Of the four subfields, Linguistics is probably the least recognized. Linguistics is the study of language. The purpose of this study is to discover the origins of language, the structural meaning of languages, the history of a language, the relationships between languages, and the inherent meaning of certain aspects of the spoken and written word. 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 Historical documents are the best source of research for historical linguists.