working with yourself: towards a human ecosystem
my gut bacteria and meeeeee
Your body is its own ecosystem.
You are not the only one that lives in your body, and that’s not the fault of a few stray parasites that you can get rid of with a juice cleanse and some antibiotics for a few weeks either: it’s by design. To many of us, the idea that our body is not our own can feel all too scary, but these other residents of our body aren’t here purely – or even majorly – in antagonism.
All the way back in 1885, Louis Pasteur (yes, the milk guy) argued that the presence of certain microbes is vital to complex life due to co-evolution of these two different modes of life. Yet, with the advent of enormously instrumental antibiotics and antiviral drugs, we’ve fallen away from the idea that microbes are vital to our own life until recent years when gut microbiome research and pop science has taken off.
The term “holobiont” is used in studies of coral, lichen and organisms with distinct sexual and asexual stages, but it has begun to also be applied to the human body. With “holo” meaning whole, and “biont” a mode of life, this word describes a host and the organism living within and around it.
For us, the human holobiont can describe our body and all the microbes living within the walls of our skin. It’s putting the metaphor of an ecosystem onto the body we live in, and the lense of an ecologist on in medical settings.
While estimates vary widely, there is consensus that human cells are outnumbered within our own body. Some call for a 10:1 ratio of non-human bacterial to human cells, while a 2016 study lowered that ratio, measuring bacterial and human cells at close to the same amounts. This study estimated bacterial cells in the average person to amount to about 0.20 kg, which is about 0.4 lb. Roughly, that’s the weight of a head of lettuce made up of bacteria in your body.
We wouldn’t survive without these other individuals in our body. There has never been a time that our microbiome can survive without us, nor one that we can survive without it.
Before birth, we are dependent on our mother and cannot exist as an individual without her (or other human aid). At birth, we are instantly barraged with a flourishing microbial community in the vaginal canal and we co-evolved with microbes we normally get through the vaginal canal. Babies born via C-section are exposed to fewer and different microbes at birth because of the different environment they enter the world through, and this has been linked to higher rates of asthma, allergies, and obesity. C-section babies are so much more likely to get sick from these kinds of diseases that there are efforts to introduce microbiota to C-section babies early in life.
We also need microorganisms living inside our body to help us digest certain food, and to help prevent attacks on our body. Our gut microbiota facilitates digestion of what our own digestion systems cannot (like fiber). A healthy gut microbiome means a wide array of digestive enzymes different from those our body produces on its own. In this way, gut microorganisms function as a community of cells capable of cooperation with our body to help achieve the same goals of digestion. Our gut microbiome is so integral to our life that some call this group of symbiotic organisms in our digestive system its own “microbial organ” in our body.
Not only do bacteria help us survive, but they also can influence us in return. A fecal transplant (the vehicle for a microbial community transplant, like a group of noxious bathroom hitchhikers) has been shown to transfer not just bacteria, but also behavioral patterns from donor to recipient. Anxiety-like behavior and depressive symptoms can be transferred in this manner. Microbes in our body can communicate with our cells and our brain, with recent studies showing that gut bacteria utilize the nervous system to influence the host’s emotional and behavioral responses. Even our minds, which are often spoken of as separate from our body, cannot be extradited from the biome of our body.
Of course, having a host of bacteria, archea, viruses, and fungi living within the thin barrier of our skin can also cause a host of problems. Even our gut microbiota can harm us when their composition is out-of-flux. All of us have been the victim of a particularly nasty microbe-induced cold, flu, or infection. But as viewing these other organisms as invasive species trying to survive and reproduce in the foreign landscape of our body instead of an evildooer coming to hurt us, we are able to focus on holistic remedies that can provide long-term relief.
For example, my father has gone through some health troubles recently, and different drugs or surgeries targeting the problem have been helpful in the short-term, but the pain has come back. The problem? Not the medical help he was given, but the pieces of the puzzle he wasn’t listening too, the fact that some lifestyle changes like a better diet and more exercise would help the pain too. After his most recent surgery, he’s taken on physical therapy homework with a greater gusto and started steps to improving overall lifestyle health. This latest surgery has given him a lot better long-term recovery than his previous cortisone shots and smaller surgeries, partially because of the different procedure, but partially because of his change in response. If we view our medical problems not as something separate from us, but something we have to work with and against — just as one would with an invasive species, or an underpopulated predator species on the brink of extinction — we can better address these issues.
Through our natal hitchikers, gut microbiome, and even the harmful effects of bacteria on our bodies, we can better view ourselves, integrate the oft-seperated body and soul, by viewing ourselves as an ecosystem, a holobiont.
Arguments against the framing of the human body as a holobiont raise the fact that the gut microbiome fluctuates throughout our life with our diet — but an ecosystem fluctuates throughout time too as animals are born, die, and immigrate and emigrate. They also bring up that gut microbiomes between humans vary without dramatic effects on this hosts. Bringing this concern into the realm of community ecology also brings up problems. A hardwood forest on a mountain in Vermont will have a slightly different species composition than one in the lowlands of New York — or even at a different elevation point on the mountain. But these hardwood forests are still all the same biome; that diversity within one biome is part of what makes ecology so interesting and hard to study.
If our body is not one organism, but an ecosystem of many, the path to reach this current place of holobiont homeostasis had to be one of cooperation of these individuals over time. In the late twentieth century, microbiologist Lynn Margulis was instrumental in the discovery and proliferation of the now widely-accepted theory of endosymbiosis. This idea, which she published first in 1967 under the name Lynn Sagan, outlines a bacterial origin for eukaryotes (all plants, animals, fungi, and some others): the modern eukaryotic cell was a result of secondary symbiosis between bacterial cells. A proto-eukaryotic cell engulfed independent bacterial cells, and parts of these bacterial cells has stayed within these eukaryotic cells to this day. Margulis suggested that mitochondria and chloroplasts (food-making organs for animal and plant cells) come from bacterial organisms that were consumed, and still retain many of their unique bacterial characteristics. This symbiosis allowed for rapid diversification and proliferation of eukaryotes that wouldn’t have been possible without the cooperation of these bacterial partners. Further, endosymbiosis can be seen as a representation of how well the different individuals in our human holobiont are cooperating for a common goal of survival. Down to the insides of our cells, we have never been one individual, and that’s not to mention intracellular parasites and symbiotes.
If the majority of cells in our body are not human, and those that still carry the scars of a different past, where do we draw the line between one individual and the next? Some would argue that a grove of aspens is one organism, as they are genetically identical, reproducing without seed but instead via vegetative shoots. Others still might argue that a colony of bees or ants act so eusocially that they should be considered closer to one superorganism rather than a litany of individuals. Why would humans be an exception in this complicated reality of defining an individual?
I cannot say with confidence that I am one whole existing within my body, but rather a collection of moving parts that create one superorganism. I’ve felt both at competition and cooperation with the other elements of my body’s ecosystem — and this interplay between competition and cooperation within our human environment is the basis for development and evolution in this ecosystem.