If we are to talk about the boundary between linguisticity and non-linguisticity then we must include some remarks about grammaticalization because the study of it can provide a more fine-grained boundary.
Grammaticalization is part of a physical theory of language - and so it is part of our theory as well - as it describes the evolution and emergences of several features in language. Grammaticalization is the study of emergent features - changes in construction to be understood as structural or organizational - that we categorize as grammatical relative to non-grammatical forms[44]. Opinions vary as to what should be included in a definition of grammaticalization. For the purpose of our research, grammaticalization will refer to the emergence of syntax in language.
Traditional inquiry in the field tends to describe grammaticalization as the evolution of morphemes by phonemic transformation [44]; a somewhat unidirectional process of the erosion of lexical uses through the loss of phonemic parts. Early lexical vocabulary undergoes reduction - shortening of expressions - and relies increasingly on affixes - at the end of vocables - to produce effects in linguistic interactions. Others, like Hurford [30], argue against the unidirectional trend towards increasingly abstracted forms for lexical vocabulary. There are examples that show re-lexicalization, that is, vocabulary that migrate into a different category, hence regaining an independent status, through affixes mutation [22].
The processes by which grammaticalization occurs are, as of yet, not transparent. However, most linguists will agree that all dynamics leading to grammaticalization should be included in this area of research[23].
The description of grammaticalization phenomena may be extended to include the dynamics that describe the emergence of linguistic forms from pre-linguistic forms. This is not an obvious extension because linguists do not all agree on the nature of the evolutionary process by which our linguisticity was produced. Changes in vocabulary may indicate a kind of evolution. However it may be that the source of these changes is the consequence of the fine tuning of a Language Acquisition Device (LAD). This device would be genetically programmed and would not be subject to evolutionary forces[30].
So far, grammaticalization theory has concentrated mostly on the changes in lexical linguistic forms and meanings, and the subsequent formal and semantic changes that such material undergoes over time. These kinds of changes are observable in all human languages.