From Edward Montserin:

Re: More men than women in ancient African exodus, geneticists say (Dec. 21): No concept on TRIBES, How they formed, why they formed.

All Animals (human as well) formed, to protect the pack, tribe, etc.

As the tribe became to large to control, the alpha male would disperse the younger males out of the tribe as they were a threat to him. When the younger males formed their own tribe. Tribal war Begun, re sites, areas to be conquered.