A naturalistic approach to the study of the relationship between biology and culture is to be meant in relation to a unitary conception of human beings. In this article we argue that an account of such conception needs to appeal to a co-evolutive view of the relations between biology and culture. However, this idea of coevolution, if it does not want to be just commonsense explanation, must make two specific conceptual assumptions: a conception of culture (at least of some of its characteristics) as a form of biological adaptation; an interpretation of cultural beliefs and behaviors as an «ecological niche» in which individuals are immersed and to which they must adapt to survive. The case study that supports the general hypothesis that we make in this article concerns the biocognitive foundations of religious beliefs and behaviors. For by construing these beliefs and behaviors as cultural ecological niches, it is possible to argue that the biocognitive systems that subserve religion can be conceived as biological adaptations due to natural selection.