( It ( ( is ( ( their ( striking resemblance ) ) ( to ( ( the ( single-celled protists ) ) ( called ( choanoflagellates ( that ( make ( ( many scientists ) ( believe ( that ( choanoflagellates ( are ( ( the ( sister group ) ) ( to ( the animals ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (PRP It)) (VP (VBZ is) (NP (NP (PRP$ their) (JJ striking) (NN resemblance)) (PP (TO to) (NP (NP (DT the) (JJ single-celled) (NNS protists)) (VP (VBN called) (NP (NP (NNS choanoflagellates)) (SBAR (WHNP (WDT that)) (S (VP (VBP make) (SBAR (S (NP (JJ many) (NNS scientists)) (VP (VBP believe) (SBAR (IN that) (S (NP (NNS choanoflagellates)) (VP (VBP are) (NP (NP (DT the) (JJ sister) (NN group)) (PP (TO to) (NP (DT the) (NNS animals))))))))))))))))))) (. .)))
It is their striking resemblance to the single-celled protists called choanoflagellates that make many scientists believe that choanoflagellates are the sister group to  the animals .
(ROOT (S (NP (JJ Many) (NNS scientists)) (VP (VBP think) (SBAR (S (NP (NNS protists)) (VP (VBP are) (NP (DT the) (JJS oldest) (NNS eukaryotes)))))) (. .)))