When one of the headwaters of a huge river, such as the Mississippi, is declared to be the "source" of the river, you may wonder why, when there are multiple large streams merging at various points along the way, each a major river in its own right. The Arkansas, Tennessee, Ohio, and Missouri Rivers all merge into the Mississippi -- so why is the "source" declared to be a spring which flows down into the Missouri ? Is it geographic distance ? Altitude ? Volume of flow ? Whatever the answer, there's bound to be a certain arbitrariness to the definition of 'source'.
So it is with "mainstream" civilization. Our own historical sources takes us back to Rome, and to Greece before that. The Greeks got their alphabet from the Phoenicians (after older scripts had fallen out of use) and some of our notions of law and civic order came from older Mesopotamian cultures. But along the way we incorporated lots of those things from other cultures -- numerals and, especially profoundly, place notation from India via the Arabs. Paper -- and paper money -- from China, as well as gunpowder, which led to guns, and profoundly shaped the course of Western culture. Rockets as pyrotechnics also came from China, but became real weapons in the hands of an Indian prince and his artisans (metal-bodied rockets, scarcely distinguishable from the "Congreve" rocket, which got the credit). Almost every aspect of knowledge which the Greeks preserved in their surviving writings were continued by the Arabs while the Dark Ages gripped Europe -- especially mathematics and medicine (Europeans continued citing Arab doctors for centuries). Maybe what describes the "mainstream" of civilization is not the place, language, or culture of origin, but the fact that it is OPEN to any rational input, and judges by practical results. RWNJs, especially those of a religious bent (or religiously "bent" ), like to refer loudly and frequently to "Judaeo-Christian" civilization/tradition/culture, (which became "Judaeo-Christian" in their rhetoric only to fend off charges of anti-Jewish bigotry) but it took a lot more than Judaism or Christianity to produce a highly technological, science-informed civilization -- indeed, religion has more often served as a dead weight on the back of progress. Maybe we should call it Western civilization since it's now more of a brand than a descriptive term, and is an inarguably profound influence on every culture today (whether they embrace it or not, they must react to it, and it shapes the world even against their wishes) while still being influenced by those it influences. Maybe someday we will just call it "global" culture, but for now I think many people would find that objectionable, thinking it diminishes their own culture(s), rather than (actually) inviting it into the mix. The one rule for joining this "club" is that you must allow your beliefs to be challenged, which is the antithesis of most RWNJ behavior. So while they are trying to run out in front and wave a baton, they are not the ones leading the parade. It makes them feel good to believe it, though.
Maybe someday we'll make enough progress it will just be known as "The Culture".