Voluntary segregation from the rest of our brothers and sisters around the world. This means borders that other tribes can't cross, arguments and wars over where the border lines and hunting rights are. Population growth causing a tribes need too expand their borders therefore invading another tribes territory. Greed and jealousy if one tribe thinks that the other has greener pastures than they do. Different cultures and beliefs emerging, causing a lack of tolerance towards their different ways of life. Until mankind (man especially) learn that borders and tribalism aren't really necessary and that we can all live together sharing and helping each other, we wont see an end too the tribal, border, religious and racist wars that have plauged the earth for thousands of years.