I think what is linear is the medium of civilization not necessarily the people who later came to benefit from it.
Well, while this is true for "History", I mean the written one, because we have trace of transmission
culture by writing, which "writing" give explanation about living systems, laws, construct buildings such as
houses, temples etc, religions and enlargement of population, when we go more back and back through millennia,
this linear approach is seriously difficult and subject to many and many speculation due to "lack" of Beyond
any doubts findings, as well as the a pproach and interpretation of who found reperts, dated them etc etc.
During the last 20 / 30 years many findings come to the light and new phisical methods were refined,which
start to rewrite what what believed till middle of previous century, the "linear system and classification"
which were applied before, show its weakness.
WHo new about "Denisovan" more than 30 years ago ? Who know that Nearderthals were more and more skilled
than what nowadays we realise they were ?
Archaeological classifications where / still are based only and esclusiveli on findings, and that lead towatrd linear
classification and issue some "interpretation".... till new findings come to the light which often
crashed the "linear clasification system".
Not only, the Mitochondrial DNA tracing and findings start to brought new not tough light on migrations,
as long as well that of climtic changes which occours.
Then the "linear" time line applied by archaeologyst must be reviewed and revisited posing to ourself,
the Historical Researchers, the obbligation to apply the method of the " 5 W ", in many details and
question: macro and micro geo-phisycal condition, and macro and micro climatic events and changements,
which brought us a "new way to consider" findings.
Specifically in the question of Ubaid culture, one question is important:
From South Iraq going to North Iraq Ubaid people (Who) had to walk a long distance in a not very comfortable land,
an alluvional plan, or to boating against a certainly strong river stream(at that time) to the North:
Why ? Where it appened ? For What purposes they did it ? When ?
Asnwer to such " 5 W" question will lead toward what most probably went on.
The same questio which you try to answer about Ubaid is anologue to: just for a mere example of course, among many of such analogue question around the world, which still nowadays have not a satisfied answer yet:
Crete - from where come the Minoans ? - From where come the "Miceneans"...... still an open question
to which the answer, if you examine the precendt historic and archaeologic findings, could be only that
such "Cultures" were the result of mixing cultures which during flows of time lead toward the identifiable
ones, thank to "writings", inst'it ?
Etruscan: is a true example of that, if you carefully examine them with the approaching methods I said.
Of course Lee that are just my prsonal opinion (and NOT inside the mainstream) toughs of mine.
You can not solve a problem with the same kind of thinking that you used to create it.