Evolution of the mammalian placenta revealed by phylogenetic analysis.

The placenta is essential for the success of therian mammalian reproduction. Intense selective pressure has shaped changes in placental anatomy and function during mammalian cladogenesis. Here we challenge the view that the hemochorial placenta is a derived feature in haplorhine primates. Using phylogenetic and statistical analyses of molecular and morphological data, we demonstrate that the ancestral eutherian mammalian placenta had the distinctive features of (i) hemochorial placental interface, (ii) a discoid shape, and (iii) a labyrinthine maternofetal interdigitation. These results reveal that the first eutherians had a deeply invasive placenta and imply that the major role of the placenta in sustaining pregnancy and promoting gestational development existed throughout the eutherian lineage that descended to humans from the last common ancestor of placental mammals. The ancestral state reconstructions demonstrate both clade-specific patterns of placentation and specific cases of convergent evolution within individual eutherian clades. Determining the mammalian pattern of change in placental morphology is important for understanding the evolutionary pressures faced by these lineages. The effects of selection pressures on the efficiency of placentation may stem from changes in nutritional demand, gestational length, number of embryos per pregnancy, uterine shape, and maternal body constitution. The influence of these factors on placental development needs further investigation.