The mysterious cells in our bodies that do not belong to us

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

Excerpt: "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.

These cross-generational transfers are bidirectional. As fetal cells cross the placenta into maternal tissues, a small number of maternal cells migrate into fetal tissues, where they can persist into adulthood.

Genetic swaps, then, might occur several times throughout a life. Some researchers believe that people may be miniature mosaics of many of their relatives, via chains of pregnancy: their older siblings, perhaps, or their maternal grandmother, or any aunts and uncles their grandmother might have conceived before their mother was born. “It’s like you carry your entire family inside of you,” Francisco Úbeda de Torres, an evolutionary biologist at the Royal Holloway University of London, told me."