Much of what was discussed in this thread is mentioned in this thread (minus pointless bickering):
Scouts being sent out to discover new lands, new tribes, new cities, new resources
Different sites having different resources, and therefore facilitating trade
Migration of peoples from village to village, city to city
Trade and migration leading to ideas and knowledge spreading
The one about exogamy is a new one though, and an interesting one. I guess any nearby tribe that you have contact with, has a chance of wanting their tribesmember to marry your tribesmember… unless they have a culture that doesn’t allow that (exogamy=no)…?