Sentence:  Sir Julian Huxley in his book Uniqueness Of Man makes the novel point that just as man <WSD>is</WSD> unique in being the only animal which requires a long period of infancy and childhood under family protection , so is he the only animal who has a long period after the decline of his procreativity . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)