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The social brain hypothesis argues that the human brain did not increase in size to solve increasingly complex problems, but as a means of surviving and reproducing in complex social groups (Dunbar, 2009). The social brain hypothesis is one of the most largely held views when it comes to explaining primate encephalization. However, an analysis of new phylogeny and more primate samples shows that differences in human and non-human primate brain evolution come down to diet, not sociality.

Diet is one of the most important factors in regards to brain and body size. The more high-quality food an animal has, the bigger its brain and body will be. Using a larger sample (3 times as large, 140 primates), more recent phylogenies (which show inferred evolutionary relationships amongst species, not which species is ‘more evolved’ than another), and updated statistical techniques, Decasien, Williams, and Higham (2017) show that diet best predicts brain size in primates, not social factors after controlling for body size and phylogeny (humans were not used because we are an outlier).

The social scheme they used consisted of solitary, pair-living, harem polygyny (one or two males, “a number of females” and offspring), and polygynandry (males and females have multiple breeding partners during the mating season). The diet scheme they used consisted of folivore (leaf-eater), frugivore-folivore (fruit and leaf eater), frugivore (fruit-eater) and omnivore (meat- and plant-eaters).

None of the sociality measures used in the study showed a relative increase in primate brain size variation, whereas diet did. Omnivores have bigger brains than frugivores. Frugivores had bigger brains than folivores. This is because animal protein/fruit contains higher quality energy when compared to leaves. Bigger brains can only evolve if there is sufficient and high-quality energy being consumed. The predicted difference in neurons between frugivores and folivores as predicted by Herculano-Houzel’s neuronal scaling rules was 1.08 billion.

The authors conclude that frugivorous primates have larger brains due to the cognitive demands of “(1) necessity of spatial information storage and retrieval; (2) cognitive demands of ‘extractive foraging’ of fruits and seeds; and (3) higher energy turnover and enhanced diet quality for energy needed during fetal brain growth.” (Decasien, Williams, and Higham, 2017). Clearly, frugivory provided some selection pressures, and, of course, the energy needed to power a larger brain.

The key here is the ability to overcome metabolic constraints. Without that, as seen with the primates that consumed a lower-quality diet, brain size—and therefore neuronal count—was relatively smaller/lower in those primates. Overall brain size best predicts cognitive ability across non-human primates—not encephalization quotient (Deaner et al, 2007). Primate brains increase approximately isometrically as a function of neuron number and its overall size with no change in neuronal density or neuronal/glial cell ratio with increasing brain size (in contrast to rodent brains) (Herculano-Houzel, 2007). If brain size best predicts cognitive ability across human primates and primate brain size increases isometrically as a function of neuron number with no change in neuronal density with increasing brain size, then primates with larger brains would need to have a higher quality diet to afford more neurons.

The results from DeCasien, Williams, and Higham (2017) call into question the social brain hypothesis. The recent expansion of the cerebellum co-evolved with tool-use (Vandervert, 2016), suggesting that our ability to use technology (to crush and mash foods, for instance) was at least as important as sociality throughout our evolution.

The authors conclude that both human and non-human primate brain evolution was driven by increased foraging capability which then may have provided the “scaffolding” for the development of social skills. Increased caloric consumption can afford larger brains with more neurons and more efficient metabolisms. It’s no surprise that frugivorous primates had larger brains than folivorous primates. Just as Fonseca-Azevedo and Herculano-Houzel (2012) observed, primates that consumed a higher quality diet had larger brains.

In sum, this points in the opposite direction of the social brain hypothesis. This is evidence for differing cognitive demands placed on getting foods. Those who could easily get food (folivores) had smaller brains than those who had to work for it (frugivores, omnivores). However, to power a bigger brain the primate needs the energy from the food that takes the complex behavior—and thus larger brain—to obtain. This lends credence to Lieberman’s (2013) hypothesis that bipedalism arose after we came out of the trees and needed to forage for fruit to survive.

Brain size in non-human primates is predicted by diet, not social factors, after controlling for body size and phylogeny. Diet is the most important factor in the evolution of species. With a lower quality diet, larger brains with more neurons (in primates, 1 billion neurons takes 6 kcal per day to power) would not evolve. Brain size is predicated on a high-quality diet, and without it, primates—including us—would not be here today. Diet needs to be talked about a lot more when it comes to primate evolution. If we would have continued to eat leaves and not adopt cooking, we would still have smaller brains and many of the things that immediately came after cooking would not have occurred.

Since we are primates we have the right morphology to manipulate our environment and forage for higher quality foods. But those primates with access to foods with higher quality have larger brains and are thus more intelligent (however, there are instances where primate brain size increases and decreases and it comes back to, of course, diet). Sociality comes AFTER having larger brains driven by nutritional factors—and would not be possible without that. Social factors drove our evolution—no doubt about it. But the importance of diet throughout hominin evolution cannot be understated. Without our high-quality diet, we’d still be like our hominin ancestors such as Lucy and her predecessors. Higher quality diet—not sociality, drives primate brain size.

References

DeCasien, A. R., Williams, S. A. & Higham, J. P. Primate brain size is predicted by diet but not sociality. Nat. Ecol. Evol. 1, 0112 (2017).

Deaner, R. O., Isler, K., Burkart, J., & Schaik, C. V. (2007). Overall Brain Size, and Not Encephalization Quotient, Best Predicts Cognitive Ability across Non-Human Primates. Brain, Behavior and Evolution,70(2), 115-124. doi:10.1159/000102973

Dunbar, R. (2009). The social brain hypothesis and its implications for social evolution. Annals of Human Biology,36(5), 562-572.

Fonseca-Azevedo, K., & Herculano-Houzel, S. (2012). Metabolic constraint imposes tradeoff between body size and number of brain neurons in human evolution. Proceedings of the National Academy of Sciences,109(45), 18571-18576. doi:10.1073/pnas.1206390109

Herculano-Houzel, S. (2007). Encephalization, neuronal excess, and neuronal index in rodents. Anat. Rec. 290, 1280–1287.

Lieberman, D. (2013). The story of the human body: evolution, health and disease. London: Penguin Books.

Vandervert, L. (2016). The prominent role of the cerebellum in the learning, origin and advancement of culture. Cerebellum & Ataxias,3(1). doi:10.1186/s40673-016-0049-z

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The debate about cooking’s role in human evolution is ongoing. Some people may rightly say “Cooking it not a selection pressure.” This is true. However, it doesn’t say much. The advent of cooking was one of the most important events in human history as it released the constraint on brain size due to predigesting our food outside the body. This seminal event in our history here on earth is one of the main reasons we are here today. In the articles I wrote two months ago on how and why we are so intelligent, I forgot to bring up two important things—the thermic effect of food (TEF) and our gut microbiota and its relationship with our brain. The importance of these variables in regards to cooking cannot be overstated. The subject tonight is cooking and how it benefitted us metabolically and our gut microbiota that partly drive our brain and behavior.

Cooking was beneficial to us not only because it released constraints on brain size due to how nutrient-rich meat was as well as other foodstuffs that were then cooked, but because it’s possible to extract more energy out of cooked food compared to non-cooked food. When erectus began controlling fire around 1-1.5 mya (Herculano-Houzel, 2016: 192) this allowed for the digestion of higher-quality foods (meat, tubers, etc) and this is the so-called ‘prime mover’ for the brain size increase in hominids over the past 3my.

The introduction of cooked/mashed foods changed the shape of the ridges on our skull which serve as attachments for the facial muscles responsible for chewing. The saggital crest on the cranium and zygomatic eminences in the cheeks exist in great apes but not us. Further, molars and canine teeth reduced in size while brain size double in erectus. Our jaw bones decreasing in size shows that we didn’t need to have as forceful of a bit due to the introduction of cooked foods 1-1.5 mya (Herculano-Houzel,2016: 193).

Along with the introduction to a diet with softer foods, smaller teeth and intestines then followed. So brain size and teeth size are not correlated per se, neither are brain size and gut size. However, the relationship between all three is cooking: cooking denatures the protein contained in the food and breaks down cell walls, gelatinizing the collagen in the meat allowing for easier chewing and digestion. So the fact that tooth size and brain size do not have a relationship throughout our evolution is not a blow to the cooking hypothesis. The introduction of softer foods is the cause for both the decrease in tooth size and gut size. Cooking is a driver of all three.

Fonseca-Azevedo and Herculano-Houzel (2012) showed that the availability of kcal from a raw diet is so limiting that without a way to overcome this limitation, modern Man would not have been able to evolve. Our brains would not have emerged if not for the advent of cooking. Indeed, Herculano-Houzel and Kaas (2011) showed that the outler is not our brains being bigger than our bodies, great apes have bodies too big for their brains, reversing a long-held belief on our brain-body relationship. Cellular scaling rules apply for all primates, so knowing this, the Colobinae (old-world monkeys) and the Pongidae (gorillas, chimpanzees, and orangutans) favored increases in body size, in line with the ancestor that we share with great apes, while our lineages showed gains in brain size and not body size, possibl due to a metabolic limitation of having both a big brain and body. Indeed, the amount of neurons a brain can hold along with how big a body can realistically get impedes the relationship between the brain and body. You can have either brains or brawns, you can’t have both.

We should then look for when genetic changes in our genome occurred from cooking. Carmody et al (2016) show that these genetic changes occured around 275-765kya. We know that differing nutrients change gene expression, so, over time, if these changes in gene expression were beneficial to the hominin lineage, there would be positive selection for the gene expression. Carmody et al (2016) took 24 mice and fed them either cooked or raw foods for 5 days. Two hours into the 5th day, mice were ‘sacrificed’ (killed) and their liver tissue was harvested and immediately (within 60 seconds of death) were flash frozen for later analysis. They evaluated differential gene expression for cooked/raw food, calorie intake (raw/fed), energy balance of the consusmer (weight gain/loss over 5 days of feeding), and food type (meat/tuber). The diet consisted of either organic lean beaf round eye toast or sweet potato tubers cooked or raw. They gave restricted rations to evaluate the effect of a cooked diet with negative energy status (this is important).

They cooked the meat until it gelatinized (around 70 degress celsius), which is equivalent to medium well-done. They were then given the same diets, cooked/raw, free-fed or restricted sweet potato tubers or meat. The mice were weighed during periods of inactivity and the food they refused to eat was weighed to monitor fresh weight than freeze-fried to monitor dry weight.

The most interesting part of this experiment, in my opinion, was that the mice that were free-fed with cooked diets consumed less kcal than the mice that were free-fed raw diets. They discovered that free-fed cooked diets led to the maintenance of body weight, whereas the free-fed raw diet led to weight loss. This confirms that cooked food gives more energy than raw food, which was itelf a critical driver in our evolution as humans.

When they looked at the livers of the sacrificed mice, they found that the mice that were fed meat showed liver gene expression patterns that were more similar to mice fed a human diet than mice that were fed tuber. The mice that were fed cooked food showed similar gene expression to mice fed a human diet and more similar to the human liver than in the mice fed the raw food. Even more interestingly, the mice fed tuber or raw foods exhibited liver expression patterns more similar to mice fed a chimpanzee diet and gene expression patterns noticed in non-human primates. Their analysis on the gene expression from cooked/raw diets compared to another data set showed that these genes that were expressed went under selection between 275-765kya.

Food type and preparation were associated with significant changes in gene expression, but those related to cooking were shown to have evidence of possible selection in the timeframe state by Carmody et al. These results also show that along with cooking increasing the bioavailability of foods, habitual cooking would have led to less energy spent on immune upregulation. This energy could then be used for other bodily processes—like our increasing brain size/neuronal count.

Carmody et al show that the biological evidence for cooking is 2mya, archaeological evidence 1mya, hearths 300kya, not too many Neanderthals controlled fire until 40 kya, and the earliest direct evidence we have of cooking appears around 50kya. We can obviously look at physiological, metabolic and diet differences between hominins and infer what was eaten. Now with looking at changes in gene expression, we can pinpoint when the positive selection began to occur. The biological evidence, in my opinion, is the best evidence. We don’t need direct physical evidence of cooking, we can make inferences based on certain pieces of knowledge we have. All in all, this new study by Carmody et al show that 1) cooking definitely predated modern humans and 2) many different hominins practiced cooking. This evidence shows that cooking for ancient hominins occurred way earlier than the archaeological record suggest.

Now, remember how the mice free-fed on a cooked meat diet ate less yet maintained their weight? There is a reason for this. Protein is the most filling macro (followed by fat, fiber then CHO). So it’s no surprise that the mice at less of the cooked meat. What was a surprise was that the mice maintained their weight eating less kcal then the mice that ate a raw foods diet. This is yet more evidence that cooking released us from the metabolic constraints of a raw, plant-based diet.

For those who have some knowledge of human metabolism, you may have heard of the thermic effect of food. The thermic effect of food is the amount of energy expenditure above the basal metabolic rate due to the cost of processing food and its storage. So if you’re cooking food before you ingest it, you bypass a lot of the processing that happens internally after digestion, allowing you to extract close to 100 percent of the kcal contained in the food. Due to cooking’s effects on foods, since we our bodies have to use some of the energy we consume to function and process the kcal, getting higher quality food was beneficial to us since we could have more for our bodily functions and to power our growing brains. Since we were able to get higher quality calories from cooked food, the effects of TEF weren’t as large, which was yet another constraint that we bypassed with a cooked diet. A cooked diet is more efficient than a raw one in more ways than one.

One more thing I forgot to mention in my series of articles on the benefits of cooking and human evolution is the effect it had on our microbiome. The completion of the Human Microbome Project (HMP) was imperative to our understanding of the trillions of bacteria that live in our guts. It was commonly stated that the bacteria in our guts outnumbered regular bacteria with a 10:1 ratio. However, Sender, Fuchs, and Milo (2016) showed that on average, there is about a 1:1 ratio with about 30 trillion normal bacteria and 39 trillion gut bacteria, some people possibly having double the amount of gut bacteria in comparison to regular bacteria, but nowhere on the level of 10:1 that has been stated for the past 40 years.

The human microbiome has undergone a substantial change since the divergance of humans and chimpanzees (Moeller et al, 2014). Over the course of our evolutionary history, our microbiome has become specialized to animal-based diets. Wild apes have way more diversity in their gut microbiota than humans do, indicating that we have experienced a depletion in our microbiota since our divergence with chimpanzees. This comes as no surprise. With the introduction to cooked foods, our microbiota became adapted to a new selective pressure. Over time, our gut microbiota became less diverse but more and more specialized to consume the food we were eating. So the introduction to a cooked diet both changed our gut microbiota as well as giving our bodies enough energy to power itself and its processes, the brain and our gut microbiota that are imperative for our development.

All that being said, some people may say “Cooking isn’t a selective pressure; neither is bipedalism nor tool-making”, and they would be correct. However, human tool-making capacities reflect increased information-processing capabilities (Gibson, 2012). So, clearly, there were some changes in our brains before the use of tools. This change was the advent of bipedalism which allowed our bodies to conserve 75 percent more energy in comparison to knuckle-walking (Sockol, Racihlen, and Pontzer, 2007). This was yet another constraint that we bypassed and allowed our brains to grow bigger. When we left the trees, we then became bipedal and that therefore increased the availability of edible foodstuffs for us. This increased our brain size, and as we learned to make tools, that increased our information-processing capabilities.

Cooking, of course, is not a selective pressure. What cooking did, however, was release the use from the metabolic constraints of a raw, plant-based diet and allowed us to extract all of the nutrients from whatever cooked food we ate. This event—one of the most important in human history—would only have been possible with the advent of bipedalism. After we became bipedal we could then manipulate our environement and make tools.

I figure I may as well touch on the Expensive Tissue Hypothesis (ETH; Aiello and Wheeler, 1995) while I’m at it. The ETH states that since our guts are metabolically expensive tissue—as well as our brains—that there was a trade off in our evolutionary history between our brains and guts. However, Navarette, Schaik and Isler (2011) showed that the negative correlation was with fat-free mass and brain size—not with the gut and brain size. However, as I noted earlier in this article, our guts reduced in size due to diet quality, e.g., softer foods. So while the correlation is there for the brain size increase/gut reduction, it is not causal. Diet explains the gut reduction and brain size increase, but the brain size increase did not cause the gut reduction.

In sum, genetic changes from cooking occured between 275-765kya. But we controlled fire and began to cook between 1-2mya (archaeological evidence says 1-1.5 mya while biological evidence says 2 mya). Cooking led to differences in gene expression and then positive selection in the hominin lineage. Mice that were fed a raw diet showed gene expression similar to a chimpanzee fed a raw diet while mice fed a cooked diet showed gene expression like that of a human. This is huge for the cooking hypothesis. What this shows is that while the gene expression occurred while we started cooking, the actual positive selection didn’t occur in our genomes for about 1my after we began cooking. This is more evidence that cooking released us from metabolic constraints, as mice that were fed a cooked diet maintained their weight even when eating less kcal than mice fed raw foods.

When thinking about the evolution of Man and our relationship with fire, we should not forget about how the body uses some of the kcal is ingests for bodily processes. Furthermore, we cannot forget about our microbiome which evolved for an animal-based diet. Those two things both cost caloric energy. The advent of cooking released us from the energetic constraints of a raw, plant-based diet as well as gave our microbiome higher quality energy. When we take both the TEF and our microbiome into account, we can then begin to put 2 and 2 together and state that along with cooking freeing us from the metabolic constraints that apes have to go through due to their diet, it also benefitted our microbiome and gave our bodies higher quality energy to power it.

We would not be here without cooking. Thank cooking for our dominance on this planet.

References

Aiello, L. C., & Wheeler, P. (1995). The Expensive-Tissue Hypothesis: The Brain and the Digestive System in Human and Primate Evolution. Current Anthropology,36(2), 199-221. doi:10.1086/204350

Carmody, R. N., Dannemann, M., Briggs, A. W., Nickel, B., Groopman, E. E., Wrangham, R. W., & Kelso, J. (2016). Genetic Evidence of Human Adaptation to a Cooked Diet. Genome Biology and Evolution,8(4), 1091-1103. doi:10.1093/gbe/evw059

Fonseca-Azevedo, K., & Herculano-Houzel, S. (2012). Metabolic constraint imposes tradeoff between body size and number of brain neurons in human evolution. Proceedings of the National Academy of Sciences,109(45), 18571-18576. doi:10.1073/pnas.1206390109

Gibson, K. R. (2012). Human tool-making capacities reflect increased information-processing capacities: Continuity resides in the eyes of the beholder. Behavioral and Brain Sciences,35(04), 225-226. doi:10.1017/s0140525x11002007

Herculano-Houzel, S. (2016). The Human Advantage: A New Understanding of How Our Brains Became Remarkable. doi:10.7551/mitpress/9780262034258.001.0001

Herculano-Houzel, S., & Kaas, J. H. (2011). Gorilla and Orangutan Brains Conform to the Primate Cellular Scaling Rules: Implications for Human Evolution.

Moeller AH, Li Y, Mpoudi Ngole E, Ahuka-Mundeke S, Lonsdorf EV, Pusey AE, et al. Rapid changes in the gut microbiome during human evolution. Proceedings of the National Academy of Sciences. 2014;111(46):16431–35.

Navarrete, A., Schaik, C. P., & Isler, K. (2011). Energetics and the evolution of human brain size. Nature,480(7375), 91-93. doi:10.1038/nature10629

Sender, R., Fuchs, S., & Milo, R. (2016). Revised estimates for the number of human and bacteria cells in the body. doi:10.1101/036103

Sockol, M. D., Raichlen, D. A., & Pontzer, H. (2007). Chimpanzee locomotor energetics and the origin of human bipedalism. Proceedings of the National Academy of Sciences,104(30), 12265-12269. doi:10.1073/pnas.0703267104