By David Robson 18 September 2015
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.
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.
Brothers 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.’”
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.’”
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”, left). 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.
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. More of Ariko Inaoka's photography, including her portraits of the Icelandic twins Erna and Hrefna, can be found here.
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