( They ( ( are ( part ( of ( the endoskeleton ) ) ) ) . ) )
(ROOT (S (NP (PRP They)) (VP (VBP are) (NP (NP (NN part)) (PP (IN of) (NP (DT the) (NN endoskeleton))))) (. .)))
They are part  of the endoskeleton.
(ROOT (S (NP (NNS Humans)) (VP (VBP possess) (NP (DT a) (PRN (-LRB- -LRB-) (NP (NN n)) (-RRB- -RRB-)) (NN endoskeleton))) (. .)))