“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.

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.

All of that makes microchimerism—named in homage to the part-lion, part-goat, part-dragon chimera of Greek myth—more common than pregnancy itself. It’s thought to affect every person who has carried an embryo, even if briefly, and anyone who has ever inhabited a womb.

In many cases, microchimeric cells are thought to be present at concentrations on the order of one in 1 million—levels that, “for a lot of biological assays, is approaching or at the limit of detection,” Sing Sing Way, an immunologist and a pediatrician at Cincinnati Children’s Hospital, told me.

Some scientists have argued that cells so sparse and inconsistent couldn’t possibly have meaningful effects. Even among microchimerism researchers, hypotheses about what these cells do—if anything at all—remain “highly controversial,” Way said. But many experts contend that microchimeric cells aren’t just passive passengers, adrift in someone else’s genomic sea. They are genetically distinct entities in a foreign residence, with their own evolutionary motivations that may clash with their landlord’s. And they might hold sway over many aspects of health: our susceptibility to infectious or autoimmune disease, the success of pregnancies, maybe even behavior. If these cells turn out to be as important as some scientists believe they are, they might be one of the most underappreciated architects of human life.

The perks of microchimerism for mothers have been tougher to pin down. One likely possibility is that the more thoroughly embryonic cells infiltrate the mother’s body, the better she might be able to tolerate her fetus’s tissue, reducing her chances of miscarriage or a high-risk birth. “I really think it’s a baby’s insurance policy on the mom,” Amy Boddy, a biological anthropologist at UC Santa Barbara, told me. “Like, ‘Hey, don’t attack.’”

Microchimerism may not always be kind to moms. Nelson and others have found that, long-term, women with more fetal cells are also more likely to develop certain kinds of autoimmune disease, perhaps because their children’s cells are mistakenly reassessed by certain postpartum bodies as unwanted invaders.

These questions are so difficult to answer, Way told me, because microchimeric cells are so challenging to study. They might be in all of us, but they’re still rare, and frequently hidden in tough-to-access internal tissues. Researchers can’t yet say whether the cells actively deploy to predetermined sites or are pulled into specific organs by maternal cells—or just follow the natural flow of blood like river sediments. There’s also no consensus on how much microchimerism a body can tolerate. In a vacuum of evidence, even microchimerism researchers are steeling themselves for a letdown. “A very large part of me is prepared to think that most if not all microchimerism is completely benign,” Melissa Wilson, a computational evolutionary biologist at Arizona State University, told me.

One option, Burlingham told me, might be to infuse organ-transplant patients with cells from their mother, which could, like tiny ambassadors, coax the body into accepting any new tissue. Microchimerism-inspired therapies could help ease the burdens of high-risk pregnancies,

Even amid these uncertainties, the experts I spoke with stand by microchimerism’s likely importance: The cells are so persistent, so ubiquitous, so evolutionarily ancient, Boddy told me, that they must have an effect. The simple fact that they’re allowed to stick around for decades, while they grow and develop and change, could have a lot to teach us about immunity—and our understanding of ourselves. “In my mind, it does alter my concept of who I am,” Bianchi, who herself has given birth to a son, told me. Although he’s since grown up, she’s never without him, nor he without her.