In the Eurasian region, the definite article appared first in the
Near East and rapidly thereafter Greek also got this feature. Later
Latin/Romance also got it, and then the Germanic languages.
The Slavonic languages have not yet got this feature, except those
bordering to the Greek area.
The southern Uralic Languages also have definite articles, but not
yet Finnish, Lappish and Rstonian.
It is therefore quite logical to think that the idea of using
demonstrative pronouns as a definite article spread by language
contact and through bilingual speakers.
The develpmnet of a system of articles is not very common, even
though it has occured in other areas to. Appart from people in the
middle east, also some Amerindian tribes also got this idea. And
also people in the Pacific Islands.
> You are attributing changes that have taken place over an
> enormous span of time to a single cause that was much more
> localized in time and space; so far you haven't made a case
> that can be taken seriously.
>
> Brian
>