The Effects of Extra-Somatic Weapons on the Evolution of Human Cooperation towards Non-Kin

Human cooperation and altruism towards non-kin is a major evolutionary puzzle, as is ‘strong reciprocity’ where no present or future rewards accrue to the co-operator/altruist. Here, we test the hypothesis that the development of extra-somatic weapons could have influenced the evolution of human cooperative behaviour, thus providing a new explanation for these two puzzles. Widespread weapons use could have made disputes within hominin groups far more lethal and also equalized power between individuals. In such a cultural niche non-cooperators might well have become involved in such lethal disputes at a higher frequency than cooperators, thereby increasing the relative fitness of genes associated with cooperative behaviour. We employ two versions of the evolutionary Iterated Prisoner's Dilemma (IPD) model – one where weapons use is simulated and one where it is not. We then measured the performance of 25 IPD strategies to evaluate the effects of weapons use on them. We found that cooperative strategies performed significantly better, and non-cooperative strategies significantly worse, under simulated weapons use. Importantly, the performance of an ‘Always Cooperate’ IPD strategy, equivalent to that of ‘strong reciprocity’, improved significantly more than that of all other cooperative strategies. We conclude that the development of extra-somatic weapons throws new light on the evolution of human altruistic and cooperative behaviour, and particularly ‘strong reciprocity’. The notion that distinctively human altruism and cooperation could have been an adaptive trait in a past environment that is no longer evident in the modern world provides a novel addition to theory that seeks to account for this major evolutionary puzzle.人类对非亲属的合作与利他行为，以及“强互惠（strong reciprocity）”——即合作者与利他者无法获得当下或未来回报的行为模式——均为演化生物学领域的重大未解谜题。本文旨在检验下述假说：体外武器（extra-somatic weapons）的发展或对人类合作行为的演化产生了影响，从而为这两大谜题提供全新解释。普遍的武器使用场景，既可能加剧古人类群体内部争端的致命性，也能平衡个体间的权力差距。在这类文化生态位中，非合作者卷入致命争端的频率大概率高于合作者，进而提升了与合作行为相关的基因的相对适合度。我们采用了两种版本的演化型迭代囚徒困境（Iterated Prisoner's Dilemma, IPD）模型：其一模拟了武器使用场景，另一则未引入该设定。随后我们测试了25种IPD策略的表现，以评估武器使用对各类策略的影响。研究结果显示，在模拟武器使用的场景下，合作型策略的表现显著更优，而非合作型策略的表现则显著更差。尤为重要的是，与强互惠行为逻辑一致的"Always Cooperate"（始终合作）IPD策略，其表现提升幅度显著高于其他所有合作型策略。综上，我们认为体外武器的演化发展为人类利他与合作行为的演化，尤其是强互惠行为的起源，提供了新的阐释视角。“人类独有的利他与合作行为或许曾是过往环境中的适应性性状，但这一效应在现代社会中已不复存在”这一观点，为解释上述重大演化谜题的相关理论体系提供了全新的补充维度。