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Catching Fire: How Cooking Made Us Human

Gene ExpressionBy Razib KhanJune 21, 2009 8:21 PM


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A few weeks ago I commented on Richard Wrangham's discussion with Robert Wright. Though most of the conversation was given over to the arguments in Wrangham's latest book Catching Fire: How Cooking Made Us Human, I focused on the older Demonic Males: Apes and the Origins of Human Violence. One of the main reasons is that the latter was a book I had read. A few days ago I managed to get through Catching Fire. Though the content wasn't particularly surprising or novel, Wrangham has been articulating the general model for years, the details were of interest and at ~200 pages it was a quick read. In some ways it resembles typical X-Made-Man books, such as Robin Dunbar's Grooming, Gossip, and the Evolution of Language, and so can get tiresome. Rather than focusing language, or human sociality, tool use, or the consumption of meat, Wrangham naturally draws everything back into the use of fire and cooking. Despite this the argument is so heavily larded with specific detail that even if you find the emphasis on the overall thesis a bit heavy-handed it's worth a read. Obviously publishers would be less than excited by a book titled "How cooking, language, tools and bipedality made us human," but I think some of these other factors are left implicit in the text. Catching Fire's basic thesis is interlaced throughout the narrative, but there's a definite disciplinary progression. Naturally Wrangham starts with basic chemisty, physiology and anatomy. Contemporary experiments and observations loom large. It may not be On Food and Cooking: The Science and Lore of the Kitchen, but it comes close at moments. Then he proceeds to paleoanthropology and other assorted historical sciences. Finally he gets to the soft meaty heart of the implications of his thesis for us moderns, surveying the treacherous terrain of ethnography and cultural anthropology. There's also an epilogue which seems to be tacked on for some public policy relevance, and has somewhat the feel of Michael Pollan's latest book. The first few chapters which focus on chemistry and physiology are the most interesting for low-level datavores. A clear and compelling case is made that cooking food makes it so that an organism can extract the maximum amount of energy per unit. I was interested to find out that Claude Lévi-Strauss argued that human taming of fire was critical for psyhcological reasons, in particular setting us apart from beasts. That is, there were no material consequences of human control of fire, something which might surprise anyone who knows the plot of The Jungle Book. Wrangham presents some paleontological data in regards to fire use which dates to nearly 1 million years in the past, which supports his contention that the control of fire and the emergence of cooking likely was a feature of Homo erectus. There are other circumstanial pieces of information which buttress this contention. To me the most persuasive one was the likelihood that Homo erectus slept on the ground because of their full bipedality, and the only plausible way they could defend themselves against predation by large animals would be a communal fire. Additionally apparently there are other clear signs of dental and digestive changes in erectus which suggests a sort of "great leap forward" vis-a-vis Homo habilus. I do not know much about paleoanthropology when it come to fossils, but this is something that I have found to be generally true. Homo erectus was a large and efficient bipedal machine, when it comes to goings on below the neck our own species has only changed marginally. Though our cranial capacities have increased in the past few million years, Homo erectus was likely qualitatively different from habilus in this regard. The former was more a man and the latter was more an ape. Of course there's another model for changes in terms of consumption that explains the erectine leap forward, that our lineage shifted from being a predominantly vegetarian one as are all primates, to a meat eating one. The arguments are laid out in books such as The Hunting Apes: Meat Eating and the Origins of Human Behavior. Many of the physiological and anatomical details which suggest a shift to cooking have also been argued for a shift to meat, so Wrangham must review both models and make a case for why cooking is superior to simply meat. First, the case for why there was likely some sort of dietary shift. Humans, for our size, have small guts, small teeth, small jaws, etc. We're simply not very adept at consuming large quantities of low quality food, as is evident when starving human populations make recourse to grass, bark, and such. Our cousins the gorillas, and to a lesser extent the chimps, are. The small gut is a critical point, because it is metabolically an expensive organ. The digestive system has to use energy to generate energy. Another organ which is expensive is our brain, which utilizes about 20% of our body's metabolism, as opposed to 10% for a typical mammal. How can we get the math to work out? Consume food which is much more energetically rich, and which is easier to process. Meat would seem to fall into this category, and true enough carnivores have much smaller digestive systems, and often relatively large brains compared to herbivores. Bu there are differences between the digestive processes of conventional carnivores and humans which imply that the analogy is imperfect. A look at human teeth makes it clear we're not specialized in the same way cats or dogs are, and our stomaches do not seem to break down meat through physical action as in these carnivores. Meat may be rich in energy, but it is still relatively tough, and increasing surface area for digestive acids to work is critical. Humans do eat raw meat, but they are selective, preferring extremely soft and tender tissue, or meat which has been processed in some manner (e.g., burger patties). Reprocessing meat physically requires time and energy. Wrangham observes that this is one reason that chimps are opportunistic in their hunting, they simply don't get as much out of it as they otherwise would because of the difficulties of tearing into and consuming raw muscle. But how much of an energetic difference does this make? Through experiments which track the metabolism of various organisms it is reported that shredding meat increases net energy extraction by 12.5%, and cooking it increases it by 12.5%, and the two are additive so shredded and cooked meat may impart 1/4 more calories per unit than raw unprocessed meat. This is an enormous difference. This is probably a low bound estimate because the methods used did not seem to account well for how much energy shredding and processing would take, as well as the opportunity cost of these activities. The combination of cutting with tools and cooking seem like a sure fire way to increase the amount of calories per unit, and reduce the energy expended in preprocessing. Additionally, both cooking and shredding are important for vegetable matters. In extant small-scale hunter-gatherer societies there is a wide range of proportion of calories obtained from meat and vegetables, and the range is constrained by ecology and not human preference. The fact that humans can live on predominantly meat and vegetable diets, combined with our generalist dentition, suggests that an emphasis on the processing of food in addition to its form is the correct. We simply aren't as constricted as pandas or cats. Surprisingly most organisms also preferred foods cooked to non-cooked. This seems peculiar as humans are the only organisms who consume food cooked on a regular basis. What gives here? There are general differences between cooked and uncooked foods, and most species have an innate ability to discern when their food is more palatable. Similarly, though most of the fruits that chimpanzees in the wild consume are rather lacking in sweetness, they habituate themselves naturally to the much sweeter fruits which humans provide them. Their preferences are general and extend naturally to novel fruits and vegetables. As a point of possible future research Wrangham observes that cooking tends to generate chemical byproducts, some of which are known to be carcinogenic, and it is therefore likely that we have developed adaptations to battle the negative effects of cooking. There isn't much exploration of possibilities, but it seems likely that biochemists and geneticists may be able to elucidate the mechanisms which distinguish us from our relatives in this dimension. Obviously 1/4 more calories per unit has an adaptive significance. Aside from the anatomical and behavioral changes this might entail, there is an obvious natural experiment which might test what happens when humans shift away from cooked food. The "raw food" movement. Raw foodists tend to be thin. The reason behind this is simple: they gain fewer calories per unit from the foods they consume, and generally the foods they consume are lower in calories to begin with even if cooked. It is important to note that modern raw food enthusiasts have access to a wide array of goods thanks to modern agriculture, and can supplement their diet in a manner impossible in a premodern period such as adding a great deal of oil to their diet. Some research suggests that 1/3 of the calories for some raw foodists derives from these oils. I'm not particularly interested in the nutritional consequences, rather, more relevant are reports that 1/2 of female raw foodists stop menstruating and many of the males claim reduced sexual appetite. Raw foodists are generally loony New Age types, and apparently many believe that menstruation and ejaculation are processes which evolved to expel toxins from their bodies, so their decrease of sexual function is an indicator that they are no longer ingesting toxins. But from an evolutionary perspective this is a very strong argument that the ancient human diet was likely to not be raw; if modern raw foodists who have access to a wide variety of items exhibit such low potential reproductive fitness it is certain that pre-modern populations on the Malthusian margin would be far more likely to simply go extinct. It seems that no matter what you an say for the positive thesis, these sorts of data go a long way toward eliminating the competing models from the set of the plausible. Much of the second half of the book ranges over the implications of the high calorie and low energy processing requirements enabled by fire. By analogy, fire is a technology, and replaced human activity in terms of making food more palatable for our digestive system. Instead of chewing for 3 or 4 hours, we simply cook for a time and we can chew the food with relative ease. Similarly, steam replaced muscle power, and so made land shipment economically competitive with water transport for the first time in human history. This change had many consequences. In Catching Fire Richard Wrangham elaborates a theory for how cooking produced our normal sexual division of labor and more generally resulted in patriarchy in most small-scale societies. You may, or may not, believe it, and similar stories have been told about tools, or language, or the importance of meat acquisition. To some extent the cultural anthropologial inferences are interesting only once you accept the role of fire and cooking as a catalyst for human evolution, you may be able to construct plausible alternative explanations based on tool production and use. Unlike the earlier sections on the chemistry and physiology of food production, processing and consumption, the later chapters are difficult to unpack as they involve more detailed speculations which hinge upon many moving parts. Still fascinating, but in many ways less concrete and more "Just So." The last chapter jumps back to the beginning to some extent. A strong case is laid out that modern food labeling methodologies are imprecise, and may be resulting in the obesity epidemic we see around us. The research reported above strongly imply that the physical structure of food affects how much net caloric input we receive. Therefore, grains high in fiber tend to deliver fewer calories than extremely finely milled flour. To some extent this correlates with the glycemic index. The data here are numerous, and makes biological sense. I suspect few would quibble with the assertion in the abstract. Of course a 10-20% difference in calories between two food sources of the same substance of different higher order structures might be tolerable for "government work," but the extra caloric intake may make a great deal of difference in terms of long term weight gain. Reducing or increasing intake 10-20% may have an enormous effect on the caloric margins. Modern industrial processed foods are extremely rich and efficient sources of calories per unit. In contrast, "whole foods" tend to be much less efficient because of their complex structure, and so are much more expensive per calorie unit. The reason raw foodists lose weight so quickly is that they eat "whole foods" necessarily because of their dietary constraints. Wrangham doesn't engage in a jeremiad against the modern food industry, that isn't his brief, rather, he is making a case that assays of calories should be reconsidered. Approximations which might justifiable in terms of basic science may have extremely significant public health impacts in the applied domain. As with most science the "moral" of the story is for you to draw. Though I think I can infer the author's general attitudes when it comes to processed foods there isn't a wholesale assault upon them. After all one reason that processed foods are "bad" for us is that they "fit" with the capacities of our digestive system so well! Fire & cooking were simply the first steps in the long progression toward the industrialization of food processing. Similarly, the specialization enabled and enforced by cooking may simply be a precursor to the modern economy, where specialization results in gains of efficiency and productivity. There are critics who might claim that the specialization as gone too far.

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