (VB live) (PP (IN on) (NP (NN methane) (CC and) (NN ammonia)))))))))))))) (. .)))
The first life forms (about 3 billion years ago ) may have been bacteria which were able to live on methane and ammonia .
(ROOT (S (NP (DT The) (JJ first) (VBG living) (NNS cells)) (VP (MD may) (VP (VB have) (VP (VBN evolved) (ADVP (NP (QP (RB around) (CD 4) (CD billion)) (NNS years)) (RB ago))))) (. .)))