A virus first perspective is presented as an alternative hypothesis to explain the role of various endogenized retroviruses in the origin of the mammalian placenta. It is argued that virus-host persistence is a key determinant of host survival and the various ERVs involved have directly affected virus-host persistence.
Keywords: Virus evolution; endogenous retroviruses; host evolution; placenta.
© 2016 APMIS. Published by John Wiley & Sons Ltd.