At least 1 in 8 of us is carrying around the cells of a twin we consumed in utero

I know, it's a lot to digest.

As many as 12 percent of us actually started out as two cells in utero. But long before our would-be sibling could even grow a limb, we went all Cain and Abel on it. The cells took root somewhere in our developing body instead and grew into human tissue. Foreign human cells have been found in kidneys, livers, and the brain. And it's not just siblings either. Mothers have consumed the stem cells of their would-be children as well.

This rather unsettling news comes from a massive paper called, "Humans As Superorganisms," which David Wallace-Wells helpfully unpacks in New York magazine.

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And to put this phenomenon — which scientists have creepily named the "parasitic" or "vanishing" twin — into context, Wallace-Wells leads with a mind-blowing, real-life tale that goes something like this:

Protective services threatened to take away a woman's three children when a maternity test revealed she wasn't the mother. This was most shocking to the woman since she conceived and delivered all three of these babies. This woman eventually discovered the true biological mother of her children was in fact (wait for it) her unborn twin!

Unbeknownst to this woman, or even her parents, she consumed the cells of the human embryo she was sharing the womb with all those decades ago and those cells developed into the eggs that produced those children. How's that for some family drama?

If you'd like to hear more about the unborn sibling you didn't know existed, as well as the other interesting and surprising facts that I learned this week, listen to this episode of "This week I learned" below. And, if you like what you hear, you can subscribe to The Week's podcasts on iTunes, SoundCloud, and Stitcher. —Lauren Hansen