Convergent evolution of placental villi in primates and ungulates: Are some placentas more efficient than others?

The placenta is a fetal organ which connects the fetus to its mother while in the womb. The functions of the placenta are to transfer food from maternal blood to the fetus and clean the fetal blood. The structure of the placenta determines how efficiently it can function. Here, we define placental efficiency as the rate at which the placenta transfers nutrients to the fetus in relation to placental size.
While the mammalian placenta only evolved once, mammalian placentas come in a remarkable array of shapes and sizes. The variety of placental structures is much more pronounced than for any other organ. The large number of different placental types suggests that different species face different requirements in pregnancy. However, we do not understand what these requirements are, and if we do not know what they are we cannot address them. This is important because the requirements which shaped placental structure in the past will also affect human and animal health today. Poor placental function does not just affect the health of the fetus, it affects the health of the mother and of the child throughout its life.
Placentas are categorised based on their overall shape, the structure of the interface with the mother, and the number of tissue layers between the maternal and fetal blood. The diversity of placental structures suggests that the requirements for successful reproduction differ for different species. Otherwise, only one 'most efficient' type of placenta would have evolved.
This project will study placentas that contain finger-like projections called villi. Placental villi evolved independently twice, once in primates (e.g. humans) and once in ungulates (e.g. cows and sheep). In humans, placental villi are in direct contact with maternal blood. By contrast, in cows and sheep, the placental villi are embedded in maternal tissue and have no contact with maternal blood, which means nutrients have to travel further to reach the baby. The common ancestor of...