One of the main differences between human life today (for most people in developed countries) and life in the EEA is the abundant availability of food independently of place and season. In the state of nature, food is relatively scarce much of the time, making energy conservation paramount and forcing difficult energy expenditure tradeoffs between metabolically costly tissues, processes, and behaviors. As we shall see, increased access to nutrients suggests several promising enhancement opportunities. We have also gained access to important new nondietary resources, including improved protection against physical threats, obstetric assistance, better temperature control, and increased availability of information. Let us examine how these new resources are relevant to potential enhancements of the brain and the immune system.
The human brain constitutes only 2% of body mass yet accounts for about 20% of total energy expenditure. Combined, the brain, heart, gastrointestinal tract, kidneys, and liver consume 70% of basal metabolism. This forces tradeoffs between the size and capacity of these organs and between allocation of time and energy to activities other than searching for food in greater quantity or quality (Aiello et al. 2001; Fish and Lockwood 2003).
Unsurprisingly, we find that, in evolutionary lineages where nutritional demands are high and cognitive demands low (such as bats hunting in uncluttered environments), relative brain size is correspondingly smaller (Niven 2005). In humans, brain size correlates positively with cognitive capacity (. « 0.33) (McDaniel 2005).
Holding brain mass constant, a greater level of mental activity might also enable us to apply our brains more effectively to process information and solve problems. The brain, however, requires extra energy when we exert mental effort, reducing the normally tightly regulated blood glucose level by about 5% (0.2 mmol/l) for short (<15 min) efforts and more for longer exertions (Scholey et al. 2001; Fairclough and Houston 2004). Conversely, increasing blood glucose levels has been shown to improve cognitive performance in demanding tasks (Korol and Gold 1998; Manning et al. 1998; Martin and Benton 1999; Winder and Borrill 1998).
The metabolic problem is exacerbated during prenatal and early childhood growth where brain development requires extra energy. Brain metabolism accounts for a staggering 60% of total metabolism in newborns (Holliday 1986), exacerbating the competitive situation between the mother and child for nutritional resources—an unpleasant tradeoff (Martin 1996). Children with greater birth weight have a cognitive advantage (Matte 2001).
Another constraint on prenatal cerebral development is the size of the human birth canal (itself constrained by bipedalism), which historically placed severe constraints on the head size of newborns (Trevathan 1987). These constraints are partly obviated by modern obstetrics and the availability of caesarian section. One way of reducing head size at birth and perinatal energy demands would be to extend the period of postnatal maturation. However, delayed maturation was vastly riskier in the EEA than it is now.
What all this suggests is that cognitive enhancements might be possible if we can find interventions that recalibrate these legacy tradeoffs in ways that are more optimal in the contemporary world. For example, suppose we could discover interventions that moderately increased brain growth during gestation, or slightly prolonged the period of brain growth during infancy, or that triggered an increase in available mental energy. Applying the EOC to these hypothetical interventions, we get a green light. We can see why these enhancements would have been maladaptive in the EEA and why they may nevertheless have become entirely beneficial now that the underlying tradeoffs have changed as a result of the availability of new resources. If the “downside” of getting more mental energy is that we would burn more calories, many of us would pounce at the opportunity.
Not all cognitive enhancement interventions get an immediate green light from the above argument. Stimulants like caffeine and modafinil enable increased wakefulness and control over sleep patterns (Caldwell 2001). But sleep serves various (poorly understood) functions other than to conserve energy (Siegel 2005). If the explanation for why we do not sleep less than we do has to do with these other functions, then reducing sleep might well have more problematic side effects than increasing the amount of calories we need to consume. For any particular intervention, such as the administration of some drug, we also of course need to consider the possibility of contingent side effects, i.e., that the drug might have effects on the body other than simply retuning the target tradeoff.
-  Increasing oxygen levels (another requirement for metabolism) also improves cognition.