The origin of platelets enabled the evolution of eutherian placentation

Abstract

Invasive placentation with extended pregnancy is a shared derived characteristic unique to eutherian mammals that possess a highly effective system of haemostasis, platelets. These are found in all mammals but no other group of animals. We propose that platelets and megakaryocytes (large polyploid nucleated bone marrow cells that produce platelets) evolved from an ancestral 2 N thrombocyte by polyploidization and that the possession of platelets enabled the evolution of invasive placentation. This could explain why invasive placentation is limited to mammals.

Keywords: evolution; mammal; megakaryocyte; placenta; platelet.

Figure 1.

Phylogenetic relationships of the major clades of mammals and the taxonomic distribution of haemostatic and reproductive characteristics. Platelets and megakaryocytes are found in all three clades of mammals but not in reptiles. Therians, i.e. eutherians and marsupials, share viviparity. In reptiles, the mode of reproduction is variable. Only eutherians have a haemochorial placenta. This condition is ancestral in eutherians, but there are some derived groups that have re-evolved non-invasive, epitheliochorial placentation: dots are shaded, with darker shading at the bottom, indicating an ancestral condition.

Figure 2.

The need for haemostasis in a minimally invasive haemochorial animal D-. novemcintus, which belongs to the eutherian clade most distantly related to humans. (a) The minimally invasive placenta of armadillo in third month gestation. The thin threads indicated by yellow arrows are the projections of the placenta entering the endometrium to the left. Arrow heads indicate penetration. The invasion through haemochorial is minimally destructive. (b) Postpartum uterus of armadillo, showing copious coagulated blood in the uterine cavity, indicating the need for effective haemostasis.