Fun fact, Syncytin-1 is the syncytin used by primates, other branches of mammals have other forms of syncytins, meaning that the event of being infected by a retrovirus that gives mammals proteins necessary for the placenta happened several times in history following the original event and replaced the original syncytin.

(Not sure those proteins are also called syncytins in other mammals but I'm not a biologist so I'm using the limited vocabulary I have)

How do we know it happened many times, vs a single ancient origin with non-lethal random mutation experienced by different branches since then?

I don't recall where I read it but I recall that the nucleus of a cell started out as a captive bacterium.

Not likely for nucleus - https://doi.org/10.3389/fmicb.2020.571831

Gemmata obscuriglobus is pretty interesting, and one could imagine how you go from it to nucleus https://en.wikipedia.org/wiki/Gemmata_obscuriglobus

Interesting, thank you!