Where do grammatical morphemes come from? On the development of grammatical markers from lexical expressions, demonstratives, and question words

Research on grammaticalization has shown that grammatical morphemes are commonly derived from content words, notably from nouns and verbs. For instance, adpositions are frequently derived from body part terms (cf. Heine et al. 1991) and future tense auxiliaries are often based on motion verbs (cf. Bybee et al. 1994). Of course, not every grammatical marker can be traced back to a content word; very often there is not sufficient historical data to determine the source of a particular morpheme. But since the development of grammatical markers from nouns and verbs is very frequent, it has become a standard assumption of grammaticalization theory that all grammatical morphemes are eventually derived from a lexical source: