You forgot the indefinite article "a" How is human speech a very important of human endeavor, and why? and you can leave out the why because you already have how in the sentence. That said, to answer your question, Human speech is a very impportant aspect in human endeavor because it allows us to combine our efforts by providing a communication tool and it helps us to learn from the past and thus build on the knowledge of our predecessors. You might also say that there can be no civilization, houses, cities books etc withou language Hope that helps John

Think about it in simple terms. Without an ability to communicate humans are trapped within themselves with little potential for growth. In other words, you can't expect a plant to grow by its own means, it requires outside help and influence. With language (from simple grunts to the grammatical complexities of English, Arabic, Mandarin, Russian, or whatever) you are able to describe what you are feeling internally. Your hopes, fears, ambitions are all things that are internally a part of you- language helps these things escape. Language is mutual.