It’s probably not anywhere near as simple and certain as Catherine Jessus is making out. Viruses don’t likely do enough to create placentas. But the main point is, this definitely isn’t yer old biology teacher’s Split-the-Desk Rant for Darwin!!! Stay tuned.
So was there a single “ancestral mammal” or is it a common design (convergent evolution)? Either is quite plausible today.
… in which we encounter the remarkable phenomenon of microchimerism (“These cells find their way into mother’s tissue and start acting like the tissue in which they find themselves. This process is known as feto-maternal microchimerism”)