When Darkness Comes — Robert Swindells

Jealousy and ambition divide a tribe so much so that it splits into two communities - each of whom has to strive to become self-sufficient. Their failure almost leads to their destruction. It's up to the chief's son and the other children of the tribe to overcome the adults' prejudices and attempt reconciliation. When they are threatened by raiders from a stronger, more skilful race, survival is all. But will the tribes be able to reunite in time?