According to cultural anthropologist and ethnographer Raymond C. Kelly, the earliest hunter-gatherer societies of "Homo erectus" population density was probably low enough to avoid armed conflict. The development of the throwing-spear, together with ambush hunting techniques, made potential violence between hunting parties very costly, dictating cooperation and maintenance of low population densities to prevent competition for resources. This behavior may have accelerated the migration out of Africa of "H. erectus" some 1.8 million years ago as a natural consequence of conflict avoidance.