Most scientists believe the evolution of humans has a history nearly as long as life itself. Anatomically modern humans and all other life that has existed on the planet first came about from the single-celled microorganisms that emerged approximately four billion years ago. Through the processes of mutation and natural selection, all forms of life developed, and this continuous lineage of life makes it difficult to say precisely when one species completely separates from another. In other words, scientists still debate when a human became a human rather than the ancestor species that came before.

Among paleontologists, the question of the human propensity for warfare in prehistoric eons has persisted. Primitive conflict that in time grew into a modern military phenomenon has become an increasing avenue of study. Scientists seek to ascertain whether the distant ancestral line of humans is genetically disposed toward the act of war, or whether social and geographical development have created a circumstantial environment for large-scale societal collisions.

Thomas Hobbes, author of Leviathan (1651), described the Paleolithic world view as a “war against all”. This notion was challenged by Jean-Jacques Rousseau’s A Discourse on Inequality (1755) and The Social Contract (1762). Raymond C. Kelly, an anthropologist and ethnologist who has written extensively on societal inequality and subsequent warfare, suggests that among the hunter-gatherer groups of Homo erectus, the population density was low enough to avoid armed conflict in most cases. In the same vein, a perception has persisted that during this less populated time of Earth’s history, life among the Homo species was relatively peaceful. Archaeologists have supported this theory through early cave art, little of which ever depicts humans hunting or killing each other explicitly. Kelly theorized that the migration out of Africa by Homo erectus 1.8 million years ago was “a natural consequence of conflict avoidance”. He believes that this general period of “Paleolithic warlessness” was to persist until the appearance of Homo sapiens approximately 350,000 years ago, and that it began with the occurrence of “economic and social shifts associated with sedentism”.

However, depictions of humans pierced with arrows began to appear in the Aurignacian-Périgordian eras (30,000 years ago), and in the early Magdalenian era (17,000 years ago). A work of Mesolithic art (20,000 to 10,000 years ago) shows an explicit battle between groups of archers, and in Valencia, a group of three archers are seen surrounded by four of the similarly armed enemy in the Cova del Roure la Vella in Castellón. In the Ares del Maestrat in Alcañiz of Aragon, another work depicts warriors fleeing a group of eight archers, while a similar work at Val del Charco del Agua in Aragon shows seven archers with plumed headgear. Other examples show warriors in lines and columns with a “distinctly garbed leader at the front”. In a sense, it’s entirely reasonable to believe that the earliest people were violent and engaged in warfare the same way people did throughout more recent eras".