This chapter examines how environmental variability and language evolution created feedback loops that amplified human behavioural plasticity. First, environmental variability during the Pleistocene reinforced selective pressure favouring human prosociality and intergroup peaceful cooperation, alongside the evolution of cumulative culture and, later, selection against reactive aggression. Rapidly shifting environmental conditions favoured behavioural versatility in the genus Homo as cumulative culture was emerging. This, coupled with the selection against reactive aggression (‘self-domestication’) and the resulting increase in prosociality over the past ~315,000/300,000 years, then enabled Homo sapiens communities to establish increasingly extensive networks of exchange for accessing distant resources, providing a key advantage over other human species in surviving Pleistocene climate fluctuations. Second, Proto-language and later the modern faculty of language interacted closely with cumulative cultural evolution and ‘self-domestication’, amplifying both intergroup peaceful cooperation and lethal conflict and playing a central role in their co-evolution. Language-enabled cumulative cultural evolution further augmented and refined the human capacity for intergroup lethal conflict; it allowed for more effective cultural teaching across generations of the know-how employed for coalitionary killings, underpinned the cumulative evolution of weapon technology, and introduced and then expanded cultural incentives associated with warfare such as status and prestige. At the same time, cultural practices such as inter-tribal marriages, cultural kinship, long-distance exchange networks, and peacemaking and conflict resolution practices between groups fostered larger-scale peaceful cooperation.