Those / who believed (that drama evolved out of ritual) /also argue (that those rites contained the seed of theater) /because music dance,masks,and costumes were almost always used/.
who believed that drama evolved out of ritual 为定语从句,先行词为前面的those
that drama evolved out of ritual为定语从句中的宾语从句
that those rites contained the seed of theater 为主句中的宾语从句,主语是those
because music dance,masks,and costumes were almost always used.是状语从句