Paleoanthropologists have long asked questions about the social system of extinct hominins. Primatology’s ability to answer these questions has been notoriously limited. Here, we use comparative data on extant nonhuman primates to reconstruct the social system of Homo erectus, based on the fact that this species lived on the open savanna, despite the presence of various large carnivores, and ate the meat of large vertebrate prey. First, we show that savanna-living species form larger groups than forest-living species, but only if compared with arboreal ones. Second, we show that such species differ in that males confront (counter-attack) predators instead of groups simply fleeing and mobbing. Based on a review of primate observations, we infer that the feasibility of confrontation depends on the predator-prey size ratio, effectiveness of weaponry, and number of males. We therefore conclude that H. erectus lived in very large, and often cohesive, groups, presumably with endogamy of both sexes and male-female friendships. Confrontational defense easily leads to the new lifestyle of confrontational scavenging. Extant primate data can therefore contribute to reconstructing hominin lifestyles.