There's a good chance you're a human chimera

Chimerism, the phenomenon of having parts of a body made from a completely different being, is a much more common occurrence than previously thought. Are you a mother? Were you gestated by a mother? I'm betting that at least one of those things is true. Which means an entirely separate person could be part of you right now.

A chimera is the mythological term given to animals that had other animals' parts. A lion with a snake for a tail was a chimera. People all over the world told tales of these fantastical beasts. An equally remarkable story is being told today in medical papers. Humans bear marks of chimerism, too. Although their stolen body parts only come from other humans, the results can be dramatic.

There was a case, not too long ago, when a blood test showed that a woman, and the children she had actually born, were not mother and child. This woman continued to undergo blood tests trying to prove that she was the mother of her own children, even when one such test showed that she was not the mother of the child she had just birthed. Either this woman was undergoing some fairly advanced medical procedures on her own, or something was up. Eventually it was found that this woman had had a fraternal twin. She didn't remember it, because it was while they were both basically blastocysts when that twin had ceased to be. Her twin had been absorbed into her, and the combined tissue created a composite body, despite having different DNA. A tissue sample taken from the woman's thyroid later showed that she was the children's biological mother. It's just that her ovaries belonged to that long-lost twin. Another woman found out that her children weren't carrying her DNA when she needed a kidney transplant and neither was a match. Again, her ovaries were made from different DNA. Both mothers were chimeras.

And it turns out a lot of mothers are chimeras. Not as completely as these women were, but fetal stem cells are tenacious things. They stay in the body of the mother, and have even been shown to slip into that sacred space where we believe that all of human individuality resides; the brain. It's possible that part of every mother's brain literally is that of her child. And if you're a child, it's likely that part of you is your mother. A mother's cells cross the placenta during pregnancy and squat in the liver, bloodstream, thymus gland, and that old sentimental favorite, the heart.

Okay, it isn't eagle wings, but chimerism is all the more strange for not being visible. It's quite likely that, by any genetic measure, you carry around the cells of another human being. You carry them around perpetually, as an indispensable and inseparable part of you. You may be housing another being in your body, and some other human is housing your cells. Something to keep in mind if you ever have your kids' blood tested.

Top Image: Michael Reeve

Via ABC, Discover, Genetics and Evolution, and How to Defeat Your Own Clone.