( ( ( ( ( Caecilians ( evolved ( from ( a ( tetrapod ancestor ) ) ) ) ) , ) but ) ( they ( ( lost ( their legs ) ) ( during ( ( the course ) ( of ( their evolution ) ) ) ) ) ) ) . )
(ROOT (S (S (NP (NNS Caecilians)) (VP (VBD evolved) (PP (IN from) (NP (DT a) (JJ tetrapod) (NN ancestor))))) (, ,) (CC but) (S (NP (PRP they)) (VP (VBD lost) (NP (PRP$ their) (NNS legs)) (PP (IN during) (NP (NP (DT the) (NN course)) (PP (IN of) (NP (PRP$ their) (NN evolution))))))) (. .)))
Caecilians evolved from a tetrapod ancestor, but they lost their legs during  the course of their evolution.
(ROOT (S (NP (NNS Caecillians)) (VP (VBD evolved) (PP (IN from) (NP (JJ tetrapod) (NNS ancestors)))) (. .)))