Prehistoric warfare refers to war that occurred between societies without recorded history. The existence — and the definition — of war in humanity's hypothetical state of nature has been a controversial topic in the history of ideas at least since Thomas Hobbes in Leviathan (1651) argued a "war of all against all", a view directly challenged by Jean-Jacques Rousseau in a Discourse on Inequality (1755) and The Social Contract (1762). The debate over human nature continues, spanning contemporary anthropology, archaeology, ethnography, history, political science, psychology, primatology, and philosophy in such divergent books as Azar Gat's War in Human Civilization and Raymond C. Kelly's Warless Societies and the Origin of War. For the purposes of this article, "prehistoric war" will be broadly defined as a state of organized lethal aggression between autonomous preliterate communities. Paleolithic According to cultural anthropologist and ethnographer Raymond C. Kelly, population density among the earliest hunter-gatherer societies of Homo erectus was probably low enough to avoid armed conflict. The development of the throwing-spear and ambush hunting techniques required cooperation, which made potential violence between hunting parties very costly. The need to prevent competition for resources by maintenance of low population densities may have accelerated the migration out of Africa of H. erectus some 1.8 million years ago as a natural consequence of conflict avoidance. Hypotheses which suggest that genocidal violence may have caused the extinction of the Neanderthals have been offered by several authors, including Jared Diamond and Ronald Wright. The hypothesis that early humans violently replaced Neanderthals was first proposed by French paleontologist Marcellin Boule (the first person to publish an analysis of a Neanderthal) in 1912. However, several scholars have formed alternative theories as to why the Neanderthals died out, which means there is no clear consensus as to what caused their extinction within the scientific community. Kelly believes that this period of "Paleolithic warlessness" persisted until well after the appearance of Homo sapiens some 315,000 years ago, ending only at the occurrence of economic and social shifts associated with sedentism, when new conditions incentivized organized raiding of settlements. None of the many cave paintings of the Upper Paleolithic depicts people attacking other people explicitly, but there are depictions of human beings pierced with arrows both of the Aurignacian-Périgordian (roughly 30,000 years old) and the early Magdalenian (c. 17,000 years old), possibly representing "spontaneous confrontations over game resources" in which hostile trespassers were killed; however,