You think that your body and mind belong to you alone. In fact, you are a mixture of many organisms, with the possibility that this includes components from another person within you, says science journalist David Robson. Back in the day, it was easy to see your origin. Your parents met together, and from a tiny fertilized egg you came into this world with your screams and kicks. Half of you is your mother, the other half is your father, but you are quite another.
There is one exception. This naive tale has now become much more complex. In addition to your genetic material that you took from your parents, you are nothing but a diverse mixture of other viruses and bacteria, with the possibility that there are parts of other people inside you.
Realistically, if you are a twin, you are especially likely to carry parts of your twin sibling inside your body or even your brain. Even more surprisingly, these components may influence your own behavior.
"Humans are not a single, unified entity, but rather a group of mighty creatures," says researcher Peter Kramer from the University of Padua, Italy. "Huge numbers of different kinds of living things, both human and non-human, are fighting within our bodies and relentlessly for control," he adds.
Kramer and his colleague Paula Bressan recently prepared a study published in the journal Perspectives on Psychology, in which they called on psychologists and psychiatrists to focus on identifying the ways in which these internal components might affect our behaviors.
It may seem alarming to the reader, but it has long been known that our bodies are really a mixture of many types of living things.
For example, some germs in the intestine can produce neurotransmitters in a way that affects your mood. Some scientists even believe that there is the possibility of microbes affecting your appetite for food in a way that makes you crave the foods that these microbes like.
Also, infection with parasites known as "Toxoplasma gondii" (Toxoplasma gondii)
It could lead to death. In nature, these parasites affect rats' brains, making them attract towards cats, which will provide these parasites with a suitable place to reproduce.
But humans can become infected and experience similar control over the brain: Parasites seem to make people vulnerable, and increase their risk of schizophrenia or suicide depression.
Six years ago, photojournalist Ariko Ianauca was taking pictures that show the interconnectedness of the twins Irna and Hervina from Iceland, to name a few. A third of British meat now contains such parasites - despite the fact that infection with them contributes significantly to the generation of these mental illnesses. So, "we have to stop that," says Kramer.
Leakage to relatives
In light of this information, it becomes clear to us that not all our actions are entirely of our will. This is enough for us to wonder about our sense of identity. But the concept of these components leaking into us gets even stranger when you realize that your mind has not only been invaded by tiny germs - but by other humans.
Perhaps the most vivid example is the case of conjoined twins that share a single brain, Kramer says. But even normal (separated) twins may share organs without realizing it.
In the early stages of development, cells can transfer between twins. In the past, this was considered a rare case, but now our knowledge indicates that it is an amazingly common occurrence.
For example, nearly eight percent of non-identical twins, and 21 percent of triplets, have two blood types rather than one: one is produced by the body's own cells, and the second by "foreign" cells that leaked into the body from one of the twins. And it may happen in many organs, including the brain.
But when this kind of interference is between two brains, it can have dire consequences. For example, we know very well that the arrangement of different parts of the brain plays a crucial role in brain function - but the presence of foreign tissues, directed by different genetic material with different effects, may lead to chaos in that arrangement.
This may explain, for example, why twins are less likely to use the right hand - a simple trait that usually depends on the relative arrangement of the left and right hemispheres of the brain.
Even if you never believed you had a twin, there are many other means of invading you by someone else's cells. For example, it is possible that you both grew up as two babies in the same womb, but the twins merged during the initial stages of development.
Since the process took place in those primitive stages of evolution, the cells merge into the tissues and appear to arise and develop normally, even though they carry the genetic characteristics of another person.
"You look like one person," says Kramer, "but you carry someone else's cells inside of you - practically, you've always been two people."
In another exaggerated case, a woman was astonished when they told her that she was not the natural mother of her two children. Alternatively, the cells of one of the siblings may remain in the mother's body, only to find their way into your body just for your mother to carry you.
No matter how that process takes place, it is entirely plausible that someone else's tissues could cause the brain to be created in unexpected ways, says Lee Nelson of the University of Washington, and she is currently studying whether cells from the same mother were implanted in the fetus's brain.
And Nelson adds: "The difference in the amount or type of cells, or the period of formation of the embryo in which those cells were acquired and acquired, all of these things may lead to defects."
Nelson discovered that even in adulthood, you are not immune to "human invasion." Two years ago, Nelson and William Chan from the University of Alberta in Edmonton, Canada, took slices of a woman's brain tissue and examined her genetic content for a "Y-chromosome."
About 63 percent of the tissue contained male cells.
"Not only did we find male genetic material in the human brain cells of a woman in a generalized monitoring process, but we detected it in multiple regions of the brain," Chan said.
In other words, her brain was permeated with tissues from the body of a man. One logical conclusion is that its source is a male fetus: somehow, stem cells from her son crossed through the placenta and lodged in her brain.
Strangely, this appears to reduce a woman's risk of later developing Alzheimer's disease - the exact cause remains a mystery. Some researchers have even begun to wonder whether these cells affect a mother's mentality during pregnancy.
Our knowledge of "mighty beings" is still in its infancy. So many of the implications are purely theoretical at the moment. Kramer and Bressan did not hope in their research to provide specific answers, but rather enlighten psychologists and psychiatrists about the many components that we are now made of.
"We cannot understand human behavior by taking into account the individual person or the other person only," says Kramer. "Ultimately, we have to understand them all in order to understand how we act 'we'."
For example, scientists often compare a group of twins to understand the origins of behavior. But the truth is that even dissimilar twins may have swapped a portion of their brain tissue with each other, making those results unclear.
We should be especially careful when using studies on twins to compare conditions such as schizophrenia that may arise from the wrong arrangement of the brain. That's what Bressan and Kramer say.
Nevertheless, on the whole, we should not feel hostile to these "invaders" - whatever the case, they have worked to shape who you are now.