… in other words:
a species gets infected by a virus, the virus inserts a part of its own viral-DNA into the species genome. Later on, a very complex placental structure starts emerging in the infected species – and everything is ready for modified reproduction … and, of course, the modified reproduction works flawlessly from the very first moment ! If it would not = no reproduction = no evolution …
Moreover, evolutionary biologists claim, that this cooption-magic happened not once, but at least 7 times, repeatedly and independently in evolutionary unrelated species (e.g. mammals, lizards … )
Domestication of the syncytin genes represents a dramatic example of convergent (repeated) evolution via the cooption of a retroviral gene for a key biological function in reproductive biology. In fact, syncytin domestication from a retroviral envelope gene has been previously shown to have independently occurred at least seven times during mammalian evolution
https://www.pnas.org/content/109/7/2184
and now for lizards:
An endogenous retroviral envelope syncytin and its cognate receptor identified in the viviparous placental Mabuya lizard
Retroviral envelope gene capture and exaptation for a placental function has been demonstrated in mammals. Remarkably, placental structures have also emerged on rare occasions in nonmammalian vertebrates, resulting in related modes of reproduction. The Mabuya lizard, which emerged 25 Mya, possesses a placenta closely related to that of mammals. Here, we identified a specific retroviral envelope gene capture that shows all the characteristic features of a bona fide mammalian syncytin, being conserved in Mabuya evolution, expressed in the placenta, and fusogenic. Together with the present identification of its cognate receptor, these results show that syncytin capture is not restricted to mammals and is likely to be a major driving force for placenta emergence.
https://www.ncbi.nlm.nih.gov/pubmed/29162694