The reason why mammals have a placenta and live birth is due to the incorporation of an ancient syncytium-producing retrovirus in our genomes.

Very interesting, Thank you for sharing. I just took a little look into this, and I'm definitely going to research it more.

Do you know of other cases similar to this, where a virus is incorporated into our genome? Or where I can look into it more?

You can find more about that in the lecture on retroviruses here:

https://www.youtube.com/watch?v=07Zu1sWKuuE&list=PLGhmZX2NKi...

Any favorite examples?

I hope we can un-slime them now to find out all of their secrets.