( Thus ( ( ( the ( four-chambered heart ) ) ( of ( the ( ( bird and ) mammal ) ) ) ) ( ( ( is not ) ( a ( sudden ( and ( inexplicable development ) ) ) ) ) . ) ) )
(ROOT (S (ADVP (RB Thus)) (NP (NP (DT the) (JJ four-chambered) (NN heart)) (PP (IN of) (NP (DT the) (NN bird) (CC and) (NN mammal)))) (VP (VBZ is) (RB not) (NP (DT a) (JJ sudden) (CC and) (JJ inexplicable) (NN development))) (. .)))
Thus the four-chambered heart of the bird and mammal is not a sudden and inexplicable development.
(ROOT (S (PP (IN Like) (NP (NP (DT all) (NNS tetrapods)) (PP (IN except) (NP (NNS birds) (CC and) (NNS mammals))))) (, ,) (NP (NNS frogs)) (VP (VBP possess) (NP (NP (DT a) (NN heart)) (PP (IN with) (NP (CD three) (NNS chambers))))) (. .)))