What if…
The word surprising is appearing at high frequency in sciences of the nutritive sort these days. In a recent interview, evolutionary biologist Eric Axelsson said that he expected to find differences in nervous system genes when doing systemic genome comparisons between dogs and wolves because of the behavioral differences between them. “But the ability to digest starch? ‘That was surprising,’ Axelsson [said]. ‘I mean, no one’s really been hypothesizing about any such changes. On the other hand, it makes sense.’” 36 It makes sense, in that it suggests coevolution with humans through eating together, with the ability to digest carbohydrates acting as a selective pressure driving these genetic changes—but not enough sense that Axelsson found it because he was looking for it. Other “surprising” findings from science news lately: the cells of the brain produce insulin; neural development is shaped by insulin; and brain tissue can become insulin-resistant, pushing some biomedical researchers to suggest that Alzheimer’s disease should be redescribed as “Type 3 Diabetes”—the New Scientist reviewed the evidence in an article introduced by the image of a brain made of chocolate with a piece broken off and the title, “Eat Your Way To Dementia.” 37 Eating and thinking, eating and inheriting: researchers have argued that high-fat diets in male mice can leave an imprint on the metabolisms of their daughters, even in the absence of any physical contact between fathers and offspring—perhaps mediated through the dietary alteration of epigenetic marks or molecules such as microRNAs transmitted through sperm. 38
And yet another surprise emanates from the exponentially expanding studies of the bacteria that live commensally with organisms in and on their bodies, particularly in their guts: “Recent research is revealing surprising roles for microbiomes in shaping behaviors across many animal taxa—shedding light on how behaviors from diet to social interactions affect the composition of host-associated microbial communities, and how microbes in turn influence host behavior in dramatic ways.” 39 Translation: bacteria in host bodies can directly affect their host’s behavior—at the same time, the eating and mating behaviors of the hosts can affect which bacteria live in them. This has been demonstrated in fruit flies, which show mating preferences for other flies that eat the same diets; in hyenas, whose distinctive clan-specific body odors are determined by the bacteria that live in their scent glands; in humans, whose different microbiomes differentially attract mosquitoes; and in laboratory mice, who have had their anxiety diminished with probiotics. This suggests that microbes affect speciation (by influencing mating and sociality) and disease distribution (through mosquitoes), and alter neural and endocrine activity in their hosts’ brains, affecting “how animals behave toward one another.” 40 The diversity of these examples, stretching from epigenetics to microbiology to endocrinology to behavioral ecology might seem to conflate an awful lot to the “metabolic” and the “reproductive,” but in a way this is exactly the point: these categories have been reopened to negotiation.
Thus the surprise elaborated above—of plant and insect and microbial RNA in the human body, doing who-knows-what—is not offered as proof or certainty; the results remain mixed, contested, and tenuous at this time. But the articulation of these findings as a discovery does offer a marker of both departure and return. A curious resonance occurs between Buffon’s notion of organic particles coursing through the blood, attracted to those parts of the body to which they are “most analogous,” and present-day descriptions of rice microRNA moving through the blood to bind to those parts of mammalian messenger RNAs with which they have sequence complementarity. This is the resonance of two models of the living from very different eras that both allow for the persistence of the eaten in the eater in ways that do not partition the alimentary from the historical.
Philosophers John Dupré and Maureen O’Malley recently suggested that metabolism is more useful in understanding living things and their evolution if it is seen in conjunction with reproduction. Reproduction and metabolism should be understood, they argue, at the level of systems of living matter, not as properties of “traditional,” individual organisms:
Standard discussions of characteristics of life […] tend to prioritize one or other of two fundamental but very different features of living things: the capacity to form lineages by replication and the capacity to exist as metabolically self-sustaining wholes. We suggest that this tension can best be resolved by seeing life as something that arises only at the intersection of these two features: matter is living when lineages are involved—directly or indirectly—in metabolic processes. 41
Where Dupre and O’Malley’s argument departs from the conventional use of metabolism is in their rethinking of what it means to be a “metabolic whole.” They alter the unit under consideration—from the organism, more traditionally understood, to the “living system.” The authors demonstrate through various examples that metabolism happens “collaboratively:” for example, humans do not digest and metabolize autonomously, but in concert with the numerous symbiotic bacteria that live in human bodies, the human microbiome mentioned above. These philosophers suggest that evolutionary forces act on reproductive-metabolic systems, not on “traditional,” individual organisms. A rather disembodied metabolism remains constitutive of their characterization of life as “the result of the interaction of lineage-forming, metabolically-collaborative matter, organized within different interacting levels.” 42 Metabolism itself, even in this unconventional pairing with replication, remains defined fairly straightforwardly as “the transformative biochemical reactions that sustain life processes.” 43
What if we go one step farther, and admit that these two “fundamental but very different features” of living things are generated in and by history, and are not innate to biological matter? Or at least, they are only one possible, partial description of it—historically powerful, practically actionable, and coming undone. Perhaps resolving the tension between them is to be done, in part, by recognizing the categories themselves as historically and culturally ingrained modes of apprehension of the living. Is it possible to think about the notion of nurture literally metabolizing nature without it having to be some kind of parasitism of one feature of life by another separate one? If we eat bacteria, and bacteria live in us, and their genetic material floats around in our blood stream doing who knows what, and their chemical signals interact with our gut cells and our brain cells, and probably our placental cells (for those of us who have them from time to time), and we eat corn and rice and fungi and honey and many other things that interact with chromatin configuration and gene transcription and cell division, and we inherit microbes from our families at the same time as bearing the imprint of changed gene regulation from our forebears’ diets—multiply inheriting and embodying the eaten—then, is it still possible to say that the metabolic and the reproductive are fundamentally different aspects of life, bound together for evolutionary convenience?
It is quite unclear what all these experimental surprises mean in terms of one another—what high-fat diet inheritance in mice has to do with carbohydrate digestion evolution in dogs or with microbiome-mediated mating preferences in flies to glucose-transporters in neural cells—in part because there is a manifest lack of theory to pin all these disparate findings to. There is a sense, however, of things breaking open, of new possibilities for theories of the physical basis of life, and, as I have argued above, a certain shock of recognition around deeply ingrained assumptions as to the separateness of various domains of life, assumptions exposed anew by their contradiction. What metabolism and reproduction are now in light of these changes is difficult to say. The categories of metabolism and reproduction are fraying—and this makes them unstable, rapidly changing terms in the present. At the same time it becomes possible to delineate more accurately what they were, as a very particular way of cutting up the body, and the world, into parts.
This work was funded in part by grants from the American Council of Learned Societies and the UCLA Center for the Study of Women. Thanks also to my many critical readers: Lieba Faier, Juliet Williams, Christine Chism, Sarah Stein, Barbara Fuchs, Jessica Cattelino, Purnima Mankekar, and Abigail Saguy.
- Véronique LaCapra,“Research Looks at Starchy Diet’s Role in Dog Evolution,” National Public Radio, 24 Jan. 2013.[↑]
- Bijal Trivedi, “Food For Thought: Eat Your Way to Dementia,” New Scientist 2880, 3 Sept. 2012.[↑]
- Sheau-Fang Ng, Ruby CY Lin, D. Ross Laybutt, Romain Barres, Julie A. Owens, and Margaret J. Morris. “Chronic high-fat diet in fathers programs β-cell dysfunction in female rat offspring,” Nature 467.7318 (2010): 963-966.[↑]
- Vanessa O. Ezenwa, Nicole M. Gerardo, David W. Inouye, Monica Medina, Joao B. Xavier, “Animal Behavior and the Microbiome,” Science 338 (2012): 198-199.[↑]
- Ezenwa et al. 2012: 199.[↑]
- John Dupré and Maureen A. O’Malley. “Varieties of living things: life at the intersection of lineage and metabolism,” Philosophy & Theory in Biology 1 (2009): 1-25.[↑]
- Dupré and O’Malley 2009: 16.[↑]
- Dupré and O’Malley 2009: 2.[↑]
