Mammalian pregnancy is a curious life phenomenon. Immunologically, the mechanism of pregnancy is difficult to explain because it involves the coexistence of an external foreign body (the embryo) and the host (the mother) for a period of time. How did mammals acquire the ability to become pregnant in parallel with altered immunity? Sex in the evolution of life and its impact on anthropology are major topics of discussion. In this paper, we outline (1) sex and evolution in mammals after the advent of our direct ancestors (apes) up to humans (i.e., the Cenozoic Quaternary), including anthropological aspects such as the development of the central nervous system; (2) the development of reproductive immunity during the Paleozoic era, when biodiversity developed explosively (and many sexually reproducing organisms have emerged); and (3) the characteristic reproductive strategies of mammals, including humans with the immunological aspects of viviparity. We present an overview of mammalian reproductive immunity, which is a heretical aspect of immunology.
Keywords: Sex; evolution; immune-system; placenta; viviparity.