Past simple should be used because you are talking about a finished action in the past. So you have to use developed in that context. As for the second question, both are possible with different meaning.
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pokhI really get confused in choosing present perfect over simple past in the sentences as aboveOof! Centuries ago! ~ Historical facts. ~ Simple past.
pokhWhy is above sentence correct?One of the clues is for thousands of years. The author wishes to imply that these sign languages are still used, or (perhaps) have only recently fallen into disuse.