Archive for June 2011
Writing about the high density-favoured digestion (deriving more break down products from each unit of resources acquired) and re-production (deriving more grand-offspring from each offspring produced) in the last post reminded me of two popular science books written by academics that I read this spring. In effect, they have suggested the importance of these (in one case the former, and in another the latter) in human evolution.
In Catching Fire: How Cooking Made Us Human, Richard Wrangham proposed that fire, particularly for cooking (a form of pre-digestion), made possible the distinct human suite of adaptations and that foraging versus cooking was our primordial division of labour by gender. In Chpt. 3, “the energy theory of cooking” he well documents the evidence that cooked food yields more calories than raw, no matter the type of food, but he may not be quite right about why. He thinks it is because of a) “spontaneous” (i.e. purely phenotypic) benefits as demonstrated by the fact that even captive animals gain weight from cooked diets and b) evolutionary benefits owing to the reduced costs of a smaller gut once we began cooking (pp. 39-40). Because costs are difficult to measure, the more usual evolutionary ecological or socioecological logic is to hypothesize (and ideally demonstrate empirically) what logically would be most beneficial under different ecological or social conditions and to conclude from that what would be selected for, given equivalent costs. So what favours digesting more over eating more? Most obviously, high densities i.e. crowded conditions relative to resources do of course. If resources are plentiful, eat more; if they are scarce digest more i.e. derive more break down products from each unit of resources acquired – sometimes called efficiency over productivity. It is difficult to imagine that maintaining fire and cooking is actually cheaper than just picking up and eating raw food – it is unlikely to be cheaper, just more beneficial under conditions of food scarcity. It was apparently more beneficial enough to early humans to overcome the additional costs. The situation of relative (raw) food shortage was probably related to a cooling climate and the shift from forest to savannah-dwelling. These latter were presumably initially adapted to in an earlier phase by increased meat consumption as well as by the use of a variety of tools not only for acquiring food (digging sticks, weapons etc.), but also for processing by cracking, chopping, crushing etc. which Wrangham also discusses and shows also yield more calories per unit consumed.
Now from fire to slings (and yes, there is a connection!) In The Artificial Ape: How Technology Changed the Course of Human Evolution,Timothy Taylor’s over-arching theme is that culture in the form of technology shaping genes did not begin with farming and herding for example. Instead it has been with us from the beginning. Tools came first and by a special form of artificial selection, literally evolved us. “Having possession of fire, tools, weapons and clothes, we do not need massive teeth, claws and muscles, or a long vegetable-absorbing gut” (p. 28). For some (in my view not very good reasons), he does not think that culture literally evolves, but his general thesis that culture has shaped human anatomy, physiology, development and behaviour is so obviously brilliantly right in retrospect, that I won’t pursue that aspect of his book. He adds some anatomical details about facial shape, musculature and teeth to Wrangham’s theory but his more specific thesis is about slings for carrying babies which made us effectively artificial marsupials. Slings were what made possible the care of our extremely altricial young with their grossly disproportionate brain size in foraging societies of naked apes on the move. Not only do they free arms for gathering, but he provides evidence that carrying babies in slings is more energetically efficient than in arms. And of course I note that that form of parental care, like others, is an investment in offspring quality i.e. ultimately in re-production, the production of grand-offspring.
The conclusion I draw from these two books which nicely complement each other is that cultural evolution from the beginning, particularly fire and slings, shaped the human quality strategy of devoting more resources to digestion over consumption and to re-production over production.
I have suggested (e.g. in Darwinian Sociocultural Evolution Chpt. 4 and this blog, Evolutionary Myth 8 posted in August 2010) that resource depletion and environmental degradation do not necessarily go hand in hand as is commonly thought. Specifically, I argued that low densities relative to ecological resources favour consuming/producing more and are associated with small sizes and resource depletion, while high densities favour digesting/re-producing more and are associated with large sizes and environmental degradation. (The size association is because of the greater surface area/volume ratio useful in the former case and the greater volume/surface area ratio useful in the latter case). The associations however can vary depending upon how the four terms are further interpreted. Hanging out sometimes with philosophers who are experts at analysing concepts, including scientific ones, encourages one to pay attention to and dig out these kinds of distinctions.
Consider only somatic functions as illustrated in the abstract in Figure 1. If consumption is understood as eating and excreting more (outer arrows 1 and 2) while digestion is understood as breaking down (degradative metabolism) and building up (biosynthetic metabolism) more (inner arrows 3 and 4), then sizes should be as stated – small versus large, but the former deplete and degrade the external environment while the latter deplete and degrade the internal environment. On the other hand, if consumption is understood as eating and breaking down more (left arrows 1 and 3) while digestion is understood as building up and excreting more (right arrows 4 and 2), then depletion and degradation should be as stated with the former depleting (the external and internal environments) and digestion degrading (the internal and external environments) but both sizes should be intermediate (although cost differences could shove both smaller or larger for example). An analogous break down can be applied to offspring production and re-production.
Such philosophical analysis of concepts can potentially be theoretically useful in the scientific sense. For example, the different interpretations described can be understood not just as different ways of analysing concepts, but as different ways in which genes specifying different components of life history strategies may be linked differently. For example, as originally suggested, the first breakdown could characterize heterospory or proto-genders with anisogamy while the second could characterize homospory or mating types with isogamy.