One argument is that language could not have evolved until humans were fully modern looking and equipped with proper mouth and throat anatomy. Structural linguists study the rules of language. These rules do not just involve formal grammar, but also the unconscious rules of language that permeate a population. Linguists use specialized symbols in order to transcribe language. It is through this transcription that specific aspects about the structure of the language can be dissected.
Historical linguists study the evolution of languages.
Historical documents are the best source of research for historical linguists. The goal of this type of linguist is to put languages into language families and be able to trace their origins as far back as possible. The English language, for example, belongs to the Indo-European family. It is even possible for a historical linguist to reconstruct a language that does not exist in writing. These reconstructed languages are called protolanguages .