Posted by: SLS | November 14, 2010

Summary of Anthropological Evidence for Human Diet

When I set out to create this blog, I did not want the discussions to be about me or about my personal thoughts or about what I do in my free time. I think there is already enough of that sort of extraneous clutter circulating the internet, and far too few places where frank discussions about legitimately sourced information can disseminate without bias or speculation. However, since I did create this blog to talk about muscular physiology and how to optimally express inherent traits for athletic performance, diet is inevitably a major component in sustaining a healthy metabolism and is an important discussion topic, even in a muscle specific blog such as this. The topic of human diet is very dear to me because it was the theme for my masters thesis. Therefore, I have lots to say on the subject, am a bit opinionated (opinions formed after years of diligent literature surveys), and take this topic very seriously.

Why is an evolutionary approach so important?

There are scores of arguments about whether there is such thing as an optimal human diet and then there are the related questions: what in particular makes it optimal, would it be optimal for everyone, can it apply to athletes and non-athletes, what about selective adaptations in certain populations or ethnic groups, what about people with illnesses…? The exceptions can begin to look more like the norm. I am not qualified to answer all of these questions, nor do I think even those who are so qualified find it practical or possible to blanket all the exceptions under one rule. However, I do firmly believe that an analysis of our (hominin) evolutionary experience is the first and most vital step towards a correct assessment of the types of foods humans are best adapted to consume. The reason for this is because an understanding of the hominin and hominoid evolutionary experience means we will necessarily understand the particular dietary factors that allowed hominin species to thrive and reproduce. Disease and sickness negatively affect reproductive fitness, so our diet must fall in line with keeping us as healthy as necessary to successfully produce the next generation. An evolutionary approach to diet allows us to understand the following (in no particular order):

  • food consumption over an enormous time scale
  • origin and adaptation dental morphology
  • origin and adaptation of digestive morphology
  • hominin energetics
  • developmental trajectories and timing of life-history events (infancy, childhood, adolescence, adulthood)
  • comparative developmental and adaptive strategies of close primate relatives in conjunction with ecological preference
  • vestigial traits (e.g. modern human tail bones)
  • available resources in ancient environments
  • health and disease patterns
  • fitness advantages and disadvantages
  • human biochemistry and physiology

I’m probably missing a few good points as well, but you can easily see a benefit to this exercise. I believe that to understand what we are, we must know where we came from (broadly, beginning with the primate-like mammal, Plesiadapis of the late Paleocene ~55 million years ago, but more relevantly our lineage took shape following the split of our ancestral line from the ancestral line of our closest living relative, the Chimpanzee, around 6-8 million years ago), and how we came to be Homo sapiens (the selective pressures our ancestors faced to bring about our species specific adaptations). I cannot stress how important I believe an evolutionary approach to be in understanding any organism’s biology, behavior or ecology. But perhaps that’s a personal bias? At least I am in good company…

“Nothing in biology makes sense except in the light of evolution”Theodosius Dobzhansky

Our current dietary physiology is the product of adaptations that occurred, at the least, 140 thousand years ago but to a larger extent, over 4 million years ago, when the first of Australopithecus walked the earth. Therefore, our physiology is the product of adaptations to ideally suit us in an ancient landscape going back not just 40,000 or 140,000 or 250,000 years ago, but multiple millions of years. Recently, a popular term to describe the era of our adaptive legacy in our formative years is “Paleolithic”. The Paleolithic describes a portion of time between the lower and upper Pleistocene from about 2.5 million years ago to 12,000 years ago, known as the Stone Age. The Paleolithic was ascribed to the period of time when stone tools were first created and widely used amongst hominin populations until the end of the Upper Paleolithic and beginning of advanced neolithic tools and textiles. Two and a half million years ago is a marked point in history because it heralds the accepted time of emergence of the Homo genus, beginning with Homo habilis. This is the time when an extremely significant and distinctive feature set apart the human lineage from the australopithicine precursors. Homo habilis possessed an enlarged brain, 150 cm³ larger than its direct ancestor, Australopithcus garhi. Actually, not only did brain size begin to increase, but other cranial and post-cranial morphological features began to change, curiously indicating a trade-off in metabolism. The brain grew larger while the gut became smaller.

Morphological predictors of diet

What is actually really interesting about modern human cranial morphology is its distinct lack of specific adaptations. We might conclude from this that humans are the ultimate omnivore. However, we must also bear in mind that a significant change in our dietary habits is our ability and penchant for processing our food through grinding, cutting, cooking, fermenting, and now in the technological age, genetic manufacture or chemical reconstitution. Going back to H. habilis, we see some reduction of cranial robusticity from Australopithecus: smaller mandible, smaller muscle attachment points on the top of the cranium, breadth reduction of the zygomatic arch (jaw-muscle passage), smaller anterior pillars for chewing force absorption, reduction of molar size and enamel thickness, greater occlusal relief on molars for shearing and tearing, and less flat molar surface for crushing or grinding.

All of these cranial changes indicate a change in emphasis from chewing hard brittle and fibrous substances (perhaps seeds, nuts, and vegetation), to chewing softer foods such as meat or fruit. Coinciding with reduced cranial robusticity was an increase in brain size and reduction in gut size, indicating a dietary change from low nutritive and calorically sparse roughage, to nutrient and calorie dense food, requiring less digestion. The most nutrient and calorie dense food in the wild is flesh. Aiello and Wheeler coined the “Expensive Tissue Hypothesis” to explain these changes, suggesting that the body put its metabolic resources into building and maintaining a bigger brain, rather than building and maintaining a big gut. If calorie consumption were to remain feasible in a wild and resource rare environment, a trade off had to occur among the metabolically active tissue.

Gut size is highly correlated with diet and relatively small guts are compatible only with high-quality, easy-to-digest food. … No matter what is selecting for relatively large brains in humans and other primates, they cannot be achieved without a shift to a high-quality diet unless there is a rise in the  metabolic rate. Therefore the incorporation of increasingly greater amounts of animal products into the diet was essential in the evolution of the large human brain”

-Aiello and Wheeler

Despite our close relation to the other great apes (chimps, gorillas, siamangs, gibbons, orangutans, and bonobos), our gut sizes are markedly different. Chimps thrive mostly on fruit along with leaves and flowers with the very rare consumption of meat. Gorillas are very specialized and exclusive leaf munchers, while orangutans subsist on fruit, young leaves, shoots, nuts and bark. These vegetable and cellulose heavy diets require intense digestion, and constant feeding, something that the human gut is not capable of maintaining for long. Here is a diagram showing the proportional differences in gut size between the great apes and humans. Notably, humans have the longest small intestine and shortest large intestine, along with a non-existent cecum. The cecum and large intestine are important for fermentation and breakdown of cellulose whereas the small intestine is where the majority of nutrient absorption occurs for animals that don’t rely on the breakdown of cellulose for nutrients.

Physiological predictors of diet

The human brain comprises about 25% of the adult body’s total resting metabolism (higher for children) and consumes about 100-145g glucose per day. Coupled with high activity rates, the human body demands a much higher caloric intake than any other living or non-living anthropoid, but with only a small gut to handle to high caloric load. The small gut and large brain meant that our human and Homo ancestors had to prioritize acquisition of energetically dense foods (fat yields 9 kcal per gram versus 4 kcal from protein and carbohydrate) in a wild and unpredictable environment. The exact timing of intense genus Homo species radiation 2.5 million years ago coincided with a long downward trending climate and severe temperature oscillations- the signs of an ice age.

According to ethnographic and fossil studies, carbohydrate rich foods are rare in the wild and were almost nonexistent during the Ice Age. The Caribou Inuit are a prime example of what periglacial human life may have been like 30,000 years ago, particularly with the Cro-Magnons in subarctic Eurasia. Living in the heavily glaciated Western shore of the Hudson Bay, the Inuit subsist primarily on large Arctic animal species such as caribou, bears, seals, and whales for 90% of their diet. Only during the summer months and in lower elevation do they gain access to sparse tundra vegetation. Going back in time over the Pleistocene (2.5 million years ago – near present day), we see that hominin fossil dates map closely with drastic climate change.

Speciation events may have been driven from these changes by disrupting environmental niches and changing the availability of resources. Changes would include extinction or scarcity of some resources with a dynamic environment characterized by unpredictability. Diet specialization would be selected against, and a series of what William H. Calvin described in his book as “cool, crash, and burn” events undoubtedly impacted evolutionary history. Changes in landscape necessitated different survival strategies, altered predator-prey relations, and caused rapid speciation. Populations either adapted to the climate induced changes or died. Where grasslands proved the mainstay environment, hominin populations adapted by switching dietary focus on the grazing animals, prompting higher consumption of preformed long-chain PUFA’s (polyunsaturated fatty acids), which provided the right constituents and timing for brain growth. We are indebted to meat for our brains, and now, since the advent of agriculture, brain size has actually decreased by 11% since our Paleolithic ancestors. Both the decrease in brain size and decrease in consumption of animal foods coincided with the agricultural revolution, when low quality, mass produced food replaced the diverse subsistence on wild plants and animals.

Since the brain is a large metabolically active organ that requires a lot of energy from glucose, humans adapted by deriving sugar from fat and protein in a process called gluconeogensis that occurs in the liver. Some contemporary views on evolutionary diet suggest that humans show an adaptive predisposition to insulin resistivity in the muscles (a precursor to diabetes when the condition is chronic) while maintaining the propensity to synthesize glucose from other macronutrients and shunt it directly to the brain. This idea comes from research on dolphin metabolisms because, like humans, dolphins have large brains and subsist on high protein diets. Their muscle tissue becomes fully insulin resistant to allow their derived glucose to nourish the brain. This means that high quality protein and fat sources were most likely the staple foods for early humans with limited access to sugary, starchy or otherwise high carbohydrate and low fiber foods.

Isotope analysis as indicator of early human food consumption

Isoptope analysis obtains stable carbon and nitrogen isotope ratios that can reflect main dietary contributions of protein- whether plant or animal sourced protein and type of environment. The ratios are expressed as a delta carbon or delta nitrogen atom value (δ¹³C or δ15N) and their variability indicate food sourcing (marine or terrestrial), plant photo synthetic pathways (C3, C4 and CAM), and trophic level of the organism (how high it resided in the food chain- carnivores are highest, then herbivores, then photosynthetic organisms at the bottom). Human bone analysis has shown that Paleolithic humans consumed large quantities of animal protein from land and marine sources. The study done by Richards and Trinkaus (2009) demonstrated that the trophic level of Paleolithic humans was higher than that of obligate carnivores such as the wolf or hyena. Certainly Paleolithic humans feasted on animal flesh.

Conclusion

Presented above is a summary of a few lines of researched used in reconstruction of hominin diet ecology. This is not a comprehensive summary, but hopefully still convincing support for the importance of meat in the human diet as our evolutionary legacy. Other techniques used to reconstruct hominin diets include:

  • paleoecology
  • coprolite analysis
  • tool technologies
  • bone assemblage analysis
  • archaeological assemblages
  • ethnographic studies
  • non-human primate studies
  • genetic research.

To finally address some criticism about resorting to evolutionary models for optimal modern human dietary patterns as backwards or faulty assumptions, I’d like to point out that the evolutionary model presents the diet to which we are best adapted, and indeed there is enormous evidence (not speculation) for the types of foods consumed by Paleolithic humans. We have spent the longest period of our evolutionary history as hunter-gatherers, allowing enough time for genetic, morphological, and physiological adaptation to meat consumption, and the modern reliance on high carb diets full of annual grain and refined plant products is a relatively novel experience for our Stone-Age physiology. We are now a sick people. Our teeth rot, we contract cancers, autoimmune diseases, metabolic diseases, require intense medical support, and yet we believe that many of these processes are merely natural results of aging. What we fail to realize now is that the groups of people who have maintained the diets of our Paleolithic ancestors (modern hunter-gatherer tribes) exhibit superb health and vigor compared to the agrarian populations. They do not have cavities or require braces, cancers are nearly non-existent, atherosclerosis is minimal if at all present, muscle tone maintains long into old age, and death usually results from violence or pathogen infection, or simply comes to the elderly in their sleep and is not the result of metabolic disease.

To say that human diet in evolution was merely the diet that was sufficient for survival long enough to reproduce is ignorant of the aggregate human evolutionary experience. It ignores the coincident evolution of a long life-span and post-reproductive survival in our species. Humans cannot just reproduce and die. If we did, our progeny would die with us since we experience a long period of dependency through childhood and adolescence. Even after reaching maturity, the first generation offspring is more successful when the parent generation survives to become the grandparent generation. This is the “grandmother hypothesis”. We are a social matriarchal species. The reproductive fitness of the individual is dependent on the survival of future generations. What better way to ensure survival than to hang around and provide extra care? Also, the development of our large brain and the dietary change from plants to meats also induced a longer childhood developmental period, thus placing even more emphasis on successful parental longevity. Gurven and Kaplan explain this with striking clarity in their article on “Longevity among hunter-gatherers: A cross cultural examination.” It’s likely that the “age-specific mortality pattern” of human beings also evolved “during our hunter-gatherer past.” That is, the human potential for longevity is not a product of modern living; instead, it appears to be a genetic characteristic shared by all Homo sapiens.

I believe that the weight of evolutionary evidence overwhelms criticism against the importance of evolutionary models for human diet. Instead, I believe any approach to enrich and aid human health must be viewed through an evolutionary lens lest it be  short-sighted and incomplete.


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Responses

  1. “… we see that hominin fossil dates map closely with drastic climate change.”

    Why would this be? And also what is the significance of it for your thesis?

    Fantastic article, BTW. Thanks.

  2. The purpose of explaining that species radiation can be mapped to climatic swings is to suggest that perhaps climate was a serious selective pressure on apes and hominins.

    My thesis explored whether there was any correlation between the type of macronutrient consumed and the amount of energy expended in a modern human population while comparing to a hunter-gatherer model, like we see in primate energetics in the wild. The conclusion was that essentially there is too much cultural noise to apply a biological model to modern humans.


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