RWC,
Rem is right. Paleontologists, anthropologists and archaeologists all assure us that mankind has widely populated our earth for far longer than anyone can possibly stretch the Bible's dating for Noah's flood.
For instance, anthropologists date the first settlement of the Americas by modern men to 15,000 B.P. (Before the Present) and their first settlements in Australia to 35,000 B.P. A simple reading of the Bible's own internal chronological historical records, anchored to a 587 BCE date for Jerusalem's destruction by Babylon, produces a date of about 2350 BCE for Noah's flood.
In an attempt to explain how Noah's flood may not have covered the whole earth, but still destroyed all human life on it, outside of the ark, some Christians have suggested that there may be "gaps" in the post flood genealogies recorded in Genesis chapter 11, and, if there are, they say this would allow us to date the flood much earlier than 4,350 years ago. However, though this explanation may appear to solve the scientific conflicts I have here referred to, it creates other ones which are just as difficult to explain for advocates of an anthropologically universal flood which they supposedly only drowned the land of Noah many thousands of years before the Bible itself seems to clearly indicate Noah's flood occurred.
What scientific problems am I referring to? These. The same scientists who tell us modern man has widely populated the earth for at least several tens of thousands of years also tell us that the things people living at the time of Noah (and their distant ancestors!) were involved in, according to the Bible, did not take place anywhere on earth prior to 10,000 years ago. These things include raising crops, herding animals, forging tools of copper and iron and building cities. (Read Genesis, beginning with chapter 4.)
So, anyone who says that Noah's flood could have destroyed all human life on earth except Noah and his family, at any time in mankind's history, and also maintains that the Bible's historical accounts contained in Genesis chapter 4 and afterwards are accurate, finds himself in a major clear conflict with the findings of modern science. I think that when such a thing happens, any honest Christian will be compelled to consider the possibility that his interpretation of the Genesis account of Noah's flood is in error.
It seems to me that the only reasonable position for a Christian to take on this issue is to understand that the flood of Noah's day, whenever it occurred, was neither geographically nor anthropologically universal.