Thanks Peter for bringing this issue to your blog readers.
Here are a couple of important points which you and/or your readers should address when evaluating Richard Wrangham’s hypothesis that the control of fire and subsequent consumption of cooked tubers represented an evolutionary advantage that led to the natural selection of a larger brain mass relative to body mass in hominids, a process known as encephalization:
- The metabolic cost of the human brain is 9 times (11.2 watts/kg) greater than average whole body metabolic rate (1.25 watts/kg) at rest (1). Accordingly, in modern humans with an encephalization index of 4.9 versus an encephalization index of 1.9 in non-human anthropoids (2), the fully encephalized modern human brain utilizes 20-25% of the total resting metabolic rate (RMR) whereas in other primates, this value is 8-9% (3). So, indeed the evolution of a large brain relative to body mass in hominid’s represents an energetically expensive process. And your question as to where did the increased energy come from to allow for the natural selection of our large, metabolically active brain is a good one, but certainly not the only relevant question. Let me address the energy question first.
- Below is Table 1 showing the energy density of the most likely foods which may have been available to our hominid ancestors during the time period which the genus Homo arose in Africa 2.0 to 2.3 million years ago (MYA) and during which increased encephalization began to occur (4). As you can see from the Table 1, far and away, fatty animal foods are the most concentrated energy sources whereas wild tubers and roots are the least energetically dense of all the foods listed (4). This situation remains true for modern, domesticated equivalents of wild tubers and roots as demonstrated in the Table 2 (5).
Table 1. Energy density of foods available to African hominids 2.0 – 2.3 MYA (4)
|African ruminant subcutaneous fat||745|
|Mixed, wild nuts||306|
|African ruminant liver||159|
|Mixed, edible wild plant foods||129|
|African ruminant brain||126|
|African freshwater fish||119|
|African ruminant muscle||113|
|Mixed, wild tubers/roots||96|
Source: Google images
Table 2. Energy density of common modern tuber and root vegetables (5)
|Modern Root Vegetables||(kcal/100 g)|
|Cassava (manioc, tapioca), cooked||160|
|Sweet potato, baked||103|
One of the crucial elements of the Wrangham Hypothesis is the assumption that the regular controlled (I bold and underlie this term for emphasis later on) use of fire allowed for the consumption of a formerly untapped food resource (underground roots and tubers), which in turn represented the energy source necessary (1,4) for the selection of a large, metabolically active brain.
A huge, logical hurdle to overcome for believers in the Wrangham Hypothesis is that tubers actually are not energetically dense, but rather are the least energetically dense of all foods available to our hominid ancestors during the period when encephalization occurred 2.3 to 2.0 MYA. This evidence is clearly demonstrated in Table 1. Further, cooking of starchy tubers yields caloric density values slightly greater (5-10%) than their raw values because of the removal of water, but lower than most than raw wild plant foods (Table 2) which have always been available to our African hominid ancestors, prior to increased encephalization.
A final flaw in the Wrangham Hypothesis is the failure to consider other nutritional constraints to encephalization in all mammals besides energy density. Clearly large brains per body mass convey multiple evolutionary advantages in cognition, environmental awareness and strategic advantages to increased reproductive success while avoiding mortality from all causes. Yet the evolution of large brains requires other important known nutritional factors.
The synthesis of all mammalian brain and nervous tissue requires at least two major fatty acid precursor molecules (arachidonic acid, 20:4n6 and docosahexanoic acid, 22:6n3). Whether a mouse, elephant or primate, regardless of encephalization indices, all brain and nervous tissue have invariant concentrations of these fatty acids (4, 9-11). Hence, a physiologic bottleneck has occurred in all mammals that we have unfortunately inherited from the very first mammals and constrains evolution of large, complex brains.
This evolutionary bottleneck necessary for increased encephalization in all mammals occurs in the liver. All mammals have extremely limited, or no ability, to convert plant precursor fatty acids fatty acids (linoleic acid 18:2 n6 to arachidonic acid, 20:4 n6) and (alpha linoleic acid 18:3 n3 to docosahexanoic acid, 22:6 n3) which are the necessary physiologic elements for the synthesis of brain tissue (4, 8-11). Unfortunately, the Wrangham hypothesis does not address or is completely unaware of this evidence. Tubers and roots contain no preformed arachidonic acid (AA) or docosahexanoic acid (DHA) required for the evolutionary increase in the encephalization index observed in homids.
The scavenged, de-fleshed carcasses of African ruminants represents a concentrated energy source (marrow) needed for increased encephalization, and the skull with its brain contents of these scavenged carcasses represents the most concentrated terrestrial source of DHA (4).
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