No one knows how your people came to be. Whispers among some tribes, that you had all come from one massive, super tribe, greater than all of you put together, have suggested greater origins than you could have imagined. Stories are told around campfires that you all came from this tribe as a greater people, that you went to war with one another, and fell. Where once there was many, then there were few. Even worse was the loss of knowledge. Over the generations of your people, you had forgotten so much that you are now nothing compared to your former glory.
Whatever the truth is, one fact remains. You are here. You are all great tribes. The future is bright, and it beckons you like a sweet lover, or as a fire attracts moths to it's torturous demise. Can you survive, great people? Can you once again flourish in military strength, material prosperity, science, arts, culture and religion?
It has been a somewhat easy Winter for your tribes and the Spring begins. Wise men of each tribe have all come up with little ideas to help sweeten your times as a tribe.
Add one bonus to your Tribes that are Technology appropriate. No discovering gun powder or faster than light travel right after you discover how to fashion and shoot with bows and arrows.
One more thing, role play what your tribe is doing at the moment, how are they advancing culturally, technologically, religiously? Etcetera. You have one hundred years.