Grammaticalization
Grammaticalization is an historical linguistic process whereby regular lexical items shift function (and sometimes form) and become part of the grammar structure.
An example of this in Yahgan would be the change of posture verbs into aspectual markers. Yahgan has a system of verbs which denote the posture of an entity: 'stand' mvni, 'sit' mu:tu:, 'lie' (w)i:a and others (for instance a:gulu: 'fly/jump', kvna 'float' etc.). In normal lexical usage one could say
hai ha-mu:t-ude: 'I sat'. (hai full pronoun first person singular, ha- bound version unmarked for number, -ude: past tense).
sa sa-mvni-de: 'You stood'. (sa full pronoun second singular, sa- bound).
But with the same root in their grammaticalized forms added:
hai ha-mvni-mu:t-ude: 'I stood regularly, or as a rule'.
sa sa-muhr-mvni-de: 'You often sat, or were ready to sit'.
The semantic bases of such usage seems to the degree of bodily contact with the substratum, vigilance, engagement, etc. Flying/jumping means ceasing some activity of interest entirely and going off to do something else rather suddenly, standing implies readiness to do something else as needed but attending to the activity when one can. Sitting is regular involvement in the activity, though not to the exclusion of other things that need doing. Lying (not just on but also within) is deep involvement, almost to the exclusion of other activities (English 'immersed in', being 'wrapped up in', 'in a rut' 'be up to one's neck/ears in', 'in over one's head', 'buried in (as work)' etc. have similar import). Relative height is another way to look at it- we say we are 'over' with something to mean 'done with' (equivalent to the jump/fly term in Yahgan), with a hint of relative dominance implied as well. While English has a plethora of colorful expressions for denoting such circumstances, Yahgan has reduced the system to a well defined smallish set of terms from the domain of posture verbs. Such reduction is one symptom of grammaticalization.
Such contact/engagement-based semantic clines are relatively common crosslinguistically, and the phenomenon of posture verbs changing to aspect marking morphemes is well known among linguists, though it is not the only pathway to creating such terms.
A phonetically based cline (based on both oral articulatory position and manner) can be seen in prefixes y- a- u:- in Yahgan, and combinations ya- and u:a-.
y- denotes an activity begun or intended, but not completed. a- seems to imply continuation, and u:- removal of impediments to the completion of the activity. Combined forms ya- and u:a- appear to accentuate the continuous part of the activity.
In North America, languages of the Siouan and Chemakuan families have similarly structured basic systems (Siouan prefixal, Chemakuan suffixal), but mostly with spatial reference. i- is standing out, away from some surface, a- is surface contact, and u- is containment within a surface, though there are also processual and figure/ground senses involved. Note that standing away minimizes surface contact. Other language families have distance demonstratives which follow similar phonological clines. It is possible that there may be historical connections between the y- form and ki:pa 'woman' and u:- and u:a 'man', which when verbalized apparently refer to less and more forceful or determined attempts to achieve respectively. Verbs ya:na 'to intend, wish' and wa:na 'pass, surpass', as well as u:a- 'do fiercely, forcibly' may be related, with suffixal -na on the first forms.
Horizontal movement verbs commonly change, crosslinguistically, into tense markers. Yahgan shows evidence of such shifts as well.
Read more about this topic: Yaghan Language, Grammar