Is there a human living inside of you?

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.
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.’”
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”, 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 .
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’.”