The Body Politic § SEEDMAGAZINE.COM
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Our bodies harbor 100 trillion bacterial cells, outnumbering our human cells 10 to one
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It’s also been easy for science to overlook their role in our bodies and our health.
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But there’s a growing consensus among scientists that the relationship between us and our microbes is much more of a two-way street. With new technologies that allow scientists to better identify and study the organisms that live in and on us, we’ve become aware that bacteria, though tiny, are powerful chemical factories that fundamentally affect how the human body functions. They are not simply random squatters, but organized communities that evolve with us and are passed down from generation to generation. Through research that has blurred the boundary between medical and environmental microbiology, we’re beginning to understand that because the human body constitutes their environment, these microbial communities have been forced to adapt to changes in our diets, health, and lifestyle choices. Yet they, in turn, are also part of our environments, and our bodies have adapted to them. Our dinner guests, it seems, have shaped the very path of human evolution.
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But the emerging science of human-microbe symbiosis has an even greater implication. “Human beings are not really individuals; they’re communities of organisms,” says McFall-Ngai. It’s not just that our bodies serve as a habitat for other organisms; it’s also that we function with them as a collective.
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To find a biological answer to the question “Who are we?” we might look to the human genome.
Certainly, when the Human Genome Project first produced a draft of the 3 billion-base-pair sequence, it was touted as a blueprint for human life. Less than a decade later, however, most experts recognize that our genomes capture only a part of who we are. Researchers have become aware, for example, of the influence of epigenetic phenomena — imprinting, maternal effects, and gene silencing, among others — in determining how genetic material is ultimately expressed. Now comes the notion that the genomes of microbes within us must also be considered. Our bodies are, after all, composites of human and bacterial cells, with microbes together contributing at least 1,000 times more genes to the whole. As we discover more and more roles that microbes play, it has become impossible to ignore the contribution of bacteria to the pool of genes we define as ourselves. Indeed, several scientists have begun to refer to the human body as a “superorganism” whose complexity extends far beyond what is encoded in a single genome.
Certainly, when the Human Genome Project first produced a draft of the 3 billion-base-pair sequence, it was touted as a blueprint for human life. Less than a decade later, however, most experts recognize that our genomes capture only a part of who we are. Researchers have become aware, for example, of the influence of epigenetic phenomena — imprinting, maternal effects, and gene silencing, among others — in determining how genetic material is ultimately expressed. Now comes the notion that the genomes of microbes within us must also be considered. Our bodies are, after all, composites of human and bacterial cells, with microbes together contributing at least 1,000 times more genes to the whole. As we discover more and more roles that microbes play, it has become impossible to ignore the contribution of bacteria to the pool of genes we define as ourselves. Indeed, several scientists have begun to refer to the human body as a “superorganism” whose complexity extends far beyond what is encoded in a single genome.
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In 2007, Cornell University microbiologist Ruth Ley coauthored a paper arguing that human microbiome studies could bridge the divide between biomedical and environmental microbiology.
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William Karasov, a physiologist and ecologist at University of Wisconsin–Madison, believes that the consequences of this new approach will be profound. “We’ve all been trained to think of ourselves as human,” he says. Bacteria have been considered only as the source of infections, or as something benign living in the body. But now, he says, it appears that “we are so interconnected with our microbes that anything studied before could have a microbial component that we hadn’t thought about.” It will take a major cultural shift, says Karasov, for nonmicrobiologists who study the human body to begin to take microorganisms seriously as a part of the system.
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Equally challenging, though in a different respect, will be changing long-held ideas about ourselves as independent individuals. How do we make sense of this suddenly crowded self? David Relman suggests that how well you come to terms with symbiosis “depends on how comfortable you are with not being alone.” A body that is a habitat and a continuously evolving system is not something most of us consider; the sense of a singular, continuous self is a prerequisite for sanity, at least in Western psychology. A symbiotic perspective depends on a willingness to see yourself as the product of evolutionary timescales. After all, our cells carry an ancient stamp of symbiosis in the form of mitochondria. These energy-producing organelles are the vestiges of
symbiotic bacteria that migrated into cells long ago. Even those parts of us we consider human are part bacterial. “In some ways, we’re an amalgam and a continuously evolving collective,” Relman says.
symbiotic bacteria that migrated into cells long ago. Even those parts of us we consider human are part bacterial. “In some ways, we’re an amalgam and a continuously evolving collective,” Relman says.
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He also believes that we might have something to gain by embracing our bacterial side. Bacteria are often dismissed as simpler, less sophisticated, and less worthy of our consideration. “We put a lot of weight on a life form’s ability to think independently,” Relman says, but microbes have achieved fantastic evolutionary success by operating on a very different principle. Microbial communities are filled with examples of self-sacrifice for the benefit of the larger colony.
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