Origins of warfare

The question of when, why, and under what circumstances human beings began killing one another in organized groups is among the most debated in anthropology, archaeology, and evolutionary biology. Warfare — defined here as coordinated lethal violence between groups rather than individual acts of homicide — leaves distinctive traces in the archaeological record: mass graves with embedded projectile points, defensive fortifications, skeletal trauma patterns consistent with weapons, and artistic depictions of combat.^{1, 8} Whether these traces extend continuously into the deep human past or appear only with the emergence of settled, resource-rich communities in the Holocene has profound implications for how we understand human nature, the evolution of cooperation, and the conditions under which peace is possible.^{3, 6}

Archaeological evidence for prehistoric violence

The most compelling archaeological evidence for organized intergroup violence in prehistory comes from two sites that predate the Neolithic revolution and the emergence of sedentary agricultural societies. The cemetery at Jebel Sahaba, located on the east bank of the Nile in modern Sudan, was excavated by Fred Wendorf in the 1960s and dates to approximately 13,000 to 14,000 years before present. Of the 61 individuals buried there, at least 24 (roughly 40 percent) bore evidence of violent death, including embedded stone projectile points in or adjacent to skeletal remains and healed injuries indicating prior episodes of violence.^{5, 9} A 2021 reanalysis by Crevecoeur and colleagues using micro-CT scanning and microscopic examination revealed previously undetected lesions, increasing the number of individuals with evidence of trauma and demonstrating that the violence was not a single massacre but recurrent episodes of intergroup conflict occurring over an extended period.⁹ The victims included men, women, and children of all ages, a pattern more consistent with indiscriminate raiding than with ritualized combat between warriors.

The second landmark site is Nataruk, located west of Lake Turkana in Kenya, where Mirazon Lahr and colleagues discovered the remains of at least 27 individuals dating to approximately 10,000 years before present. The skeletal remains exhibited extensive perimortem trauma including blunt-force injuries to the skull, fractured hands and knees consistent with being bound, and obsidian projectile points embedded in cranial and thoracic bones.⁴ The bodies were found in positions suggesting they had fallen where they died, with no evidence of deliberate burial. Mirazon Lahr interpreted the site as the aftermath of a raid on a small foraging group by an attacking party, noting that the presence of obsidian projectiles — a material not locally available — suggested that the attackers came from a different territory.⁴ The Nataruk evidence is significant because it demonstrates organized lethal violence among mobile hunter-gatherers who had not adopted agriculture, undermining the argument that warfare emerged only with sedentism and the accumulation of storable resources.

Beyond these two sites, evidence for pre-Neolithic violence is scattered but persistent. Skeletal trauma consistent with interpersonal violence appears in Mesolithic cemeteries across Europe, including sites in Portugal, Scandinavia, and the Danube Gorges, though distinguishing evidence of warfare from evidence of individual homicide, execution, or accidental injury remains methodologically challenging.¹⁴ Eshed and colleagues documented increasing frequencies of cranial trauma in Levantine populations through the Neolithic transition, suggesting that rates of interpersonal violence rose with sedentism and population density.¹⁵

The deep-roots hypothesis

The deep-roots hypothesis holds that coalitional lethal violence is an evolved feature of human behavior with origins extending back into the shared ancestry of humans and other great apes. Its most prominent advocate, Richard Wrangham, drew explicit parallels between lethal intergroup raiding observed in wild chimpanzee populations and patterns of small-scale warfare in human hunter-gatherer societies, arguing that both reflect a common adaptive strategy favored by natural selection under conditions where groups compete for territory and resources.^{2, 10}

Chimpanzees engage in lethal coalitional aggression in which groups of males from one community deliberately seek out and kill isolated individuals from neighboring communities. A comprehensive study by Wilson and colleagues, analyzing data from 18 chimpanzee communities observed over a combined 426 years, found that lethal aggression was a pervasive feature of chimpanzee behavior across diverse habitats and levels of human disturbance, undermining the alternative hypothesis that chimpanzee violence is an artifact of anthropogenic habitat compression or provisioning.⁷ The frequency of lethal attacks was best predicted by factors such as the number of adult males in the attacking party and the population density of neighboring communities, consistent with a strategic model in which coalitional killing serves to reduce competitors and expand access to food and mates.⁷

Wrangham argued that the parallels between chimpanzee lethal raiding and human small-scale warfare — both involving predominantly male coalitions, both targeting vulnerable individuals from out-groups, both motivated by competitive advantage — are unlikely to be coincidental and instead reflect a behavioral disposition inherited from the last common ancestor of humans and chimpanzees approximately six to seven million years ago.^{2, 10} Samuel Bowles extended this argument with a mathematical model demonstrating that if intergroup conflict was sufficiently lethal among ancestral human populations, it could have driven the evolution of altruistic behaviors directed toward in-group members, since groups composed of individuals willing to cooperate and fight on behalf of their community would outcompete less cohesive groups.⁶ In this framework, warfare and cooperation are not opposites but co-evolved traits — the capacity for extreme in-group solidarity and extreme out-group hostility developing as two sides of the same selective coin.^{6, 11}

The shallow-roots hypothesis

The shallow-roots or “invention” hypothesis counters that warfare is a relatively recent cultural phenomenon, not an evolved biological predisposition. Its leading proponent, Douglas Fry, has argued that the ethnographic record of mobile hunter-gatherer societies — the closest living analogues to ancestral human populations — reveals that most such societies lack institutionalized warfare and resolve conflicts through avoidance, mediation, or individual duels rather than organized group combat.^{3, 12} Fry distinguished between nomadic foragers, who rarely engage in true warfare, and complex or sedentary hunter-gatherers such as the Pacific Northwest Coast peoples, who did practice warfare but whose socioeconomic organization was more comparable to agricultural societies than to mobile foraging bands.³

R. Brian Ferguson went further, arguing that much of the ethnographic evidence for warfare among indigenous peoples was contaminated by the destabilizing effects of contact with expanding state societies — the “tribal zone” effect, in which colonial disruption of territories, trade networks, and political structures created conditions for violence that would not have existed in the pre-contact era.¹⁶ Ferguson challenged the chimpanzee analogy on similar grounds, arguing that lethal aggression in wild chimpanzee populations might be amplified by habitat destruction, human provisioning, and other anthropogenic stressors, though the Wilson et al. 2014 study subsequently weakened this argument by demonstrating that rates of chimpanzee lethal aggression were not correlated with measures of human impact.^{7, 16}

The shallow-roots position does not deny that prehistoric violence occurred, but maintains that organized, recurrent intergroup warfare emerged primarily with the Neolithic transition, when sedentism, food storage, territorial ownership, and population growth created conditions favoring resource competition and the defense of accumulated wealth. In this view, the capacity for violence is part of human behavioral flexibility, but the institutionalization of that capacity into warfare is a cultural development contingent on specific material conditions rather than an inevitable expression of evolved psychology.^{3, 12}

Fortifications and the intensification of warfare

Whatever the depth of warfare’s roots, the archaeological record demonstrates an unmistakable intensification of organized conflict with the onset of sedentism and agriculture. Defensive fortifications — walls, ditches, palisades, and towers — appear at some of the earliest permanent settlements, including the Pre-Pottery Neolithic site of Jericho (c. 9000 BCE), where a massive stone tower and encircling wall have been debated as either defensive structures or communal ritual monuments.^{1, 13} By the 6th and 5th millennia BCE, fortified settlements are widespread across Europe, the Near East, and China, and the archaeological signature of warfare — weapon hoards, trophy skulls, burned villages, mass graves — becomes increasingly common and unambiguous.^{1, 8}

Keeley’s landmark synthesis demonstrated that the frequency of warfare among pre-state societies documented in the ethnographic and archaeological record was far higher than many scholars had assumed, with annual death rates from warfare in some tribal societies exceeding those of the most destructive state-level conflicts of the 20th century when calculated as a percentage of population.¹ This finding challenged romantic notions of a peaceful pre-state past and supported the conclusion that organized violence, once established as a behavioral pattern, could be devastating in its consequences regardless of political complexity. Keeley argued that the perception of pre-state warfare as trivial or “primitive” stemmed from a systematic underestimation of its lethality by Western scholars influenced by assumptions about the progressive nature of violence.¹

State-level warfare and its transformation

The emergence of state-level societies after approximately 3000 BCE transformed the scale, organization, and character of warfare without changing its fundamental nature as coordinated intergroup violence. States introduced standing armies, military hierarchies, siege warfare, fortification engineering, and logistical systems for projecting force over long distances — capabilities that magnified the destructive potential of conflict by orders of magnitude.^{1, 13} The earliest textual and artistic records of state-level warfare come from Mesopotamia, Egypt, and China, where palatial archives, commemorative stelae, and tomb paintings document campaigns of conquest, the subjugation of neighboring polities, and the ritualized display of military victories.⁸

Richerson and Boyd proposed that the evolution of large-scale cooperative institutions — including armies, bureaucracies, and systems of taxation — was itself driven by intergroup competition, with cultural group selection favoring societies whose institutions enabled more effective collective action, including military coordination.¹¹ In this framework, the institutions of the state are not simply instruments of elite domination but evolved responses to the competitive pressures of intergroup conflict, refined through millennia of cultural evolution in which more effectively organized societies displaced or absorbed less effectively organized neighbors.¹¹

The origins of warfare ultimately resist simple answers. The archaeological evidence demonstrates that lethal intergroup violence predates agriculture, sedentism, and state formation by thousands of years, supporting some version of the deep-roots hypothesis. Yet the intensity, frequency, and institutional elaboration of warfare clearly increased with sedentism, resource accumulation, and political complexity, supporting the shallow-roots insight that material conditions powerfully shape the expression of violent conflict.^{1, 3, 6} The human capacity for coalitional violence appears to be ancient, rooted in primate behavioral ecology and the competitive dynamics of small-scale foraging societies. The scale and institutionalization of that violence, however, are products of the cultural and economic transformations of the past ten to twelve thousand years — transformations that turned an occasional and costly behavioral option into a recurrent and defining feature of human political life.^{6, 11}