The genetic code that goes on to create our brains, our selves, and our consciousness, is not only hereditary. Anna M. Hennessey argues that microchimerism, where non-hereditary DNA is introduced into our bodies through cells exchanged during pregnancy between the mother and fetus and vice versa, is ripe for scientific and philosophical enquiry. For Hennessey, these exchanges do not only alter our brain but our consciousness itself, and how we experience the world.

During a plenary of last year’s 30^th annual conference of The Science of Consciousness (TSC), David Chalmers began his talk with a tribute to his mother, who had recently passed away. A seminal figure in the field of consciousness studies and a key member of the first TSC conference held in 1994, Chalmers discussed his mother’s spiritual study of consciousness, his father’s scientific study in neuroscience, and their impact on his own path to studying consciousness. Beyond the intellectual, psychological, and emotional influences Chalmers’ mother had on him, however, she and he likely also influenced the neurological hardwire of each other’s brains in a less widely known process – by transferring their own cells and DNA to one another through her placenta decades ago. These cells are microchimeric cells, and the bidirectional cell transfer that occurred between Chalmers and his mother – as well as between mammals and their mothers more broadly – is called feto-maternal microchimerism.

The word “chimera” refers to a hybrid creature and stems from Greek Mythology and Hesiod’s Theogony in which he describes a female beast made up of a lion, goat, and dragon. In common parlance, the term can refer to an illusion or fantasy of the mind, while in its medical context the word indicates a part of the human body that is genetically diverse. Research on microchimerism has grown in exciting directions over the past few decades. “Maternal microchimerism” refers to the transfer of cells from mother to fetus and “fetal microchimerism” refers to the reverse. Studies of these various forms of microchimerism have shown us that we are all chimeric on some level and contain cells from our mothers and sometimes from older siblings. Women who give birth also contain the cells of their children as well as of their own mothers. Our biology challenges the very notion of a singular self.

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While the philosophy of mind has largely overlooked pregnancy and birth, biology tells a profound story of interconnected consciousness.

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Chalmers is not a materialist. In The Conscious Mind (1996), Chalmers’ first book, he famously stated that “[m]aterialism is a beautiful and compelling view of the world, but to account for consciousness, we have to go beyond the resources it provides.” Believing in the “hard problem” of consciousness, he does not consider his subjective experience of consciousness a simple correlate of his brain’s activities. But as a property dualist, he has a vision for a science of consciousness and thus believes that the mind is still intimately connected to the brain. Some of the latest research on feto-maternal microchimerism suggests both that brains could be chimeric during their most formative stages of neurodevelopment, and that for women who give birth, cells from one or multiple fetuses become part of the mother’s neurons and other parts of their brains. If there is a conscious “self” connected to the architecture of the brain, then perhaps these other “selves” bound to it should be considered. This consideration becomes even more important when we learn about how this microchimeric material may also be connected to behavioral development.

These cells, there due to pregnancy and birth, are unfortunately seldom matters of philosophical interest. However, these cells are not only fascinating for scientists but for philosophers of mind, too. Breastfeeding is also interesting in this context because the maternal microchimeric transfer of cells from mother to child, including to their brains, continues after birth through to breastfeeding. If our brains are affected by these feto-maternal cells, then it is possible that they affect our consciousness and how we experience it.

Historically, when pregnancy and childbirth have appeared as topics of interest within Western philosophy, they have been overshadowed by topics such as the ethics of abortion, cloning, or stem cell research. Although canonical Western philosophers have focused on universals in the human experience, including the universality of death, they have given much less attention to pregnancy and birth. And yet birth is deeply philosophical and intersects with many other areas of philosophy that have nothing to do with ethics. Nonwestern philosophy knows this, and we find literature in traditions such as Confucianism, Buddhism, Daoism, and Hinduism, among others, on the philosophical import of pregnancy, birth, and matters of the prenatal world.

While the philosophy of mind has largely overlooked pregnancy and birth, biology tells a profound story of interconnected consciousness. As a mother of two, I likely have cells of my children in my brain, and my children could have some of my cells in their brains. These cells are microchimeric and therefore reside as entities that are separate from our hereditary DNA, while still being fused with our organs and other biological matter. As research shows, these cells may also have an impact on not only how maternal brains function but on how prenatal and neonatal brains develop. It is therefore possible that the consciousness currently emerging from my brain is not the same as what I had before becoming pregnant with my children, impacted on some level by the presence of their selves in my body. The term “consciousness” is a complex one, though here I am referring to it as an all-encompassing concept of thought that makes up mental experiences and includes the subjective experiences of ideas.

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If our brains are connected to our selves, then there does seem to be some communal or familial element to the “self” that each of us has ended up with.

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And in the case of my children having received my maternal cells in their brains during the days they spent in my womb, those cells also had an impact on their brains’ developments that differs from the impact of hereditary DNA. If our brains are connected to our selves, then there does seem to be some communal or familial element to the “self” that each of us has ended up with. Research in psychology on the plasticity of the maternal brain as well as psychiatric studies on the transmission of intergenerational trauma are also of interest because they show that an individual’s physical and psychological selves are intertwined with the physical and psychological selves of other people. Under these conditions, it is difficult to say that the selves we have, or the consciousnesses that we experience, are completely our own.

The bidirectional transfer of cells between mothers and fetuses has been studied to some degree since the 1960s. Research on microchimerism only started to take off in the 1990s, however. It was during this time, while researching autoimmune diseases in human mothers, for example, that Dr. J. Lee Nelson, a rheumatologist and professor of medicine at the University of Washington, discovered fetal cells present in the blood and tissues of mothers, some of whom had given birth years earlier, with publications in Arthritis & Rheumatism, The Lancet and The New England Journal of Medicine.

More recently, the Nature Communications journal published a study from a research group led by Steven Schepanski and based at the University Medical Center in Hamburg, Germany, which examined maternal cell transfer to the fetus during mammalian pregnancy. Studying mice, Schepanski’s team found that maternal microchimeric cells (MMc) do transfer to the fetal brain. More astoundingly, the research claimed that these cells are not simply part of a “placental leak” during pregnancy. The MMc also impacted homeostasis of microglia (a type of brain cell) in the fetus, resulting in an increased homeostasis of the fetus’ developing brain. While emphasizing the need for more research on the topic of how MMc function in fetal brain tissue, especially in the human brain, the study did conclude that the MMc appear both to promote communication circuitry in the fetus’ prefrontal hippocampus and even to impact positively the way behavioral abilities develop neonatally in offspring.

Mice are not humans. But as Chalmers’ TSC colleague Christof Koch most recently emphasized during the same 2024 plenary meeting, the human brain and that of mice are extremely similar, and experimentation on the mouse brain is paramount in studies of interest to consciousness. Koch, Meritorious Investigator at the Allen Institute, has worked extensively over the decades on the issue of neuronal correlates of consciousness (NCC).

The bidirectional transfer of cells that occurs in feto-maternal microchimerism is lopsided, with more cells going from fetus to mother than from mother to fetus. The functions of the cell transfer are not yet clear, though studies appear to show positive, negative, and neutral effects. The presence of fetal cells in the mother relates to a lower risk of breast cancer, for example, though these cells are also implicated in the occurrence of autoimmune diseases in some post-pregnant women.

Whatever the reason for this cell transfer to the mother, the brain of a mother can contain cells from the mother’s own mother, the mother’s children, and even cells from previous pregnancies that did not result in a live birth. This means that a mother’s brain is particularly chimeric, with the cells of multiple foreign subjects possibly inhabiting the same space together.

Outside of scientific research, scholars such as Aryn Martin and Chikako Takeshita have examined impacts of microchimerism on more culturally related issues. Takeshita indicates that our understanding of an organism as a single unique genome is collapsing the more we learn about humans and their biology. Both scholars question the idea of the “self” since we now know that the human body is composed of multiple living entities.

Studies in neuroscience and psychology have provided additional evidence that the circuitry of the maternal brain grows and transforms on its own during pregnancy, a process which continues after birth, especially in correlation with activities such as breastfeeding and caretaking. The neural plasticity in the maternal brain is very high, with structural change and gray matter volume increasing in various areas of the brain. This plasticity could correlate with behavioral changes exhibited by the mother during the early postpartum period, and also be connected to the maternal care she provides later on. Some fMRI studies have shown, for example, that increased activity in the frontal regions of a mother’s brain relate to the quality of attachment connected to her own child crying.

Research in psychiatry on the transmission of intergenerational trauma has in the meantime found that an offspring’s brain development in utero can be affected by the pregnant woman’s experiences of her own childhood maltreatment. While the transmission mechanisms are unclear, Developmental Origins of Health and Disease (DOHaD) finds that during the prenatal period, long-term changes in the maternal or paternal biology of the fetus may impact its own gestation and brain development.

In his work on how we can construct a science of consciousness, Chalmers calls for the collection and integration of two forms of data. “Third-person data” refers to objective material studied in cognitive psychology and cognitive neuroscience, such as what auditory functions take place when sounds are made, the connections to neural mechanisms when hearing sounds, and how the subject reports the effects of those sounds. “First-person data” refers to the subjective experiences themselves, such as what it is to have the experience of listening to music. Chalmers believes that these two forms of data are both needed in a science of consciousness. They are not reducible to one another, but highly interrelated, so that subjective experiences correlate “systematically” with both brain processes and behavior.

Within this construction, the study of microchimeric brain cells and their connections to behavior and neural development would seemingly fit into Chalmers’ description of third-person data. And yet, there may be first-person data strongly associated with the third-person data that also arises from feto-maternal microchimerism. This first-person data could relate to the subjective experience of either the mother or of the child or both. If the homeostasis of a fetus’ developing brain is impacted by maternal microchimerism, for example, then the subjective experience of consciousness could also be impacted.

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Biologically, we are all chimeras. Our consciousnesses may on some level be chimeric too.

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When my children were very little, I started to notice a shift in my own perceptions and particularly in my aesthetic tastes. Styles of artwork I had previously been uninterested in suddenly seemed vibrant and attractive. As someone who has studied art extensively, I always assumed that this shift had to do with the passage of time and having a broader understanding of art as I got older, but the change correlated directly with my experience of becoming a mother. Perhaps this aesthetic modification in my sense experience was connected to the onset of motherhood in my life, though it could relate more to the social aspect of spending time with my children and seeing the world anew through their eyes. Or could it be that those microchimeric cells in my brain originating in my children have impacted my taste in art? Either way, this is just one example of how my subjective experience of consciousness has changed after having children and of becoming a mother.

Feto-maternal microchimerism is an issue connecting the philosophy of birth to the philosophy of mind. The phenomenon provides us with an intriguing occurrence that blurs the line of what it is to have a singular “self.” Biologically, we are all chimeras. Our consciousnesses may on some level be chimeric too.