Thursday 15 November 2012

The Raw and the Cooked: Apologies to Claude Lévi-Strauss

As you might imagine, I've been wrestling in silence with Richard Wrangham's cooking hypothesis for some time now. This is what Publishers Weekly had to say about Catching Fire, RW's popular book on the subject.
Contrary to the dogmas of raw-foods enthusiasts, cooked cuisine was central to the biological and social evolution of humanity, argues this fascinating study. Harvard biological anthropologist Wrangham (Demonic Males) dates the breakthrough in human evolution to a moment 1.8 million years ago, when, he conjectures, our forebears tamed fire and began cooking. Starting with Homo erectus—who should perhaps be renamed Homo gastronomicus—these innovations drove anatomical and physiological changes that make us adapted to eating cooked food the way cows are adapted to eating grass. By making food more digestible and easier to extract energy from, Wrangham reasons, cooking enabled hominids' jaws, teeth and guts to shrink, freeing up calories to fuel their expanding brains. It also gave rise to pair bonding and table manners, and liberated mankind from the drudgery of chewing (while chaining womankind to the stove). Wrangham's lucid, accessible treatise ranges across nutritional science, paleontology and studies of ape behavior and hunter-gatherer societies; the result is a tour de force of natural history and a profound analysis of cooking's role in daily life. More than that, Wrangham offers a provocative take on evolution—suggesting that, rather than humans creating civilized technology, civilized technology created us. Copyright © Reed Business Information, a division of Reed Elsevier Inc. All rights reserved.
Ringing praise, I'd say. I could spend weeks going after the plethora of assumptions that the author incorporates in the thesis from a post-modern anthropological point of view. Suffice it to say that I have issues with it.
     But that's not why I've called you here today. Lately, I came across another contribution to the literature on cooked food and primate brains. The only advice I could have given the authors while they were still writing is: 'Look before you leap.' Karina Fonseca-Azevedo and Suzana Herculano-Houzel, ‘Metabolic constraint imposes tradeoff between body size and number of brain neurons in human evolution.’ PNAS 109:18571-18576, 2012 (published ahead of print October 22, 2012, doi:10.1073/pnas.1206390109)* wrests a far-reaching hypothesis from some interesting brain- and body-size data on a range of primates in relation to the number of hours each species spends feeding. And here it is.
... by showing that metabolism is indeed limiting at physiologically relevant combinations of body [mass] and [brain mass], our data provide evidence that metabolic cost is limiting enough to impose tradeoffs in brain evolution, and thus offer direct support for the proposition of Wrangham (Wrangham RW, Jones JH, Laden G, Pilbeam D, Conklin-Brittain NL. 'The Raw and the Stolen. Cooking and the Ecology of Human Origins.' Curr Anthropol 40:567–594, 1999; Wrangham RW. Catching Fire: How Cooking Made Us Human, Basis Books, New York, 2009) that such a metabolic limitation was overcome in the human lineage by the advent of cooking food, which greatly increases the caloric yield of the diet, as a result of the greater ease of chewing, digestion, and absorption of foods (Urquiza-Haas T, Serio-Silva JC, Hernández-Salazar LT. 'Traditional nutritional analyses of figs overestimates intake of most nutrient fractions: A study of Ficus perforata consumed by howler monkeys (Alouatta palliata mexicana).' Am J Primatol 70:432–438, 2008; Carmody RN, Wrangham RW. 'The energetic significance of cooking.' J Hum Evol 57:379–391, 2009; Carmody RN, Weintraub GS, Wrangham RW. 'Energetic consequences of thermal and nonthermal food processing.' Proc Natl Acad Sci USA 108:19199–19203, 2011). In line with this proposition, a cooked diet is preferred by extant nonhuman great apes (Wobber V, Hare B, Wrangham R. 'Great apes prefer cooked food.' J Hum Evol 55:340–348, 2008). Although the earlier addition of raw meat to the diet of earlier hominins may also have contributed to increase its caloric content (Milton K. 'A hypothesis to explain the role of meat-eating in human evolution.' Evol Anthropol 8:11–21, 1999), raw meat is difficult to chew and ingest, whereas cooked meat is easier to chew and has a higher caloric yield (Wrangham 2009; Carmody et al. 2011). Besides increasing the caloric yield and making previous metabolic limitations irrelevant, cooking would also have increased the time available for social and more cognitively demanding activities, which in turn would impose a positive pressure for increased numbers of neurons, now affordable by the new diet. We propose that the combination of a newly affordable larger number of neurons with the accompanying time now available to use these neurons in cognitively demanding tasks that improved species fitness drove the rapid increase in numbers of brain neurons encountered in human evolution from H. erectus onward (Herculano-Houzel S, Kaas JH. 'Gorilla and orangutan brains conform to the primate cellular scaling rules: Implications for human evolution.' Brain Behav Evol 77:33–44, 2011)
This article gives me an opportunity to unpack a crucial concept in evolution that I think is lost on a great many, and to counter a few of the ideas floated in the authors' conclusion, quoted above. For example, I could remind the cooked food adherents that a circumpolar subset of the human species subsists almost entirely on raw meat, which these authors have decided is 'difficult to chew and ingest.' [I won't stoop so low as to point out to the authors and to the PNAS referees that in this characterization of raw meat the authors must in fact mean 'difficult to digest,' not 'ingest.' And since when was sushi difficult either to chew or digest?] A case in point. The people known as the Inuit in Canada, heretofore known by the derogatory label 'eaters of raw meat--or Eskimo' can ill afford to cook anything in lands that are almost bereft of combustible vegetation. [Although, I suppose, Wrangham and others would counter by saying that this would explain why the Inuit didn't invent Western, Industrialized society. They obviously lost out in that evolutionary game thanks to their diet depauperate of cooked food.]
     I could also bring up the issue of what primates actually eat, and the difference between a diet comprising largely indigestible leaves--that of gorillas, for example--and one composed largely of ripe fruit or nuts. Or, that humankind's digestive system has the architecture of a frugivore, which is what truly 'allows' shrunken guts [to quote Publishers Weekly again, but also through them, Wrangham himself]. In the same vein, gorillas do not have the gut of an obligate browser, like Bos, and thus not only do they rely on low-quality food, but also they are incapable of extracting more than a small percentage of the nutrients contained therein. If humans were to subsist on a diet of leaves, I can well imagine that we'd be chewing most of our waking lives, with nothing more material to show for it than piles of caw-caw that resemble meadow muffins or more social than a group dump site. But, enough about the Cooking 'R' Us crowd and their sometimes silly assumptions.
     The major point I want to make today has to do with the way these authors seem to be thinking about evolutionary change and natural selection. I'm talking about the idea of what's called 'selective pressure,' or more generally, 'adaptation.' In their final statement, Fonseca-Azevedo and Herculano-Houzel aver that in our fossil ancestors 'cognitively demanding tasks that improved species fitness drove the rapid increase in numbers of brain neurons' [emphasis added]. Perhaps it's just a poor lexical choice, but use of the word 'drove' in this context more than suggests that the authors believe evolutionary change amounts to natural selection for advantageous traits. Indeed, the notion that nature 'selects' anything good in a species' genome allows the authors and their compadres to make the assertions they do.
      Remember your introduction to evolution? You learned, for example, about the English moths during and after the worst depredations of the Industrial Revolution. During the time of maximum air pollution and the accumulation of what amounted to soot on tree trunks, the lighter coloured moths all disappeared and dark-coloured moths predominated. Know what was happening? No, Nature wasn't gracing the dark-coloured moths because they blended into the colours of the soot-covered tree trunks. Nature was showing its callous side by arranging it so that the light-coloured moths stood out like sore thumbs on the tree trunks where they used to blend in, and were thus being preyed on more readily, and more thoroughly than ever before, and all that was left were the darker coloured variants that had always existed, but which were always genomically swamped by the better-camouflaged lighter coloured variety. In short, evolution works by selection against, rather than for any latent or novel genetically determined trait in a population. 
     As such, I think it's clear that it's altogether misguided [nay, wrong!] to conclude that the encephalization of Homo erectus and later hominids was the result of a 'combination of a newly affordable larger number of neurons [and the] time now available to use these neurons in cognitively demanding tasks that improved species fitness [which] drove the rapid increase in numbers of brain neurons ... from H. erectus onward.' In like wise, Wrangham's major conclusion-- that 'cooking enabled hominids' jaws, teeth and guts to shrink'--is failed before it even gets off the page. Not just, mind you, for the same reason as Fonseca-Azevedo and Herculano-Houzel's thesis. Instead, there are any number of possible explanations as to why cooking might have conferred an advantage on the larger-brained hominids that were capable of using fire. One possibility that comes to mind is that spending more time making, tending, and using a fire to cook would require a sheltered and in that way protected environment, such as a cave or rock shelter, or a forest glade. That alone might have lessened the chance that bigger-brained hominids would be preyed upon. So, you see, by coat-tailing on the claims for fire use more than a million years ago [about which you know what I think], Wrangham and others have generated a host of 'just-so' evolutionary scenarios that are sufficient to explain encephalization in hominids, but which are certainly not necessary for it to have occurred.
     I'm done with cooking for now. But be relieved. I could have totally gone off on Wrangham for his ideas about Homo erectus cooking, 'table manners,' 'sexual division of labour,' 'pair bonding,' and what. ever.
* Did you ever wonder what they name the kids when two people with hyphenated last names get married? 

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  1. Another good one, Rob. I could not agree more about selection "against" and have written about it: Davidson, I. (2007). 'As large as you need and as small as you can'--implications of the brain size of Homo floresiensis. In A. Schalley & D. Khlentzos (Eds.), Mental states: evolution, function, nature (pp. 35-42). Amsterdam: John Benjamins

  2. Thanks, ID. I had you in mind while I was writing this. If I hadn't thought of independently, and well before we ever met, I'd have cited you! ;-)
    [Sorry. I hope that wasn't sleazy of me.]


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