For starters, we are chimeras: some parts of us are human, but genetically not “us”. Most, if not all, of us contain a few cells from our mother, our grandmothers and even elder siblings that infiltrated our bodies in the uterus.
Women who have carried children host such cells too. “Something like 65 per cent of women, even in their 70s, when autopsies were performed, had cells in their brains that were not theirs,” says David Linden at Johns Hopkins University in Maryland. Chimeric cells have been found to contribute to both good and bad health, for example promoting wound healing but also triggering autoimmune disease.Graham Lawton, “Why it’s the aliens living inside you that create your sense of you” at New Scientist
The “chimera” (a body comprising two different beings) is an ancient concept; the decorative plate pictured here dates from 350-340 B.C.
But our alien invasion is broader than that, Lawton says: Our body is full of and surrounded by a cloud of bacteria, fungi, protists, archaea and viruses. Our bodies average 30 trillion human cells and 38 trillion one-celled microbes. We are outnumbered, so to speak. And we depend on bacteria to digest, for example.
So far so good. But do these “aliens” really create our sense of self, as Lawton’s title implies? He cites zoologist Thomas Bosch at the University of Kiel in Germany, who argues, as per a 2018 open-access paper,
Today, the three classical biological explanations of the individual self––the immune system, the brain, the genome––are being challenged by the new field of microbiome research. Evidence shows that our resident microbes orchestrate the adaptive immune system, influence the brain, and contribute more gene functions than our own genome. The realization that humans are not individual, discrete entities but rather the outcome of ever-changing interactions with microorganisms has consequences beyond the biological disciplines. In particular, it calls into question the assumption that distinctive human traits set us apart from all other animals––and therefore also the traditional disciplinary divisions between the arts and the sciences.Rees T, Bosch T, Douglas AE (2018) How the microbiome challenges our concept of self. PLoS Biol 16(2): e2005358. https://doi.org/10.1371/journal.pbio.2005358
The authors are quite clear about what they understand our microbial companions to mean — the idea that humans are “more than mere nature” is an “untenable idea”:
As we see it, it is important but not enough to argue that “we have never been individuals” ––or to suggest that human and microbial worlds are inseparably “entangled” [30–32]. What is needed, in addition, is a whole new configuration of research, one where arts and science are combined. The challenge is 2-fold. Researchers in the life sciences have to learn that the stakes of their research are bigger than their expertise, and researchers in the arts have to learn to think the human––philosophy, politics, and poetry––beyond the now untenable idea that humans are more than mere nature.Rees T, Bosch T, Douglas AE (2018) How the microbiome challenges our concept of self. PLoS Biol 16(2): e2005358. https://doi.org/10.1371/journal.pbio.2005358
Everything we know about human beings shows that we are more than “mere nature.” Otherwise, we wouldn’t even be asking about it.
As a result, perhaps, the paper attracted a Cautionary Note from other scholars: “ While we strongly support interdisciplinary collaboration between the arts and sciences, the authors overinflate the microbiome’s influence in ways that are conceptually and empirically problematic.”
One of the essay’s authors (Douglas) has effectively refuted claims that the microbiome and host form a single evolutionary individual . The question of physiological unity is less clear. When the authors argue that our microbiomes “co-constitute” who we are, they most probably mean host and microbiome are physiologically and developmentally united, and that hosts such as humans would not function without their microbes. We agree that causal relationships between individual microbes or small groups of microbes and the host are very likely to impact how the host survives and thrives in daily life. But suggesting that the whole microbiome is causal is problematic (e.g., “the microbiome also plays a central role in the three processes that have traditionally been said to define the human self”). We simply don’t know what most of our microbes are doing, how much they contribute, and whether their effects are indispensable to human biology. This is not merely a matter of missing knowledge. Many microbiome participants are known empirically and theoretically to be transient, opportunistic, or even inactive (e.g., [5–7]). They are thus as much “human constituting” as many other organismal aspects of our environment, such as ticks or fleas.Parke EC, Calcott B, O’Malley MA (2018) A cautionary note for claims about the microbiome’s impact on the “self”. PLoS Biol 16(9): e2006654. https://doi.org/10.1371/journal.pbio.2006654
Doubtless, our microbiome influences our emotions and at times interferes with rational thought. But, apart from a need to promote a naturalist view of the human being (nature is all there is), there is no reason to believe it.
Yes, microbes are aliens to us but they do not make us what we are. They are along for the ride and, usually, the benefits.
You may also wish to read: Why do many scientists see cells as intelligent? Bacteria appear to show intelligent behavior. But what about individual cells in our bodies?