Tribes are traditionally hyperextended family units, united by blood, language, and culture. Bands within those tribes are basically bloodlines within the larger tribe ("Houses", in Shakespeare-speak.) Neighbouring tribes might have similar beliefs and language, but there will be differences that caused a tribal split sometime back in the distant past. That's how evolution works, those differences are the seeds of new subspecies (races) and eventually, species.
Tribes are traditionally hyperextended family units, united by blood, language, and culture. Bands within those tribes are basically bloodlines within the larger tribe ("Houses", in Shakespeare-speak.) Neighbouring tribes might have similar beliefs and language, but there will be differences that caused a tribal split sometime back in the distant past. That's how evolution works, those differences are the seeds of new subspecies (races) and eventually, species.