( ( ( ( ( ( During ( the ( first ( ( 2 billion ) years ) ) ) ) ( , ( ( only ( single ( cell microorganisms ) ) ) existed ) ) ) , ) but ) ( ( once ( ( cells ( with nuclei ) ) ( developed ( ( ( about ( a billion ) ) years ) ago ) ) ) ) ( , ( ( ( increasingly complex ) ( multicellular organisms ) ) evolved ) ) ) ) . )
(ROOT (S (S (PP (IN During) (NP (DT the) (JJ first) (QP (CD 2) (CD billion)) (NNS years))) (, ,) (NP (RB only) (JJ single) (NN cell) (NNS microorganisms)) (VP (VBD existed))) (, ,) (CC but) (S (SBAR (IN once) (S (NP (NP (NNS cells)) (PP (IN with) (NP (NNS nuclei)))) (VP (VBD developed) (ADVP (NP (QP (RB about) (DT a) (CD billion)) (NNS years)) (RB ago))))) (, ,) (NP (ADJP (RB increasingly) (JJ complex)) (NN multicellular) (NNS organisms)) (VP (VBD evolved))) (. .)))
During the first 2 billion years, only single cell microorganisms existed, but once cells with nuclei developed about a billion years ago, increasingly complex multicellular organisms evolved.
(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))))) (. .)))