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As for the virus/placenta thing, I recall reading a similar thing about wasps, though I couldn't provide any citations, that the method by which a wasp stops the immune system of a host (such as a catepiller) from attacking its eggs/larvae is viral in origin, and remarkably similar in some ways to the way a mother's body doesn't attack an organism that in any other circumstances would read as a parasite.
Interesting stuff.
This line of thinking is just hopeful projections of desired causality.
Such changes are replicated more successfully (more offspring) making the change/mutation more resilient to disappearing from the genepool.
Over time any organism that's living/thriving is going to have a lot of these mutations stacked on top of one another in a resilient way(size of population with the same mutations). Any mutations that are disadvantageous to natural selection and proliferation are weeded out of the genepool (go extinct)
I don't know if you're being somehow pedantic for some definition of "eye" or "evolve", or if you're one of those "Intelligent Design" creeps.
Evolution can be very easy given time. Trees independently evolved from non-tree plants something like 100 times, for example.
The GP is skeptical that one can go from A, a chance interaction between two systems, through a continuous uninterrupted, robust, repeatable chain of unguided interactions, and arrive at Z, the placenta. An organ whose function and elegance is unrivalled by anything we as a species have made with logic and thought. And why should he not be skeptical, everything we've practically achieved in terms of technological progress has been the product of mind and intelligence, any claims to be able to do the same thing without that should be scrutinised very closely.
Before modern medicine, it and the rest of this elegant system had a roughly ~2-3 in 100 chance of killing the mother in any particular pregnancy.
I detect a creationist.
It asserts that some virus genes made it into our genome, and that some of those were useful in the eventual evolution of the placenta.
That mechanism is well documented: https://en.wikipedia.org/wiki/Endogenous_retrovirus
> the idea that it can ... somehow get into our sperm and egg in order to self propagate into another organism... is ridiculous
Again, that's precisely what endrogenous retroviruses do. From my link:
"Rarely, retroviral integration may occur in a germline cell that goes on to develop into a viable organism. This organism will carry the inserted retroviral genome as an integral part of its own genome—an "endogenous" retrovirus (ERV) that may be inherited by its offspring as a novel allele. Many ERVs have persisted in the genome of their hosts for millions of years."