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Comparative studies of placentation across mammals have been greatly facilitated by the availability of phylogenetic trees for the entire range of placental mammals based on molecular data. The three basic placenta types defined by Grosser on grounds of increasing invasiveness epitheliochorial, endotheliochorial, and hemochorial have routinely served as a basis for comparative discussions. Primates differ starkly from all other placental mammal orders because the two extreme kinds of placentation are represented, while the moderately invasive is not. Reliable inference of the primitive condition is an essential starting-point for successful reconstruction of the evolution of placentation in placental mammals. Having considered the general background to evolution of the placenta in primates, it is now possible to focus specifically on catarrhines Old World monkeys, apes, and humans. The architecture of the placenta is notably different in New World monkeys (platyrrhines), which diverged from catarrhines at least 40 mya.