In chapter 4 of Genesis we begin to get into what many believe to be the genealogy of man from Adam to Abram, a period some have put at 4,000 years, with extreme ages given to the main characters. I believe, instead, that what we are dealing is a tribal geneaology, common in Middle Eastern cultures, where the main person in the family is its’ patriarch, or founding father.*
There were two groupings of people in Middle Eastern society (then and now): those who occupied a single area of land (cities, for lack of a better word) whose primary interest was agricultural; nomads who wandered between the cities and were herdsmen who needed large tracts of land for their animals to graze on, or merchants bringing goods from other peoples. In this type of culture it became important to know who you were dealing with, so these family tribes adopted the name of their patriarch and would frequently keep this name long after his death. We continue this today with corproations named after their founders: Westinghouse, Suzuki, and Cadbury. That way people would know that they are still dealing with the same group (family or tribe). This is why we see the names of people as having extraordinary lifespans: Adam lived 930 years; it wasn’t an individual named Adam who lived this long; rather, it was his tribal descendants who continued identifying themselves with him, long after his death, much like John Smith would identify himself with the Smith family.
Divisions occur within families for many reasons, and this causes one side of the division to separate and claim its’ own identity. In tribal cultures this can occur as well, disagreements between two sides or overgrowing the available resources are just two reasons this might occur. What we see when the Bible speaks of the “sons of …” is a division among the tribe that resulted in a new group breaking off and forming its’ own identity (taking it’s name from it’s founder), while the original group may or may not continue. For a culture where stories were passed along verbally from generation to generation it would be easier to remember that the tribe of Adam lasted 930 years, then passed the torch along to the tribe of Seth, than to remember possibly 40 people between Adam and Seth.
This seems even more plausible when you notice that only major descendents from each patriarch are mentioned, with everyone else being noted as “and…had other sons and daughters”, likely indicating divisions that did not last; only the most prominent, or final, tribes would be remembered. It is not until after Noah (the last of the long-lived patriarchs) that we get more detailed genealogies, and even here the “sons” listed for Noah’s descendants are more a list of Middle Eastern tribes/nations: Cush, Egypt, and Canaan, to list just a few. His three sons (Shem, Ham, and Japheth) are each assigned parts of the old world as being their founders: Shem, the Semitic people of the Middle East; Ham, the Hamites who populated the lands of the south (Africa); and Japheth, the lands north of the Middle East (Europe and Asia).
So, to briefly summarize, the genealogical list from Adam to Noah comprises a list of family tribes, after Noah we see the dissolution of most of the large tribal groups, and the beginning of nations. Family tribes do not completely disappear, Israel will continue with this family-tribe structure until we see the twelve tribes form into two nations: Judah and Israel.
* The Story of Chaldea – Ragozin, Zenaide A.