A Recipe for Sentience: The Energetics of Intelligence

This article was originally written for a collaborative blog known as Science in my Fiction, where writers could submit articles about various points of scientific knowledge that could be used to inform fantasy or science fiction writing.  It is the only piece I have complete that was not written either just for my circle of friends or for a technical audience.

The website eventually went defunct due to a lack of updates, so I have reproduced it in full here.


“No man can be wise on an empty stomach.”

Mary Anne Evans, under the pseudonym George Eliot

Humans have been suffering from a bit of a self-image problem for the last half century.

First we were Man the Tool-Maker, with our ability to reshape natural objects to serve a purpose acting to  separate us from the brute beasts.  This image was rudely shattered by Jane Goodall’s discovery in the 1960s that chimpanzees also craft and use tools, such as stripping leaves from a twig to fish termites out of their nest to eat.

Then we were Man the Hunter.  We’d lost our tool-making uniqueness but we still had our ability to kill, dismember, and eat much larger animals with even simple tools, and it was thought that this ability unlocked enough energy in our diet to fuel the growth of larger body size and larger brains1.  Of course, then we found out that although it is not a large component of the diet, chimpanzees eat enough meat to act as significant predators on other primates in their forest homes.

So meat eating by itself doesn’t seem to make us as distinct from our closest living relatives as we had previously thought, and the argument of what makes us special has since moved on to language.  That does leave a standing question, though: if it wasn’t meat-eating that allowed us to get bigger and more intelligent, what was it, exactly?

While there is evidence in the fossil record that eating raw meat allowed humans to gain more size and intelligence, it is both unlikely that we were the hunters and that this behavioral change was enough to unlock a significant jump in brain size.  Instead, there is another hypothesis and human identity that has been gaining more traction as of late: the concept of Man the Cooking Animal, the only animal on Earth that can no longer survive on a diet of raw food because of the energy demands of its enormous brain2.

Napoleon is famously said to have declared that an army marches on its stomach (at least, after what may be a loose translation).  That is, the power of an army is limited by the amount of food that a society can divert to it.  What we have come to realize more recently is that this same limitation exists inside the body, be it human, animal, or speculative alien species.  No matter what the diet, a creature will only have a fixed amount of energy available to divert to activities such as maintaining a warm-blooded body temperature (homeothermy), digestion, reproduction, and the growth and maintenance of tissues.  We can track some of these changes in the human line in the fossil record, but others must at best be more speculative due to the difficulty of preserving evidence of behavioral changes (which of course, do not fossilize) as well as limited research on modern examples.  We’ll start by looking at the evolutionary pathway of humans to see what information is currently available.

The Woodland Ape and the Handy Man

The oldest definite human ancestors that we can unequivocally identify as part of our line lie in the genus Australopithecus.  These have been identified by some authors as woodland apes, to distinguish these more dryland inhabitants from the forest apes that survive today in Africa’s jungles (chimpanzees, bonobos, and gorillas).  They are much smaller than a modern human, only as tall as a child, but they have already evolved to walk upright.  They still show adaptations for climbing that were lost in later species, suggesting they probably escaped into the trees at night to avoid ground predators, as modern chimps do.  Their brains were not much larger than a modern chimpanzee’s, and their teeth are very heavy, even pig-like, as an adaptation to a tough diet of fibrous plant material – probably roots, tubers, and corms, perhaps dug from plants growing at the water’s edge2,3.

The hominoids thought to have first started eating meat are Homo habilis, the “handy man”, and the distinction between them and the older Austrilopithecus group from which they descended are not very large.  The two are close enough that Homo habilis has been suggested it might be more properly renamed to Australopithecus habilis, while the interspecies variation suggests to some researchers that what we now call habilis may represent more than one species4Whatever its proper taxonomic designation, H. habilis shows a modest increase in brain size and evidence that it was using simple stone tools to butcher large mammals, probably those left behind by the many carnivorous mammals that lived on the savannahs and woodlands alongside it.

The transition between H. habilis and H. erectus is far more distinctive, with a reduction in tooth size, jaw size, and gut size, and an increase in brain volume.  They are also believed to have been larger, but the small number of available hominid fossils makes this difficult to verify.  H. erectus is also the first human to have been found outside of Africa.  While the habilis-erectus split has been attributed to the eating of significant amounts of meat in the Man-the-Hunter scenario (recall that habilis, despite its tool-using ability for deconstructing large animals, does not appear to have hunted them), the anthropologist Richard Wrangham has suggested that the turnover instead indicates the first place at which humans began to cook2,3.  Because the oldest solid evidence of cooking is far younger than the oldest known fossils of erectus, what follows is largely based on linking scraps of evidence from modern humans and ancient fossils using what is known as the Expensive-Tissue Hypothesis.

Brains versus Guts: The Expensive-Tissue Hypothesis

The Expensive-Tissue Hypothesis was first proposed by Leslie Aiello and Peter Wheeler in 19955, and it goes something like this: large brains evolve in creatures that live in groups because intelligence is important to creating and maintaining the social groups.  This is known as the social brain hypothesis, and it helps to explain why animals that live socially have larger brains than their more solitary relatives.  However, not all social primates, or even social animals, have particularly large brains.  Horses, for example, are social animals not known for their excessively large brain capacity, and much the same can be said for lemurs.  Meanwhile, apes have larger brains than most monkeys.  This can’t be accounted for purely by the social brain hypothesis, since by itself it would suggest that all social primates and perhaps all social animals should have very big brains, rather than the variation we see between species and groups.  What does account for the difference is the size of the gut and, by extension, the quality of the diet.

Both brains and guts fit the bill for expensive body tissues.  In humans, the brain uses about 20% of the energy we expend while resting (the basal metabolic rate, or BMR) to feed an organ that only makes up 2.5% of our body weight2.  This number goes down in species with smaller brains, but it is still disproportionately high in social, big-brained animals.  Aiello and Wheeler note that one way to get around this lockstep rule is to increase the metabolic requirements of the species5 (i.e., throw more calories at the problem), but humans don’t do this, and neither do other great apes.  Our metabolic rates are exactly what one would expect for primates of our size.  The only other route is to decrease the energy flow to other tissues, and among the social primates only the gut tissue shows substantial variation in its proportion of body weight.  In fact, the correlation between smaller guts and larger brains lined up quite well in the data then available for monkeys, gibbons, and humans5.  Monkeys and other animals that feed on low-quality diets containing significant amount of indigestible fibers or dangerous plant toxins have very large guts to handle the problem and must expend a significant amount of their BMR on digestion, and have less extra energy to shunt to operate a large brain.  Fruit-eating primates such as chimpanzees and spider monkeys have smaller guts to handle their more easily-digested food, and so have larger brains.  Humans spend the least amount of time eating of any living primate, with equally short digestion times as food speeds through a relatively small gut.  And ours, of course, are the largest brains of all2.

These tradeoffs are not hard-linked to intestinal or brain size, and have been demonstrated in other species.  For example, there is a South American fish species with a tiny gut that uses most of its energy intake to power a surprisingly large brain, while birds with smaller guts often use the energy savings not to build larger brains, but larger, stronger wing muscles2.  Similarly, muscle mass could be shed instead of gut mass to grow a larger brain or to cut overall energy costs.  The latter strategy is the one taken up by tree-dwelling sloths to survive on a very poor diet of tough, phytotoxin-rich leaves, and although it makes them move like rusty wind-up toys it also allows them to live on lower-quality food than most leaf-eating mammals.

Modern humans have, to a degree, taken this approach as well.  When compared to one of our last surviving relatives, H. neanderthalensis, humans have a skeletal structure that paleontologists describe as “gracile:” light bones for our body size, anchoring smaller muscles than our shorter, heavier relatives.  Lower muscle and bone mass in H. sapiens gives us an average energy cost on the order of 1720 calories a day for males and 1400 calories a day for females in modern cold-adapted populations, which are thought to have similar metabolic adaptations for cold weather as the as extinct Neanderthals.  By contrast, H. neanderthalis has been estimated to need 4000-7000 calories a day for males and 3000-5000 calories for females, with the higher costs reflecting the colder winter months6.

Cooked versus Raw

 At the point where human brain size first increases dramatically (H. erectus, as you might recall), both guts and teeth reduce significantly while the brain increases.  The expensive tissue hypothesis explains the tradeoff between guts and brains, but cooking provides a possible explanation for how both the teeth and the guts could reduce so significantly while still feeding a big brain.

Data on the energetics of cooked food are currently limited, but the experiments that have been performed so far indicate that the softer and more processed the food the more net calories are extracted, since less calories need to be spent on digestion.  A Japanese experiment with rats showed that they gained more weight on laboratory blocks that had been puffed up like a breakfast cereal versus rats on normal blocks, even though the total calories in the food were the same and the rats spent the same amount of energy on exercise2.  Similarly, experiments with pythons show that they expend about 12% more energy breaking down whole meat than either meat that has been cooked or meat that has been finely ground.  The two treatments reduce energy cost independently of each other, meaning that snakes fed ground, cooked meat used almost 24% less energy than pythons fed whole raw meat or rats2.

There is even less data on how humans utilize cooked food versus raw food.  Because it only recently occurred to us that we might not be able to eat raw food diets like other animals, only a few studies exist.  So far the most extensive is the Giessen Raw Food study performed in Germany, which used questionnaires to collect data from 513 raw foodists in Germany who eat anywhere from a 75% to 100% raw food diet.  The data are startling.  Modern humans appear to do extremely poorly on diets that our close relatives, the forest apes, would get sleek and fat on.  Body weights fall dramatically when we eat a significant amount of raw food, to the point where almost a third of those eating nothing but raw had body weights suggesting chronic energy deficiency.  About half of the women on total raw food diets had so little energy to spare that they had completely ceased to menstruate, and 10% had such irregular cycles that they were likely to be completely unable to conceive at their current energy levels2.   Mind you, these are  modern first-world people with the advantage of high-tech processing equipment to reduce the energy cost of eating whole foods, far less energy expenditure required to gather that food, and a cornucopia of modern domestic plants that have been selectively bred to produce larger fruits and vegetables with and lower fiber and toxin contents than their wild counterparts.  The outcome looks more dismal for a theoretical raw-food-eating human ancestor living  before the dawn of civilization and supermarkets.

Fantastic Implications

 What this all ultimately suggests is that there are tradeoffs in the bodies of intelligent creatures that we may not have given much consideration: namely, that to build a bigger brain you either need a much higher level of caloric intake and burn (high BMR) or the size and energy costs in something in the body have to give.  Certain organs do not appear to have much wiggle room for size reduction, as Aiello and Wheeler discovered; hearts for warm-blooded organisms need to be a certain size to provide enough blood throughout the body, and similarly lungs must be a particular size to provide enough surface area for oxygen to diffuse into the blood.  However, gut size can fluctuate dramatically depending on the requirements of the diet, and musculature can also reduce to cut energy costs.

Humans seem to have done an end-run around some of the energy constraints of digestion by letting the cultural behaviors of cooking and processing do the work for them, freeing up energy for increased brain size following social brain hypothesis patterns.  This is pretty classic human adaptive behavior, the same thing that lets us live in environments ranging from arctic to deep desert, and should therefore not come as a great surprise.  It does, however, give us something to think about when building intelligent races from whole cloth: what energy constraints would they run up against, and assuming they didn’t take the human path of supplanting biological evolution with culture, how would they then get around them?

Fantasy monsters and evil humanoids in stories tend to be described as larger and stronger than humans (sometimes quite significantly so) and as raw meat eaters, particularly of humanoid meat.  There’s a good psychological reason for doing so – both of these characteristics tap into ancient fears, one of the time period not so long ago when humans could end up as prey for large mammalian predators, and the other a deep-seated terror of cannibalism without a heavy dose of ritualism to keep it in check.  However, both the Neanderthal example and the Expensive Tissue Hypothesis suggest that such a species would be very difficult to produce; there’s a very good reason why large mammalian predators, whatever their intelligence level, are rare.  It wouldn’t be a large shift, however, to take a monstrous race and model them after a hybrid of Neanderthal and grizzly bear, making them omnivores that can supplement their favored meat diet with plant foods and use cooking to reduce the energy costs of digestion.  Or perhaps their high caloric needs and obligate carnivory could become a plot point, driving them to be highly expansionistic simply in order to keep their people fed, and to view anything not of their own race as a potential meal.

On the science fiction front, it presents limitations that should be kept in mind for any sapient alien.  To build a large brain, either body mass has to give somewhere (muscle, bone, guts) or the caloric intake needs to increase to keep pace with the higher energy costs.  Perhaps an alien race more intelligent than humans would be able to do so by becoming even more gracile, with fragile bones and muscles that may work on a slightly smaller, lower-gravity planet.  Or perhaps they reduce their energy needs by being an aquatic race, since animals that swim generally use a lower energy budget for locomotion than animals that fly or run7.

From such a core idea, whole worlds can be spun: low-gravity planets that demand less energy for terrestrial locomotion; great undersea empires in either a fantastic or an alien setting, where water buoys the body and reduces energy costs enough for sapience; or creatures driven by hunger and a decidedly human propensity for expansion that spread, locust-like, across continents, much as we did long ago when we first left our African cradle.

Food for thought, indeed.


  1. Stanford, C.B., 2001.  The Hunting Apes: Meat Eating and the Origins of Human Behavior.   Princeton, NJ: Princeton University Press.
  2. Wrangham, R., 2009. Catching Fire: How Cooking Made us Human. New York, NY: Basic Books.
  3. —-, 2001. “Out of the Pan, into the fire: from ape to human. ”  Tree of Origin: What Primate Behavior Can Tell us About Human Social Evolution.  Ed.  F.B.M. de Waal.   Cambridge, MA:  Harvard University Press.   119-143.
  4. Miller, J.A., 1991. “Does brain size variability provide evidence of multiple species in Homo habilis?American Journal of Physical Anthropology 84(4): 385-398.
  5. Aiello, L.C. and P. Wheeler, 1995. “The Expensive-Tissue Hypothesis: The Brain and the Digestive System in Human and Primate Evolution.” Current Anthropology 36(2): 199-221.
  6. Snodgrass, J.J., and W.R. Leonard, 2009. “Neanderthal Energetics Revisited: Insights into Population Dynamics and Life History Evolution.” PaleoAnthropology 2009: 220-237.
  7. Schmidt-Nielsen, K., 1972. “Locomotion: Energy cost of swimming, flying, and running.” Science 177: 222-228.

Ethnographic Fieldnotes: Jikoji Retreat

This fragment of writing represents in-the-field data collection (specifically, participant observation) or an anthropological paper on how Zen Buddhism has evolved in America.  I was curious how it managed to avoid the aura of otherness experienced by other religious minorities – Zen Buddhists in the Bay Area do not view themselves as outsiders any more than the surrounding society does, which was very puzzling to me since the practitioners are effectively atheistic in their religious traditions.  Despite this, the reactions that they garner from the public at large are very different from those who profess direct atheism or agnosticism.

It turns out that the way in which Zen was imported into the U.S. was key to its current standing in the culture, where it is viewed as an individual philosophy of living rather than a religion and thus is likely to attract followers from the more privileged end of society.

Jikoji Retreat is a beautiful Zen center located in the Santa Cruz mountains less than an hour’s drive from my house.  These observations date from February 2011, when an unusual cold snap had pulled the snowline down below 3,000 feet.


Jikoji Retreat was chosen in particular because its particular style of Buddhism (Soto Zen) is also seen in Hannah, my informant, and because this particular retreat welcomes visitors to its Sunday services.  It is located in the Santa Cruz Mountains along Skyline Boulevard (Highway 35) which, although not particularly far away from where I live, is rather difficult to access because of the nature of mountain roads.  With this in mind I left for services shortly before 9:00 AM, both to give myself extra travel time due to the mountain roads and also in case I became lost on the way – an idea that occurred to me more than once driving up Highway 9 into the mountains, since I had forgotten how dependent I had become these days on GPS receivers such as Garmin to navigate for me.

Jikoji is located off the entrance to public open space, and the road down to it is largely gravel and dirt.  Because of its location on the crest of the mountains the area is heavily forested with coast range broadleaf trees such as California bay, live oak, and madrone, all with their trunks coated with a thick layer of moss and (because it had been so cold lately) a dusting of snow.  I arrived early on a very cold morning and found a place to park off the main entry road, as far out of the path of traffic as possible, and took a look around.

I was fortunate to encounter one of the people who works at Jikoji, who showed me where the important service sites were and introduced me to the student teacher who was presenting the talk.  Since I had arrived some time beforehand, there was time to step inside the Zendo (after removing my shoes) and for her to show me how to properly sit zazen.  Normally they use a small round pillow (the zafu), which they sit on about 1/3 of the way to the edge, cross their legs to provide a platform or tripod for support, and hold their hands at stomach level with the thumbs touching and the fingertips resting on top of one another (to hold the “mudra,” although I am unclear what this means).  Unlike other meditation types I have experienced this is performed with the back straight so the ears are over the shoulders and with the eyes open, tilted down at a 45-degree angle with the chin tucked slightly in.  I am told this is to maintain the link between the inner and outer self, although in my experience it also provides a benefit in preventing me from falling asleep as I have done in other meditative attempts.

Incense and candles are lit in front of a statue of the Buddha at one end of the room, a bell is rung three times, and then we sit facing the wall on the pillows and meditate for 40 minutes.  Since the topic of meditation seems to be personal, there is no predetermined topic for meditation – the student instructor mostly told me to attempt to step outside of my own thoughts, and let them go.  Because I was unclear what to meditate on (and I have difficulty meditating to begin with) I mostly focused on the sounds of the birds outside – the screeching of scrub jays, the deep caw of a crow, and the call, somewhere distant, of a raptor.  To get my mind to silence I focused on the image of a bay nut, picked up from the ground outside as I looked around, somewhat lost, on my initial arrival, with its thin shell crumbling off to expose the nut within.  While the plant community around Skyline is likely not its original climax community, it is still as close to nature as one gets around here, and so this is what I meditated on primarily.  Although I still experienced severe difficulty maintaining inner quiet, I did experience in this meditative state the form of general feeling of wellness and calm that I occasionally feel just before falling asleep or during heavy periods of intense data entry, some of the few times my mind clears much at all.  This is likely similar to the meditative state that humans once experienced when our sleep schedules were not affected by artificial light, when we would sleep four hours and wake up for an hour or two of quiet restfulness and reflection before sleeping again, a pattern known as segmented or biphasic sleep. The lull sensation continued after the initial zazen period when we perform a chant while sitting, the text of which is provided below:

Dai-sai geda pu-ku

                Muso fuku den-e

                Hi-bu nyori kyo

                Ko-do shoshu jo


                How great is the kesa

                A virtuous garden far beyond form and emptiness

                I will wear the Tatagata’s teaching

                And save all sentient beings


                 Dai-sai geda pu-ku

                Muso fuku den-e

                Hi-bu nyori kyo

                Ko-do shoshu jo

This is followed by standing and facing the altar as the student teacher re-lights the incense and then we perform three full bows from a standing position – that is, a prone position with knees flexed under the body and head on the floor resting between the hands, elbows tucked in to the sides.  After this first period of zazen is the kinhin, or walking meditation, where everyone in the zendo forms a line around the edges of the room and walks, slowly, with hands folded or in a praying position in front of them, for ten minutes unti the bell is rung again.  After this first period we returned to the zazen position to continue meditation for another forty minutes.

Although the feeling of reflective well-being continued through the kinhin and the first part of the second zazen, by the end of it I felt about ready to explode from attempting to be so still for so long and to attempt to clear my mind (a common issue I have with meditation, and the main reason I typically avoid it).  However, this period seemed to last less time than the first period of zazen, or perhaps I simply wasn’t being as good at blanking out my thoughts.  At any rate, the ringing of the bell and the monotone repeat of the chant provided a respite from my restlessness and we took a brief ten-minute break to walk outside, talk, or in my case to stay sitting on the floor (but off the zafu pillow, which by now was becoming intensely uncomfortable).

The service itself was a discussion on the need to practice mindfulness in everyday life, presented both with examples drawn from the sutras as well as more psychological language – i.e. the repetition on the wheel of life (a paper model of which was passed around) is likened to our psychological need to repeat certain actions in our life, particularly destructive ones, and learning to let go of this is couched almost in self-help terms as well as in terms of the ideals of Buddhism.  This was also seen in the idea of performing inquiry meditation, wherein the period of zazen is spent first allowing the mind to settle and rest, and then looking into one’s past to see where these patterns of repetition of destructive behavior are seen.  The idea is to lose the illusion of self as separate from the rest of the world, but to do so one has to not only learn to let go of material or immaterial things (dislike, pride, hubris, materialism) but also to not only love all living things, but also turn the love and kindness that is espoused by the teachings to one’s own self.  I noted that the guest teacher frequently was looking at me as she said this, most likely because she knew I was new, but also perhaps because I was one of the few people with their eyes open and watching her during the service.

Service is presented with all participants sitting on the floor, many on the zafu pillows but in my case and a few others simply sitting on the mats used for meditation.  We begin by standing, then reading the sutra presented to us on pre-printed paper as a low, monotone chant.  This time around the sutra appeared to be focused on the ideal of extending love to all living things and invoked the name of Avalokitesvara (or Kwan Seum Bosal/Kwan Yin, the bodhisattva of compassion).  Once the reading is done we perform the deep bow another three times and then sit, this time with everyone facing away from the wall and towards the center of the zendo.  We read another chant at the beginning of the service, a final chant at the end of the service, and then repeated the Four Vows:

Sentient beings are numberless, I vow to liberate them

                Desires are inexhaustible, I vow to put an end to them

                The Dharmas are boundless, I vow to master them

                The Buddha’s Way is Unsurpassable, I vow to become it.

The guest teacher was dressed in traditional Zen monk robes, black on the outside with a white inner collar (perhaps undergarments as well) and with a brown cloth draped around the body and hung around the left shoulder.  The teacher was barefoot, also typical of monks, but she was neither male nor had the shaved head often seen as traditional; however, she also was white, as were most of the participants besides me and one other person (who perhaps was of Middle Eastern descent).  Participants ranged in age from perhaps their late 20s to their 50s (a rough estimate based on visual observation).  I also noted items that I typically only see displayed by higher social classes: Gore-tex jackets against the cold, reusable plastic or steel Nalgene water bottles, the wool socks usually sold at REI to be worn in hiking boots.  There also was a specific sort of bib or neck item worn by some of the participants, dark blue cloth with a white square in front, often with writing on it, which they tucked their hands under during kinhin.  I am unclear as to the purpose of this item although it is apparently quite sacred, since during lunch one of the other participants turned it backwards and explained that to clean the item, it would need to be spot-washed by hand while burning cedar incense.

The talk portion of the service was particularly of interest, for here the other participants asked questions regarding the service, but the atmosphere was much more relaxed.  Particularly near the end, it gave way to friendly joking both on the part of the participants and the student teacher.  This ended at about 12:30, when after some difficult to answer questions the teacher laughed, pointed at her watch, and said it was time for lunch!

The social lunch that was provided in the small side kitchen was typical American fare, albeit vegetarian: wheat (gluten-free?) rigatoni with a marinara sauce, garlic bread, salad, and apple pies that some of the participants had brought for dessert.  During this period I explained more of what I was doing there during my visit, and listened to the other participants as they talked with one another.  Notably, this is when I discovered what some of them did: one works for Oracle, another works for Apple, and notably neither seem to be especially happy with their job.  For them, it sounds as if the meditative services are their escape, particularly now that the economy has made leaving unhappy jobs a much more dangerous affair.  Two appear to be doing stand-up comedy on the side, one is a musician for a hobby, and it is unclear what all of the others do as I could not track all of the side conversations going on with approximately twelve people in a very small kitchen.

After lunch the group broke up to help push a car back onto the road that had slid off just prior to service, and people started moving back into their cars to head home.  By this point the temperature had warmed slightly and much of the remaining snow had melted off, although the air still had the acrid smell of snow to it, and the brick stairway that I climbed to get back to my car was still slippery with ice.