Just a mere hypothesis...
Doesn't *es- "to be" include the
same *-s- that is the suffix found in sigmatic aorist and future? Than the
primary root *e- can be compared with a plenty of similar Nostratic forms, e.g.
Turkic e- (e-di/ e-r-di "was" etc.), Mongolic a- (a-mui "is"
etc.), Finno-Ugric *e-/*o- (Komi e-m "there is", Fin. o-n < *o-m
"is").
Since the function of *-s- in IE sigmatic
aorist and future seems to transform continuative/iterative verbal stems into
momentative/inchoative ones, we may suppose that the primary copula *e- "to
be", when used with the *-s-, had to mean "to become", but
afterwards the former could disappeare, and than the latter began to be used
instead of it...
This, of course, can explain, why in many IE
branches (the most of them!) *es- has no future or/and aorist forms, using
instead something like *bhu:-, primarily meaning "to grow". Cf. Rus.
budu, Lith. busiu "I shall be", Pers. budam, Lat. fui "I
was" etc. - everywhere present forms being from *es-. And even if the
future and aorist forms of the root *es- do exist, they tend to be not sigmatic.
This we see, for example, in Latin, where the future ero: < *eso: is thematic
vs. athematic present, and in Greek, where the future esomai is thematic and
middle vs. athematic and active present.
Any objections?
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Vadim Ponaryadov