Abstract
BACKGROUND: At present, there are so many living things on the earth. Most of these organisms have a reproductive strategy called sexual reproduction. Among organisms that reproduce sexually, mammals have an extremely complex and seemingly unnatural method of reproduction, or viviparity. METHODS: As an approach to understanding the nature of viviparity, the author have tried to outline the common life phenomena of embryos, cancers, and parasites based on the literature to date, with internal parasites as the keyword. MAIN FINDINGS: Embryo, cancer, and parasite are constituted as a systemic interaction with the host (mother). Based on these facts, the author proposed the hypothesis that in the case of mammals, "the fetus is essentially harmful to the mother", and that the parasitic fetus grows by skillfully evading the mother's foreign body exclusion mechanism. CONCLUSION: Comparative studies of "embryos", "cancers", and "parasites" as foreign bodies have the potential to produce unexpected discoveries in their respective fields. It is important to consider the evolutionary time axis that the basic structure of our mammalian body arose over 200 million years from the Mesozoic Triassic, the period immediately after the Paleozoic Era, when life on Earth became massively extinct.