I think this article is a bit misleading. The placenta did not evolve from an ancient virus. A protein found in primate placentas (but, interestingly, not fou

I think this article is a bit misleading. The placenta did not evolve from an ancient virus. A protein found in primate placentas (but, interestingly, not found in other placental mammals) is clearly related to endogenous retrovirus proteins. (I do not think that HERV and HIV have much to do with each other -- they are very different viruses). But it is pretty interesting that primates started using this protein relatively late in mammalian evolution.