The parasite-stress theory of sociality, the behavioral immune system, and human social and cognitive uniqueness.
Abstract
Parasite adversity was an important source of Darwinian selection in human evolutionary history because parasites selected for a diversity of human behavioral parasite-defenses in addition to the numerous defenses provided by the classical human immune system. We argue for a broader view of behavioral immunity than has been emphasized recently. We propose a new hypothesis for the evolution of the unique cognitive and social adaptations of humans. We argue that, in human evolutionary history, as weaponry and other technologies reduced the importance of the physical environment, typical ecological challenges, and predators as agents of selection shaping mental and social traits, parasites became more important selection agents on these traits. Also, we suggest that the reduction of natural selection in the context of predation in human evolutionary history resulted in selection favoring high pathogenicity in human parasites because predation is focused on debilitated prey and hence selects against pathogenicity in the parasites involved. The new hypothesis argues that, in human evolutionary history, temporal variation in local parasite adversity gave rise to frequent change and complexity in the values that are adaptive for individuals to use in their social navigation in relation to the degree of local disease stress, and that selection in this context accounts for the evolution of aspects of human uniqueness in cognitive ability and sociality. The new hypothesis is akin to the social brain hypothesis but is contrary to the version of that hypothesis deemphasizing parasites. Tests of the new hypothesis are addressed.