The Tangled Tree

The Tangled Tree by David Quammen has split my brain open. The first 2/3 was well-written, entertaining science history. It provided some context to the evolution discoveries that were developing while I was in college and med school.

Cover of a book: The Tangled Tree by David Quammen

Then it suddenly turned into a horror novel (for me). Last summer I was in the hospital with urosepsis from vancomycin-resistant enterococcus. It took a week of culturing and switching antibiotics before I could go home with linezolid pills. I was vaguely aware of how bacteria could horizontally transfer antibiotic resistance but now I have a much better idea of how that works and what that means. In my own bladder.


I have examined, handled, peered at countless human placentas. More-so than any other internal organ. After every delivery, I would make sure the membranes and lobes all looked intact, to verify nothing was retained in the uterus. It’s kind of weird. It’s almost the size of the newborn and they look nothing alike. That baby made this alien thing?

Newborn and a placenta lying on a sheet, still connected

On the placenta, there’s a layer called the syncytiotrophoblast, as thin as can be, protecting this foreign embryo, halfway consisting of someone else, from the mother’s immune system. The father couldn’t donate a kidney to the mother, probably can’t donate blood to the mother, yet this embryo that is 50% dad manages to hang out for 40 weeks.

We’re taught that this is normal, and why not? We’re surrounded by mammals that came into the world attached to a placenta. The reason is because the syncitiotrophoblast is a gigantic sheet, one cell-layer thick that serves as armor against the mother’s immune system. It also secretes hormones to control the mother’s reproductive organs. 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!
Some platypus type thing got infected by a retrovirus, the viral DNA was incorporated, and now there is a pivotal gene that can now be used to evolve the placenta! (there were other viral genes acquired along the line)

HOLY F&%K! Why did I not know this? Why doesn’t everybody know this? YOU ARE PART VIRUS! The virus part of you allowed you to attach and then detach from your mother! She didn’t have to make a nest and sit on your egg allowing her (and her mother, grandmother, etc.) go around and do awesome stuff!

Pregnant woman standing at a desk with a computer, looking at a sheet of paper

Think of that when you see a pregnant woman, (or any other pregnant mammal). She’s doing whatever she’s doing, not brooding a nest of eggs.

Like any great science book, the knowledge Tangled Tree put in my brain is outweighed by the new questions I have. I learned other fun stuff and I think you should read it. I can’t guarantee it will be as personally meaningful but enjoyably informative.


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