More specifically, the gene that enables syncytiotrophoblasts to do what they do enabled placentas to be compatible with an adaptive immune system. So the evout

More specifically, the gene that enables syncytiotrophoblasts to do what they do enabled placentas to be compatible with an adaptive immune system. So the evoution of the placenta needs to be seen in the immune system context, a more simple immune system may not have a problem with a non-self embryo in the body. So the virus gene could be one of the factors that enabled a proto-placent to evolve along an adaptive immune system. [Note this is pure speculation, I haven't read up on the evolutionary timescales of either.]