You carry literal pieces of your mom—and maybe your grandma, and your siblings, and your aunts and uncles.

Some 24 years ago, Diana Bianchi peered into a microscope at a piece of human thyroid and saw something that instantly gave her goosebumps. The sample had come from a woman who was chromosomally XX. But through the lens, Bianchi saw the unmistakable glimmer of Y chromosomes—dozens and dozens of them. “Clearly,” Bianchi told me, “part of her thyroid was entirely male.”

The reason, Bianchi suspected, was pregnancy. Years ago, the patient had carried a male embryo, whose cells had at some point wandered out of the womb. They’d ended up in his mother’s thyroid—and, almost certainly, a bunch of other organs too—and taken on the identities and functions of the female cells that surrounded them so they could work in synchrony. Bianchi, now the director of the Eunice Kennedy Shriver National Institute of Child Health and Human Development, was astonished: “Her thyroid had been entirely remodeled by her son’s cells,” she said.

The woman’s case wasn’t a one-off. Just about every time an embryo implants and begins to grow, it dispatches bits of itself into the body housing it. The depositions begin at least as early as four or five weeks into gestation. And they settle into just about every sliver of our anatomy where scientists have checked—the heart, the lungs, the breast, the colon, the kidney, the liver, the brain. From there, the cells might linger, grow, and divide for decades, or even, as many scientists suspect, for a lifetime, assimilating into the person that conceived them. They can almost be thought of as evolution’s original organ transplant, J. Lee Nelson, of the Fred Hutchinson Cancer Center in Seattle, told me. Microchimerism may be the most common way in which genetically identical cells mature and develop inside two bodies at once.

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