The following text is reference to question.  

        Thanks to the field of linguistics we know much about the development of the 5,000 plus languages in existence today. We ___(1)___ their grammar and pronunciation and ___(2)___ how their spoken and written forms ___(3)___ over time. ___(4)___, we understand the origins of the Indo-European group of languages, which includes Norwegian, Hindi and English, and can trace them back to tribes in eastern Europe in about 3000 BC. 

        So, we have mapped out a great deal of the history of language, but there are still areas we know little about. Experts are beginning to look to the field of evolutionary biology to ___(5)___ how the human species developed to be able to use language. So far, there are far more questions and half-theories than answers.  

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