Chapter 5: Brain Foods

Having Your Cake and Efficiently Digesting It Too.

In Chapter Five, Wrangham discusses how our genus Homo experienced an evolutionary trade off, giving up big guts in order to grow big brains. Diverting more of our Basal Metabolic Rate (BMR) toward fueling our calorie-hungry brains, we had to compromise by shrinking the size of calorie-consuming stomach. Brains and stomachs, along with hearts, kidneys and livers, are the most expensive organs in the human body in terms of consumption of metabolic energy (Wrangham).  Aiello and Wheeler (1995), summarize this nicely in the following chart, which was reprinted (Billings, 1999):

Furthermore, the same source states:

“Since gut size is associated with dietary quality (DQ), and the gut must shrink to support encephalization, this suggests that a high-quality diet is required for encephalization. That is, a higher-quality diet (more easily digested, and liberating more energy/nutrients per unit of digestive energy expended) allows a smaller gut, which frees energy for encephalization.”

But was shrinking the size of the gastrointestinal track the only adaption that could have facilitated encephalization? I wanted to conduct a bit more research into morphological or behavioral options that could have allowed us to grow bigger brains, and to evaluate their validity.

Option 1. The Other Big Spenders

As previously mentioned, the heart, kidneys and liver also consume a large proportion of our energy. However shrinking any of these organs could have much greater consequences. The brain depends on glucose processed by the liver, so it has to be of a large enough size to function properly. This is especially important considering the brain has no glucose stores of its own and depends on that which flow through the blood stream (Aiello and Wheeler, 1995). The brain, if it was to increase in volume, would need even more glucose, so it is best not to shrink the liver. Aiello and Wheeler (1995) also state that shrinking the heart may compromise circulation which brings a constant flow of oxygen and nutrients to the brain, another unviable option. Finally, the kidneys have high metabolic rates as well, but compromising their function of removing waste products from the blood stream and processing urine would be especially dangerous in the habitats where water is limited, which is likely the case with Africa hominids (Aiello and Wheeler, 1995).

Option 2. Cheaper Cuts

Perhaps a reduction in skeletal muscle could balance out the energy budget? Not quite, because muscle tissue has a much lower BMR, so to offset the brain’s high BMR, much more muscle tissue would have to be eliminated, at least 70% (Aiello and Wheeler, 1995).  

Option 3: Have Your Cake and Eat it Too

Consume more calories, keep both. Sounds perfect, right? Aiello and Wells (2002) states increased BMR imply that an organism must either increase the quality or quantity of food. Increasing the quantity is unlikely, states the same source, because Homo ergaster needed adequate time for traveling, socializing and resting, not just for feeding. The same source purposes a drastic increase in diet quality, adding more soft bulbs and tubers and increasing meat consumption. And not surprisingly, the increased nutritional content of cooked food would help too—this leads back to support Wrangham’s original hypothesis. Therefore, it appears after a brief comparison that the shrinking of the GI track may be the most energy efficient way to balance out encephalization. 


Also: Wrangham brings up the curious case of the walking science experiment Mr. St. Martin, whose open-air stomach led to the discovery of the digestive process. Learn more at: radiolab.org

Resources:

Aiello L, and Wheeler P. 1995. “The Expensive-Tissue Hypothesis: The Brain and the Digestive System in Human and Primate Evolution.” Current Anthropology. 36:199-221

Aiello L, and J.C.K. Wells. 2002. “Energetics and the Evolution of the Genus Homo.”  Annual Review of Anthropology. 31:323-338.

Billings T. 1999. “Comparative Anatomy and Physiology Brought Up to Date: The Relationship of Dietary Quality and Gut Efficiency to Brain Size.” Available at www.beyondveg.com

Wrangham, R. 2009. Catching fire: how cooking made us human. New York: Basics Books.