This book has split my brain open.

I have examined, handled, peered at countless human placentas. Moreso than any other internal organ. It's kind of weird. That baby made this alien thing?

There's a layer, as thin as can be, protecting this foreign embryo, halfway consisting of someone else, from the mother's immune system. That layer is made with a protein encoded by a gene from a virus.

Our ancestors stopped laying eggs because of DNA they picked up from a virus! HOLY FUCK! Why did I not know this? Why doesn't everybody know this?

Like any great science book, the knowledge it put in my brain is outweighed by the new questions I have.