Cyber violence is verbal aggression occurring on the Internet that inflicts harm on victims. Prior research mostly considered cyber violence a manifestation of human irrationality. However，drawing on a case study from an evolutionary perspective，we argue that aggression—including violence on and off the Internet—are behaviors that fitted the human ancestral environment and thus ecologically rational. Specifically，in terms of the ultimate cause，aggression exists（i.e.，maintained by natural selection）because it has likely helped individuals resolve conflicts of interest and increased their reproductive success. In terms of the proximate mechanism，aggression is likely based on the aggressor’s computation of welfare trade-offs between the aggressor and the target，with the goal of upregulating the target’s treatment of the aggressor’s welfare or eliminating the target that is a threat to the aggressor’s welfare. Explicating the cause and mechanism of cyber violence has important policy implications and informs the study of other human online behaviors.