You asked: Wouldnt everyone have been 'Noah's direct descendants' including the sumerians?
No, that would only have been the case if the flood was a global one, which of course science assures us that it could not have been.
You wrote: I fail to see how the reasoning presented here to make noah's account older does not apply equally to any other account.
The Hebrew people were called "Shemites" because they descended primarily from Noah's son Shem. (Today that term is still used in a slightly corrupted form, "Semites," as in "antisemitic.") Noah who lived quite a while after the flood and Shem who was on board the ark would have given their direct descendants first hand eyewitness accounts of the flood. People in neighboring lands would have only heard of the events which took place in the land of Noah in a second hand way, and probably many years after the flood had taken place.
You asked: Are you advancing the idea of a geographically and anthropologically global flood circa 2350 bc? (please say no.)
No, the biblical flood was confined to the land of Noah. However, both Bible chronology and recent tree ring studies indicate that it took place in about 2350 BC.
That our earth has never been completely covered with water since land masses first arose from its primordial global sea has been firmly established by modern science in more ways than I can possibly here begin to mention. For a discussion of this subject matter see Problems with a Global Flood at http://www.talkorigins.org/faqs/faq-noahs-ark.html .
A conservative Christian's typical response to such information is to say that they choose to believe the Word of God over the findings of scientists. This certainly sounds quite noble. And I suppose I would commend them for their stance if such a stance was called for by the Bible itself. But it is not. For a careful study of the flood account in Genesis reveals that the Bible does not tell us that the flood of Noah's day was global. And an examination of the scientific "evidence" presented by Christian fundamentalists in support of a global flood, sea shells on mountain tops and the like, quickly reveals that the presenters of this so-called evidence have a very poor understanding of science. ( By the way, sea shells on mountain tops are the result of earth's plate tectonics causing land masses to slowly rise from the sea over many millions of years. This process is an ongoing occurrence and can be proven by comparing the measured heights of various mountain peaks today to their measured heights just a few years ago.)
That the Bible itself does not tell us that a global flood occurred in Noah's day can be seen from a careful examination of the text. To begin with we do well to keep in mind that the word widely translated as "earth" in the flood narrative, giving the impression that our entire planet was flooded, is often translated elsewhere in the Old Testament as "land." ( In acknowledging this fact, the translators of The New American Standard Bible chose to translate the same Hebrew word as both "land" and "earth" throughout the flood narrative.) We can certainly understand that without our modern means of global communication and global travel ancient peoples must have had a much more limited view of their world than we do today. That being the case, it seems more likely that the flood account in Genesis recounted the story of the whole "land" of Noah being flooded than the whole "earth" being flooded.
But doesn't the Bible's story of the flood say that all the high "mountains" were covered with water? And if that was true, since water seeks its own level, wouldn't that mean the whole earth had to have been flooded? For an answer to such questions we again have to look at the ancient Hebrew language. The ancient Hebrew word which has been widely translated as "mountains" in the flood narrative is translated elsewhere in the Old Testament simply as "hills." You see, the ancient Hebrews had only one word to describe what may have been either a small mound of earth or a Himalayan peak. That being the case, the flood narrative can certainly be understood as telling us that "all the high hills in the land of Noah were covered with water to a depth of about twenty feet." (see Gen. 7:20, 21)
But what about the unmistakably "universal" language used in the account? Doesn't the Bible tell us that God destroyed "all life under the heavens" (Gen. 6:17) during the flood? Yes, it does. But it also tells us that "all nations under heaven" lived in fear because of Joshua's conquest of Canaan. (Deut. 2:25) We are also told that during a famine that occurred at the time of Joseph, "The people of all the earth came to Egypt to buy grain from Joseph." (Gen. 41:57) And it tells us that at the time of Paul the good news of Jesus Christ had been "proclaimed to every creature under heaven." (Col. 1:23) Are we to believe such statements included the nations of people which then lived in North America, South America, China and Australia?
We must remember that the world of the Bible writers was a much smaller world than our world today. Their part of the earth was then for them "the whole world." We should also accept the possibility that Bible writers may, at times, have used larger than life expressions, just as we often do today. We often use figures of speech such as, "This book weighs a ton," or "I'm so hungry I could eat a horse." This common form of speech is called hyperbole. It is certainly possible that it may, at times, also have been used by Bible writers. When we use such exaggerated figures of speech for dramatic impact we are being neither inaccurate nor dishonest. The same can be said for the writers of Scripture.
But why would God have had Noah construct such a large ark if it was intended to carry only Noah, his family, and a collection of animals from his own land? Could it be that Noah was instructed to build an ark big enough to hold every person in the land that was about to be flooded! An ark with room enough for all those who might repent but didn't? We know that "God does not desire any to be destroyed but desires all to attain to repentance." How could Noah be telling a land full of people to repent and get on the ark if that ark had no room for them? God's plan of salvation today has room for everyone on earth, does it not? Should we believe that God's plan of salvation in Noah's day did not?
Another question that is sometimes asked is, "If the flood was confined to the land of Noah, why would God not have simply told Noah to take his family and pairs of animals and flee to higher ground?" Many who believe that the flood of Noah's day, as described in Genesis, was confined to the land of Noah say that the answer to this question can be found in 1 Peter 3:20,21. There we are told that Noah and his family, "were saved through water, and this water symbolizes baptism." So, they say that by choosing to save the lives of Noah and his family as they passed through the waters of the flood, God was symbolically pointing to a time when his people (Christians) would find salvation as they passed through the waters of baptism. There may also be other reasons. The Bible tells us that "Noah was a preacher of righteousness." He may very well have continued urging the residents of his land to repent and accept God's provision for their salvation right up to the day it began to rain. (Gen.7:11-13) If Noah had relocated away from the area that was to be flooded he would have been unable to offer his neighbors a way to escape God's coming judgment nearly as long as he did.
Some have also asked, "Wouldn't a 150 day flood require an enclosed area?"
First, we should keep in mind that Mesopotamia has often been described as a "trough" by geologists because it is "enclosed" by areas of higher elevation on its north, east and west sides. If part of central Mesopotamia suddenly lost elevation due to a meteor impact, which some scientists have recently said may have caused Noah's flood, and tidal waves from the Persian Gulf brought on by that same meteor impact, or another one accompanying it, drowned the land of Noah, as some now understand the epic of Gilgamesh to say, then that three sided "trough" may have temporarily turned into a four sided trough, that is until the land of Noah recovered its previous elevation, and while doing so drained its flood waters back into the Persian Gulf from which they mainly came. Remember, the Bible does not say that it was just the 40 days of rain that were responsible for the flood. It tells us that it was also and probably primarily caused by "waters of the great deep" which "burst forth." (Gen. 7:11) I say, "probably primarily caused" because that is the first cause listed.
I encourage all Christians to investigate the possibility that the Bible does not really teach that the whole earth was flooded at the time of Noah. For I believe that when Christians now promote such a teaching, a teaching which conflicts with all serious scientific evidence, they only succeed in making themselves, and Christianity, look very foolish to very many people.