Wednesday, March 10, 2010
On 'Catching Fire:' Part 1: Summary of the Material Wrangham Presents
Edit: for convenience, I'm now linking all the other sections
Wrangham's book consists of an Introduction to his 'Cooking Hypothesis,' 8 chapters in which he develops both the hypothesis itself and its implications for the shape of human social evolution, and an Epilogue, in which he uses his findings as a compassionate platform from which to join the likes of Michael Pollan in urging us to take more responsibility for our food choices.
This is a very long post! I am sharing at such length because I really believe that it is interesting and important information. Here is a brief summary of what is contained in each chapter summary, so that if you don't have time to read it all at once, you can pick and choose.
The first three chapters motivate the hypothesis that cooking food would have conferred an evolutionary advantage. Chapter 4 and in part chapter 5 argue for the very early timing of this. Chapters 5-8 present the variety of impacts which cooking may have had on human evolution, from growing larger brains to the loss of body hair to the development of patriarchy to the evolution of language. I'll spend more time on Chapters 1-4 than on 5-8 simply because they are more interesting and really contain the 'meat' of his thesis.
In Chapter 1, Wrangham looks at two 'raw food' studies and some other anecdotal evidence and concludes that humans generally do not do well on raw diets. 'We are not like other animals. In most circumstances, need cooked food.' (sic) p36. We'll come back to this in part 2, as many of his arguments are worth noting for raw foodists.
Chapter 2 looks at why it might be that humans 'fare poorly on raw diets.' The concept of evolutionary trade-offs is introduced: humans don't climb as well as apes but walk and run far better; humans' digestive tracts are far smaller than apes' and humans thus are better at processing cooked food and less able to handle large amounts of highly fibrous plant matter. In this connection, he reports that the reduction in gut size saves humans at least 10% daily energy expenditure as compared to primates, who eat around twice as much per day by weight as humans (most of the extra weight being fiber).
A key claim of the book, introduced in the introduction, examined in most detail in Ch 3 and returned to in Ch 8 and the Epilogue in an appeal to responsibility, is that cooking increases the amount of energy obtained from foods. In this connection, he shows that all kinds of animals, from insects and fish through domestic pets to all kinds of livestock, grow more rapidly on cooked food than on exclusively raw foods.
He then argues that the smaller jaw and tooth size of humans, as well as the smaller stomach and intestines, reflect an evolutionary adaptation to eating cooked food. Digestion uses up a lot of energy, so when cooking began, natural selection favored the cooked-food eaters who were thereby saving energy, and favored smaller guts because these were able to process the food at a lower energetic cost. 'The high caloric density of cooked food suggests that our stomachs can afford to be small.' p43.
The key role of cooked food is further motivated by invoking the vital importance of plant foods in the human diet: humans need fat and/or carbs and (as is well known) cannot survive on lean protein alone - and meat is very lean in the dry seasons. There were no calorically dense fruits reliably available year-round, and seasonal scarcities would have led to reliance upon fibrous, starchy plant matter such as roots. Many wild roots are difficult-to-impossible for humans to eat raw (think taro, cassava), so in this situation cooking would have conferred a huge advantage. Dr Wrangham is an experienced field anthropologist, and some of his personal observations about the kinds of wild fruit available to chimps and other apes are really worth noting for raw foodists (see Part 2 for more on all this). Humans' smaller jaws and lack of resistance to meat-borne bacteria make raw-meat-eating an implausible alternative candidate for explaining this.
In Chapter 3, Wrangham addresses the confusion in the literature concerning how cooking affects the energy yield of a food. In his words, 'I believe that the effects of cooking on energy gain are consistently positive. The mechanisms increasing energy gain in cooked food compared to raw food are reasonably well understood. Most important, cooking gelatinizes starch, denatures protein, and softens everything. As a result of these and other processes, cooking substantially increases the amount of energy we obtain from our food.' p57. This quotation summarizes the material presented in the chapter, with several studies to show superior absorption of cooked versus raw starch and protein. He adds that softness is an important piece for increased digestibility as well as the chemical changes, citing Kyoko Oka's study in which two groups of rats were fed identical foods, except for the fact that one groups' food was softened: this latter group gained weight and the other group did not, and a study on Burmese pythons showing that cooking and grinding separately reduced digestion time by 12% but together, by 23.7%, pp. 78-9.
He acknowledges that cooking can lead to vitamin loss and the creation of some toxic compounds, but concludes that since 'life is mostly concerned with energy,' even at that cost cooked food is superior because the yield of more calories makes for better survival rates and longer lives. p81. (See Part 2 for much more on this!)
Chapter 4 builds on these suggestions about the importance of cooked food as a concentrated energy source and looks at archaeological and anatomical evidence to offer a date for the introduction of cooking.
Archaeology is of limited help, although it does show the use of fire in a food context as far back as 790,000 years ago (in Israel, at a time when northern Europe was ice-bound), pp. 83-8.
The main premise of this chapter is that animals are biologically adapted to their diet, and that in response to a change in diet, 'species tend to exhibit rapid and obvious changes in their anatomy…the tight fit between food and anatomy is driven by food rather than by the animal's characteristics.' p. 89. In other words, he is arguing that the anatomical characteristics of humans (small jaws and teeth, shorter guts) that reflect cooked-food eating are an evolutionary adaptation to eating cooked food.
Wrangham notes that many animals seek out cooked food, eating seeds that have been parched by forest fires that they do not touch in the raw state. He suggests that this is a pre-adapted preference, and cites brain studies that show that hard-wired responses to taste/texture, etc, 'are integrated in the brain with learned responses to the sight and smell of food.' p.92. So, the same selection criteria could be applied to cooked foods by animals a priori, and a predisposition to select softer and more tender foods further motivates the advent of cooking. Wrangham also adduces this as an argument against the possibility that fire was controlled by hominids for any length of time without its being used for cooking.
There is no sign of fire's use before 2 million years ago. Since then, there were only three times when changes were so rapid and drastic that the species name was changed (from habilines to homo erectus, from homo
erectus to homo heidelbergensis, and from homo heidelbergensis to homo sapiens. On the basis of anatomy, he argues that the earliest of these was the relevant time at which cooking was adopted. He suggests that controlling fire and cooking meat and roots led to less reliance on tree foods, reducing the need for climbing ability, while having a fire at night made it possible to sleep on the ground rather than in the trees. Once tree climbing was no longer necessary either for food or for safe sleeping, natural selection would also have favored the anatomical changes away from climbing ability and toward higher running and walking ability.
On the basis of this, he fills in some more of the suggested picture: 'Effects of cooking include extra energy, softer food, fireside meals, a safer and more diverse set of food species, and a more predictable food supply during periods of scarcity. Cooking would therefore be expected to increase survival, especially of the vulnerable young. It should also have increased the range of edible foods, allowing extension into new biogeographical zones.' p95.
Chapters 6-8 consist of a fleshing out of the details and implications of the above quotation. First, in Chapter 5, he presents and develops the hypothesis that the reason why humans are more intelligent than other primates is also because of their use of cooked foods during evolution.
An external motivation for intelligence is social competition - but some social primates have very small brains. A major variable among animals is 'how much brainpower the body can afford. For this reason the quality of the diet has been identified as a key driver of the growth of primate brains.' pp.105-6 He believes that cooking must have been a key player in this for humans.
Citing work by Aiello and Wheeler showing that in many different species (including fish and birds), larger brains correlate with smaller guts made possible by a high-quality diet, he suggests that this sam trade-off explains the difference between primate and human brains. Humans and primates have equivalent basal metabolic rates, so it's not simply a function of higher metabolism that the human brain receives more glucose to allow more activity. As he's already discussed in earlier chapters, length of the gut is the main variable among primates. This is the 'expensive tissue hypothesis:' primates whose intestines need less fuel to do the work of digestion have more fuel to spare to power brain tissue.
He then goes over various increases in brain size in early primates, showing increase in brain size correlating with various primitive food softening techniques (cf. chimps wadding meat with leaves, and the pounding/hammering of meat), again leading to his conclusion that cooking began with homo erectus, and explains later and relatively small increases in brain size as being due to further refinements of cooking techniques.
So, his conclusion is that it was the use of cooking in evolution that enabled the human brain to become uniquely large.
In Chapter 6, he turns to the implications of cooking for development of society. Citing the enormous saving in required chewing time, he suggests that (since cross-culturally, men hunt and women gather) this would have freed up men to go hunting all day, with the safe expectation of a meal when they got home that they could safely eat on the ground (protected by the fire) and that did not require many hours of chewing. This division of labor in terms of kinds of food collected by men and women respectively is unique to humans, as is the sharing of the proceeds. Defining family and household as being based upon 'a predictable economic exchange between women and men' (p. 146), he suggests that cooking is what made this possible.
In Chapter 7 he further ties this to the development of patriarchy and the subjugation of women, based on the universal status of domestic (as opposed to ceremonial) cooking as 'women's work.' He argues that because foods are brought to camp for preparation instead of being eaten where found, cooking creates a social contract situation.
Furthermore, he hypothesizes, especially in the early days of cooking it would have been hard for a cook to protect her food from marauders, and that this motivated the development of pair bonds. 'Having a husband ensures that a woman's gathered foods will not be taken by others; having a wife ensures the man will have an evening meal.' He characterizes this scenario as a 'primitive protection racket.' p.154.
He fleshes out this picture by examining food sharing practices in various modern hunter-gatherer cultures and also reporting instantiations of male food guards, female food suppliers and respect for another's possessions amongst other animals.
Chapter 8 is the most speculative chapter. Wrangham opens it by suggesting that we don't know how deeply our DNA is affected by the technology of cooking, and cites a number of possibilities.
He notes humans' greater longevity, which correlates with better escape from predators, and suspects a role of fire in this.
He suggests that soft food could have allowed earlier weaning and thus more frequent pregnancies, affecting social behavior by mandating babysitters, and also promoting more rapid growth in the young.
Even now, hunter-gatherers have a hard time in the 'lean seasons,' but he suggests that cooking mitigates this by expanding the possibilities in terms of what is actually edible. He points out that this challenges the 'thrifty gene hypothesis,' which claims that humans evolved to store fat easily to withstand periods of famine. Instead, he says, cooking 'It implies that humans easily become obese as a result of eating exceptionally high-energy, calorie-dense food, rather than from ancient adaptation to seasonality. Great apes become obese in captivity on a rich diet of cooked food.' p181.
He hypothesizes, 'given our uniquely long exposure to ingesting them in high concentrations' pp. 181-2, that we may be better able to process Maillard molecules and other inflammatory and carcinogenic agents that are by-products of cooking. This is the flip side of our reduced tolerance (compared to other primates) for tannins and other toxins in plant foods that he discusses in Ch. 2.
In final broad speculation, he correlates the use of fire with the development of humans' wonderful running abilities, via the loss of body hair. Other primates get overheated from even short bursts of running. And, 'every species other than humans can maintain adequate body heat without fire.' p. 182. Furthermore, he suggests that the communality of cooked meals around a fire would have taught co-operation and led natural selection to favor more tolerant individuals - and that this may even have led to the development of language.
Wrangham's Epilogue shows that the modern sweet, soft, smooth, mass-produced foods are the logical conclusion of this evolutionary path, addresses the problem of the fact that calorie counts do not tell us all that we need to know, and urges greater discernment and understanding in our selection of food.
In a very compassionate perspective, he declares that the bland/mass-produced/energy-dense, micronutrient-poor foods that cause excess 'are the foods we have evolved to want.' p.195. His mobilizing cry is to eat lower-density foods: 'We need to become more aware of the calorie-raising consequences of a highly processed diet.' p.196.
He reviews all the evidence that he had discussed earlier in the book that demonstrates that the structural complexity of a food as well as its basic macronutrient profile matters for how many calories can be extracted from it. He then explains how the calorie counts universally relied upon today were arrived at - and 'the human body is not a bomb calorimeter' p.202. - and points out two major problems with it. First, it does not take into account the costs of digestion (which vary depending on food type and also vary between people, with leaner people tending to have higher digestion costs than obese people, although which way the causality goes here is unclear). Second: it assumes that the proportion of food digested/absorbed remains constant regardless of whether it is in a liquid or solid, raw or cooked, higher or lower-fiber context. He has presented lots of evidence to show that these are important variables.
So, he ends with a call to modify Atwater's convention to take into account the ways that the physical structure of a food influence its nutritional value and joins Michael Pollan in advocating real/whole foods as the basis of the diet. 'The less processed our foods, the less intense we can expect the obesity crisis to be.' p.206.
He concludes that the unique flexibility and adaptability of humans that has enabled survival on such a range of diets (from all-plant to all-meat) was a response to the uniform problems in the early environment and that this adaptability is now being challenged by the new situation of plenty, particularly plenty of very easily-digested foods. In our new living conditions, the technology that enabled our evolution and survival, taken to its logical conclusion, is damaging our health.
This is one of the things that I am going to explore in Part 2: that our living conditions are different now from in our evolutionary past. I am going to suggest that this means that just because cooked food was a key factor in our evolution to where we are now, it is not necessarily a key to our current well-being.
I also feel compelled to write some responses to his critiques of modern raw-foodism and raw-foodists: I accept the importance of his negative stance because of the hypothesis that he is developing, but I feel that even within the context of his own writing this attitude is not essential.