Check out "War in the Tribal Zone". It's a collection of papers from the 90s that partially address this question. The central thesis that ties them together is that the "tribe" is an organizational structure that people who don't have much formal hierarchy tend to adopt when confronted with organized imperialistic states. For instance, if the state wants to trade, the state tends to expect one representative who speaks for many people. This position might not exist in a society, but often they'll create that position, or someone will opportunistically claim it, to trade in desirable goods.

In another case a society might have to resist and imperial power militarily and find it necessary to have more structured military, political, and logistical organizations in order to conduct war and diplomacy.

In this framework a tribe is an organizational system that arises in societies that have little formal hierarchy, government, or differentied social and political roles in order to better interact with highly structured and hierarchical states on their borders.