it was almost identical to those used by retroviruses to produce the proteins that attach to cells they are infecting without triggering an immune response.[24] The scientists concluded that a crucial function of the placenta didn’t emerge gradually as a result of evolution by natural selection but was suddenly acquired when a retrovirus inserted its DNA into our ancestor’s genome. If one of our distant ancestors hadn’t been infected by a virus hundreds of millions of years ago, humans would reproduce by laying eggs.
