You may think your body and mind are your own. In fact, you are a fusion of many organisms - including, potentially, another person. Words by David Robson, photography by Ariko Inaoka.

This story is part of BBC Future’s “Best of 2015” list, our greatest hits of the year. Browse the full list.

-

Once upon a time, your origins were easy to understand. Your dad met your mum, they had some fun, and from a tiny fertilised egg you emerged kicking and screaming into the world. You are half your mum, half your dad – and 100% yourself.

Except, that simple tale has now become a lot more complicated. Besides your genes from parents, you are a mosaic of viruses, bacteria – and potentially, other humans. Indeed, if you are a twin, you are particularly likely to be carrying bits of your sibling within your body and brain. Stranger still, they may be influencing how you act.

A very large number of different human and non-human individuals are struggling inside us for control

“Humans are not unitary individuals but superorganisms,” says Peter Kramer at the University of Padua. “A very large number of different human and non-human individuals are all incessantly struggling inside us for control.” Together with Paola Bressan, he recently wrote a paper in the journal Perspectives in Psychological Science, calling for psychologists and psychiatrists to appreciate the ways this may influence our behaviour.

That may sound alarming, but it has long been known that our bodies are really a mishmash of many different organisms. Microbes in your gut can produce neurotransmitters that alter your mood; some scientists have even proposed that the microbes may sway your appetite, so that you crave their favourite food. An infection of a parasite called Toxoplasma gondii, meanwhile, might just lead you to your death. In nature, the microbe warps rats’ brains so that they are attracted to cats, which will then offer a cosy home for it to reproduce. But humans can be infected and subjected to the same kind of mind control too: the microbe seems to make someone risky, and increases the chance they will suffer from schizophrenia or suicidal depression. Currently, around a third of British meat carries this parasite, for instance – despite the fact an infection could contribute to these mental illnesses. “We should stop this,” says Kramer.

Infiltrating siblings

In this light, it becomes clear that our actions are not entirely our own. It’s enough to make you question your sense of identity, but the idea of infiltration becomes even more eerie when you realise that your brain has not just been invaded by tiny microbes – but also by other human beings.

Even non-conjoined twins could be sharing organs without realising it

The most visible example might be a case of conjoined twins sharing a brain, says Kramer, but even regular twins could have shared organs without realising it. During early development, cells can be passed between twins or triplets. Once considered a rare occurrence, we now know it is surprisingly common. Around 8% of non-identical twins and 21% of triplets, for example, have not one, but two blood groups: one produced by their own cells, and one produced by “alien” cells absorbed from their twin. They are, in other words, a chimera – a fusion of two bodies – and it may occur in many organs, including the brain.

Brother from another mother

Women accidentally carrying a "twin's" child

Lydia Fairchild’s paternity test was meant to be straightforward, proving to the courts that her two sons’ father was the person she said he was. When the test came back, however, Fairchild herself came up as a blank: there was no trace of her DNA in her own children.

The courts threatened to convict her of illegal surrogacy – they assumed it was a scam to gain benefits. Luckily, at around the same time, a scientific paper reported a similar case in which a woman was apparently not the biological mother of two of her three children. The reason was that she was a chimera: a case in which two twins had merged into one body early in development. Being the product of two different cell lines, some of her eggs carried a genome that was different from the rest of the body.

Needless to say, the discovery has caused Fairchild to question her own identity. “Telling my sons about this was the hardest part because I felt that part of me hadn't passed on to them,” she told the website Jezebel. “I thought, ‘Oh, I wonder if they'll really feel that I'm not quite their real mother somehow because the genes that I should've given to them, I didn't give to them.’”

A chimera brain could have serious consequences. For instance, we know that the arrangement of different brain regions can be crucial for its function – but the presence of foreign tissue, being directed by different genes carrying a different blueprint, may throw that intricate design into disarray. This may explain, for instance, why twins are less likely to be right-handed – a simple trait that normally relies on the relative organisation of the right and the left hemispheres. Perhaps chimerism has upset the balance.

Even if you do not think you ever had a twin, there are many other ways you might be invaded by another human’s cells. It’s possible, for instance, that you started off as two foetuses in the womb, but the twins merged during early development. Since it occurs at such an early age of development, the cells can become incorporated into the tissue and seem to develop normally, yet they are carrying another person’s genetic blueprint. “You look like one person, but you have the cells of another person in you – effectively, you have always been two people,” says Kramer. In one extreme case, a woman was surprised to be told that she was not the biological mother of her two children (See “Brother from another mother”). Alternatively, cells from an older sibling might stay around the mother’s body, only to find their way into your body after you are conceived.

However it happens, it’s perfectly plausible that tissue from another human could cause the brain to develop in unexpected ways, says Lee Nelson from the University of Washington. She’s currently examining whether cells from the mother herself may be implanted in the baby brain. “A difference in the amount, cell type, or the time during development at which the cells were acquired could all result in abnormalities,” she says.

Nelson has found that even as an adult, you are not immune from human invaders. A couple of years ago, Nelson and William Chan at the University of Alberta in Edmonton took slices of women’s brain tissue and screened their genome for signs of the Y-chromosome. Around 63% were harbouring male cells. “Not only did we find male DNA in female human brains as a general observation, we found it to be present in multiple brain regions,” says Chan. In other words, their brains were speckled with cells from a man’s body. One logical conclusion is that it came from a baby: somehow, her own son’s stem cells had made it through the placenta and lodged in her brain. Strangely, this seemed to decrease the chances that the mother would subsequently develop Alzheimer’s – though exactly why remains a mystery. Some researchers are even beginning to wonder whether these cells might influence a mother’s mindset during pregnancy.

Our knowledge of the human “superorganism” is still in its infancy, so many of the consequences are purely theoretical at the moment. Kramer and Bressan's aim with their paper was not to give definitive answers, but to enlighten other psychologists and psychiatrists about the many entities that make us who we are today. “We cannot understand human behaviour by considering only one or the other individual,” Kramer says. “Ultimately, we must understand them all to understand how ‘we’ behave.”

For instance, scientists often compare sets of twins to understand the origins of behaviour, but the fact that even non-identical twins may have swapped bits of brain tissue might have muddied those results. We should be particularly careful when using these twin studies to compare conditions such as schizophrenia that may arise from faulty brain organisation, Bressan and Kramer say.

In general, however, we shouldn’t feel hostile towards these invaders – after all, they made you who you are today. “I think it is now clear that our natural immigrants are with us for the long-term, for better or for worse,” says Nelson. “And I would think “for better” outweighs ‘for worse’.”

David Robson is BBC Future’s feature writer. He is @d_a_robson on Twitter. More of Ariko Inaoka's photography, including her portraits of the Icelandic twins Erna and Hrefna, can be found here.

Follow us on Facebook, TwitterGoogle+LinkedIn and Instagram