( Thus ( ( ( the ( life cycle ) ) ( of humans ) ) ( ( ( ( ( ( ( ( begins ( with ( ( the formation ) ( of ( the ( single-celled zygote ) ) ) ) ) ) ( by ( ( the fusion ) ( of ( ( egg and ) sperm ) ) ) ) ) , ) ( ( progresses ( through ( embryo ( , ( birth ( , childhood ) ) ) ) ) ) ( to ( sexual maturity ) ) ) ) , ) and ) ( completes ( ( the circle ) ( when ( ( ( the ( ( egg or ) sperm ) ) ( of ( the individual ) ) ) ( contribute ( to ( ( the formation ) ( of ( a zygote ) ) ) ) ) ) ) ) ) ) . ) ) )
(ROOT (S (ADVP (RB Thus)) (NP (NP (DT the) (NN life) (NN cycle)) (PP (IN of) (NP (NNS humans)))) (VP (VP (VBZ begins) (PP (IN with) (NP (NP (DT the) (NN formation)) (PP (IN of) (NP (DT the) (JJ single-celled) (NN zygote))))) (PP (IN by) (NP (NP (DT the) (NN fusion)) (PP (IN of) (NP (NN egg) (CC and) (NN sperm)))))) (, ,) (VP (VBZ progresses) (PP (IN through) (NP (NN embryo) (, ,) (NN birth) (, ,) (NN childhood))) (PP (TO to) (NP (JJ sexual) (NN maturity)))) (, ,) (CC and) (VP (VBZ completes) (NP (NP (DT the) (NN circle)) (SBAR (WHADVP (WRB when)) (S (NP (NP (DT the) (NN egg) (CC or) (NN sperm)) (PP (IN of) (NP (DT the) (NN individual)))) (VP (VBP contribute) (PP (TO to) (NP (NP (DT the) (NN formation)) (PP (IN of) (NP (DT a) (NN zygote))))))))))) (. .)))
Thus the life cycle of humans begins with the formation of the single-celled zygote by the fusion of egg and sperm, progresses through embryo, birth, childhood to sexual maturity, and completes the circle when the egg or sperm of the individual contribute to the formation of a zygote.
(ROOT (S (NP (NP (DT The) (VBN fertilized) (NN egg)) (SBAR (WHNP (WDT that)) (S (ADVP (RB sexually)) (VP (VBG reproducing) (S (NP (NNS organisms)) (VP (VB begin) (NP (NN life)) (SBAR (IN as) (S (VP (VBZ is) (VP (VBN known) (PP (IN as) (NP (DT a) (-LRB- -LRB-) (NN n) (-RRB- -RRB-))))))))))))) (VP (VBP zygote)) (. .)))