The protein syncytin, which is essential for formation of the placenta, originally came to the genome of our ancestors, and those of other mammals, via a retrovirus infection. Placental structures have also developed in non-mammalian vertebrates. The Mabuya lizard (pictured: image credit), which emerged 25 million years ago, has a placenta very much like those in mammals, and its development was likely driven by capture of a retroviral gene.
[Read more…] about Retroviruses turned egg-layers into live-bearers