One in eight adults who think they were born alone are carrying around, like a change purse, some residue of a sibling they never knew existed. Photo: Bobby Doherty/New York Magazine

It is not only possible, it has in fact happened that a woman who vaginally conceived a child, then vaginally delivered her, had Protective Services threaten to take the child when a maternity test showed she was not, in fact, the mother. Nor was she the mother of her second child, genetically. Or her third, whom she was still carrying throughout the dispute with her estranged boyfriend — the man who, those same tests proved, was definitively the father. Only later did Lydia Fairchild discover that the true mother of all three of her children was her twin — if twin is really the word for one human embryo more or less swallowed by another before birth. The eggs that produced those babies had been with Fairchild her whole life, but genetically they belonged to an unborn sister, unknown to her and even her parents, living on in small parts inside her — a phenomenon that poetic scientists have called “parasitic” or “vanishing” twins. These days, they tend to prefer “chimerism,” after the mythic beast assembled, like Frankenstein’s monster, from multiple animals. But, man, isn’t that even creepier?

Don’t relax — it’s not just twins. In a new paper, “Humans As Superorganisms,” Peter Kramer and Paola Bressan of the University of Padua describe a typical human body as a teeming mass of what they call “selfish entities.” Picture a tree warped by fungus, wrapped with vines, dotted at the base with mushrooms and flowers, and marked, midway up, by what the tree thought the whole time was just a knot but turns out to be a parasitic twin. This is the human superorganism — not the tree, not the tangled mess of things doing battle with it, but the whole chunk of forest — and Kramer and Bressan would like to place it at the very center of the way we think about human behavior. They are psychologists, and their paper is a call to arms to their fellow shrinks, exhorting them to take seriously as a possible cause of an enormous buffet of behavioral phenomena — from quotidian quirks, to maddeningly opaque disorders like autism, to schizophrenia — the sheer volume and weird diversity of completely crazy alien shit going on in just about all of our bodies, just about all the time.

At least one part of this superorganism theory is not all that unfamiliar, especially to anyone who remembers recent articles by Michael Pollan and others about what is often called “the brain in your gut.” That part: that our stomachs are, actually, zoos. In fact, they’re not really our stomachs. Principally, they belong to the hundred trillion bacteria enticed by evolution into your chutes-and-ladders intestinal tract, then enlisted to eat your food for you. The weirder thing is that evolution also put hundreds of millions of neurons there, which means there’s a lot of trouble to be caused by those 160 or more species of bacteria (yes, full species). And the behavioral effects are pretty startling. Take a mouse, evacuate his intestines, and repopulate them with the microbes of another mouse, and he’ll act like the other mouse — adventurous mice become timid. In humans, what is delicately called “gut flora” affects not just obesity but also anxiety, and some think it plays a role in disorders as far-ranging as MS and Parkinson’s. What role exactly? Who knows? Though there have been some attempts to treat autism with yogurt.

Okay, so, the gut is weird. But what if you lived in the gut? What if you were the gut? Kramer and Bressan want us to stop looking at our stomachs like we’re hosts to some messy guests, or homeowners too disgusted by a particular closet to ever go poking around in it, because, they write, the human superorganism isn’t something to observe from the privileged perch of the self. Instead, they suggest, it envelops the self — the environment in which and against which genes give rise to who you are, an internal environment populated nevertheless by an entire orchestra of aliens, some of them fiddling away in the brain, and each with its own evolutionary interests at stake.

What aliens do they mean? They point also to viruses and parasitic diseases — both the ones that have already colonized our DNA (depending on how you count, as much as 45 percent of it) and the new ones we could catch, like toxoplasma gondii, just for instance, which is three times as likely to be found in schizophrenics. And then there’s what Kramer and Bressan call “foreign human cells” — a sort of catchall for whenever parts of one person end up in another. Which happens a lot! There’s the way identical twins invariably share cells like Matchbox cars at the early stages of development; the way, they theorize, even fraternal twins arise from the same bundle of cells; the way a mother’s immune system can attack and partially colonize a fetus she is carrying; the way a baby can send cells back in the other direction, including stem cells, which can grow into just about anything; and the way in which that mother-child dynamic may direct the in-house genetic engineering called gene imprinting. And it also includes, yes, Lydia Fairchild’s vanishing twin, and the ones that many of us have been apparently hosting, unwittingly, our whole lives. All of which, Kramer and Bussan say, may just help explain why your son is gay. To take one of their more speculative claims.

As many as one in eight single-baby newborns, and possibly more, began life as one of two, later absorbing the other baby-to-be before anyone really noticed (including the baby, who at that point typically wouldn’t have produced so much as a single limb). Which means that one in eight adults who think they were born alone are carrying around, like a change purse, some residue of a sibling they never knew existed; in some cases, that change purse could be, inside a woman, say, a male kidney. Foreign cells can come from children too; an autopsy study of 46 mothers showed male cells in 13 of their kidneys, ten of their livers, and four of their hearts, thanks to stem cells’ having passed through the placenta, taken root where needed, and sprouted, sometimes into whole patches of tissue. In another study, this one of 59 women, 63 percent harbored male cells in their brains.

And then there are the more extreme cases, like fetus-in-fetu, a kind of twin growth almost too grotesquely surreal to mention. Almost. Possibly the gnarliest example is the case of Sanju Bhagat, an Indian man who carried his twin inside him for 36 years, until a 1999 surgery designed to relieve swelling in his stomach so massive it made him look pregnant. “To my surprise and horror, I could shake hands with somebody inside,” a presiding doctor told reporters of the procedure. “First, one limb came out, then another limb came out. Then some part of genitalia, then some part of hair, some limbs, jaws, limbs, hair.” As everyone noticed, the finger­nails were quite long.

This is not, by the way, the speculative part of the paper; it’s the real-deal part. In fact, what Kramer and Bressan have written is technically called a review, which means mostly it summarizes and analyzes existing research, enclosing in a single phenomenological circle a wide range of what had been seen as divergent subjects, grouping some pretty routine stuff with genetic anomalies that used to land you in a freak show. And then asks us to contemplate, and their colleagues to study more closely, what it all means for behavior and psychology and identity that a part of your brain really belongs to your daughter or your unborn twin — or, for that matter, a virus that’s hijacked the control panels of at least some patch of cells. There are those who will wonder just how rigorous that speculation is, and, perhaps for them, the authors have preemptively chosen to open their paper with a quotation from Star Trek: The Next Generation (the Borg cycle, of course). But in fairness they probably could’ve also used H. G. Wells, Dostoyevsky, Freud, or any of the Romantic poets. “We argue that we do not have a unitary ‘self,’ ” Kramer and Bressan write. “We argue that an incessant struggle among a very large number of ‘selfs’ — some human, some not — determines who we are.” Is it insane to say, at least as a metaphor for mosaic identity, that sounds sort of reasonable?

*This article appears in the October 5, 2015 issue of New York Magazine.