Why Did Our Brains Stop Expanding?


Tony Wright will be joining host Dennis McKenna for the live, interactive video course, “What Plants Can Teach You: Consciousness and Intelligence in Nature.” A new paradigm is emerging that recasts how we relate to and understand nature, supported by new scientific evidence. Plants instruct us through their behavior, through their interdependence with the environment, and through direct transmissions conveyed by spirit.  Along with Tony and Dennis, the course gathers  some of the leading experts in the emerging field of plant intelligence, including: Chris Kilham, Stephen Harrod Buhner, Dayna Baumeister, and Simon G. Powell. This 5-part Evolver webinar starts on June 17. Click here to learn more

The following is excerpted from Return to the Brain of Eden: Restoring the Connection between Neurochemistry and Consciousness by Tony Wright and Graham Gynn, recently published by Inner Traditions. 

 

In the forest the human brain was expanding and expanding at a phenomenal rate. Sometime at around 200,000 to 150,000 years ago, this process came to an end. The brain stopped expanding and started to shrink. This key point in our evolutionary journey has been noted but rarely addressed, and its significance comprehensively ignored.

Christopher Ruff, of John Hopkins University, and his colleagues thoroughly analyzed the fossil record to determine the evolving body mass and brain size of the various Homo species leading up to us. The results show that the assumption of a straight progression from a pea-brained ancestor to the ultrabrainy modern Homo sapiens is decidedly shaky. Hominid brains appear to have remained fairly constant in size for a long period from some 1.8 million years ago until about 600,000 years ago. But then, from 600,000 to 150,000 years before the present, fossils show that the cranial capacity of our ancestors skyrocketed. Brain mass peaked at about 1,440 grams (3.17 pounds). Since then brain mass has declined to the 1,300 grams (2.87 pounds) that is typical today (Ruff 1997).

Of course, brain size alone does not tell the whole story. Brain size also correlates with body size, and the peak of brain size roughly corresponds to the peak in archaic Homo sapiens’ body size (the Neanderthals). The decline in size of the body in Homo sapiens sapiens (modern humans get two “wises” in our name, but do we really deserve it?) over the past fifty thousand years has raised our ratio of brain-to-body size to just above Neanderthal levels. Yet we have done this by shrinking our bodies to a greater extent than our brains have shrunk. There is some evidence that our brains are still shrinking and that they may have done so over the last ten thousand years by as much as 5 percent.

This very recent period of brain shrinkage coincides with a major dietary change, for it was around this period that cereals and grain (grass seed) came to the fore. Cereals and grains may be the foundation of our diet today and responsible for the huge explosion in our numbers, but they may not be the best foods for optimal function. Indeed, studies of skeletons from early agricultural societies show that ill health accompanies the initial transition to eating more grains and cereals. Skeletons dug up from the East Coast of the United States, dating from around 1000 CE, the era when Native Americans switched to corn-based agriculture, are smaller than earlier skeletons. Studies of skeletons from other societies undergoing this transition show signs of deficiencies such as anemia. Clark Larsen, the physical anthropologist who studied the East Coast skeletons, has stated, “Just about anywhere that this transition to cereals occurs, health declines”(Larsen 2002).

It is thought that humans from such agrarian societies were lucky to live beyond thirty years. In contrast forest apes, such as chimpanzees, can live for some sixty years. We can reasonably assume that humans in the forest lived easily as long if not longer. Furthermore, if man in the forest was as long-lived or even longer-lived than chimps, it would provide a strong argument for the notion that this was both the most natural and most suitable place, particularly in terms of diet, for a human to live.

Ancestral Diets 

If the evolution of the unique human system was somehow linked with our ancestral diet, we would expect the human system still to be best adapted to something approaching this. While there is continued debate on this subject, few dissent from the view that there is an increasing problem with the food we are eating in our sophisticated, time-stressed modern world. In just one six-week period, newspaper headlines in the United Kingdom announced: “World Alert over Cancer Chemical in Cooked Food” (Daily Telegraph, May 18, 2002); “Children at Risk from the Junk Food Time Bomb” (Daily Mail, May 31, 2002); and “Anti-social Conduct May Be Linked to Diet, Says Study” (Guardian, June 26, 2002). This is a small sample of worries arising from recent research. Today, we are told we risk diabetes, heart disease, and cancers from eating the “wrong sort of food.” Weight problems caused by an addiction to high-fat and high-sugar convenience foods, or simply an ignorance of the alternatives, carry the risk of these and other diseases manifesting in later life. One in ten children under age four is now classified as obese, and health problems resulting from being overweight cost Britain some two billion pounds a year. It has been estimated that if we continue eating a “junk food” diet, in forty years time half the population will be obese. Furthermore, specialists also fear that ­anemia due to poor nutrition in early life can have long-lasting effects on a child’s mental development and learning ability.

Although longevity has increased over the last few centuries, many folk live the last years of their lives with the fear of disease, if not the actuality of it, but old age and disease do not necessarily go together. In the remote Andean highlands of Ecuador, there are communities of people who it is claimed live for 140 years or more and who remain agile and lucid right to the end. Death from heart disease and cancer is unknown in these high mountain valleys but rife in nearby towns. David Davies, an English zoologist and member of the gerontology clinic, University College, London, who has made a study of these “centenarians of the Andes,” found that the people who have the best chance of a healthy old age are those who actively use their minds and bodies, even toward the end of their life span. He looked at many elements of their life and environment, from genetic factors to the tranquility and lack of stress in their way of life. The folk who lived longest were found among those who lived on a subsistence diet, which was low in calories and animal fat. Typically, the main meal of the day was eaten in the early evening and was made up of very small wild potatoes, yukka, cottage cheese, and maize or bean gruel. Melons were eaten for dessert. Sometimes green vegetables, cabbage, or pumpkins were added to the menu, and sweet corn cobs were often taken to work for lunch. The people working in the fields ate fruit throughout the day. The climate is ideal for citrus fruits, and many other “hedgerow” fruits such as mora (like a blackberry), guava, and naranjilla are abundant, too. Meat was only eaten rarely, a type of cottage cheese was made from goat or cow milk, and eggs were eaten raw or almost raw (Davies 1975).

Though these people are very healthy and extremely long-lived, we mustn’t necessarily jump to the conclusion that this diet is perfect for the human system; their diet is restricted by the environment they live in. However, if we look at other communities of long-lived folk, the parallels are striking. The Hunzas of northeast Kashmir also live in mountainous regions and have a diet that includes wheat, barley, buckwheat, beans, chickpeas, lentils, sprouted pulses, pumpkins, cottage cheese, and fruit—the famous Hunza apricots and wild mulberries. Meat is again only eaten rarely, and because fuel is in short supply, when food is cooked it is usually steamed—a method of cooking that is the least damaging to the chemical nutrients in the food. Hunzukut males, like the people in the Andean highlands, are also reported to live to 140 years of age. So, we must conclude that these diets are, at the very least, much more suitable than the ones we depend on in the affluent industrialized countries.

There seems to be no definitive study that has so far convinced society as a whole that nutritionally we are barking up the wrong tree (or at least not picking from the right one). But there are many scraps of information that support the thesis that a more natural diet is the most beneficial option. Lymphocyte production and hence resistance to illness is boosted by consuming the nutrients that occur in optimal proportions and quantities in uncooked vegetables. There are also a huge number of cases in which raw food, particularly fruit and vegetable juices, has seemingly cured a wide range of illnesses. Migraines, skin complaints, tuberculosis, mental disorders, heart disease, cancers, and a host of other diseases have responded favorably to a diet rich in raw food. There are clinics, foundations, and institutions throughout the world that offer therapies based on “living nutrition.” Such diets are much closer to our ancestral diets than the chips, pies, and cookies that adorn most of our supermarket shelves.

As with all organisms, hominids in the course of evolution were locked into the biological matrix of their environment. Whether our diet consisted of insects, fruit, or meat, it was all biologically active material. Some primates today eat a bit more of this or that; much coverage has been given recently to meat-eating chimps, but this comprises a relatively small percentage of their diet. Despite their skill in capturing live prey, chimpanzees actually obtain about 94 percent of their annual diet from plants, primarily ripe fruits. Primate biochemistry is largely based on plants, and a plant-based diet is what hominids were eating during their evolutionary development. A pictorial representation of an early human living in the forest, lounging around eating fruit, may be more accurate than one in which he is dressed in animal skins, spear in hand, on the hostile open plains.

The lack of plant material in the fossil record has led, according to Richard Leakey, the paleoanthropologist famed for his work in Kenya, to an overemphasis on meat eating as a component of the early hominids’ life. He also finds some of the work on tooth analysis “very surprising” (Leakey, 1981, 74). The teeth of Australopithecus robustus fall into the fruit-eating category. The patterns of wear and the small scratches left on the enamel appear very similar to those of the forest-dwelling chimpanzees, yet here was a hominid that was supposed to live on the plains in an era when the climate was dry and the vegetation mainly grass. The examples of Ramapithecus teeth that have been similarly analyzed show exactly the same patterns, and the teeth of Homo habilis, the first creature to be awarded Homo status, also have smooth enamel typical of a chimpanzee. This evidence is extremely relevant. All the early hominids and their great ape cousins were mainly fruit-eaters. The teeth of Homo erectus suggest a more omnivorous diet. The enamel from their teeth shows scratches and scars that are compatible with grit damage, possibly from consuming bulbs and tubers. As a response to a cooling climate and a contraction of the forest, did this species widen its diet to adapt to a new environment? Some forest would have remained intact along the wetter river systems. Chimpanzees and gorillas survived there along with, we suspect, another hominid whose teeth were very well adapted to fruit eating—Homo sapiens.

Primates, given a choice, will select fruit in preference to any other food. Fruit is a rich, nutritious, and easily digestible food. If it is available, this is what all the great apes prefer to eat. However, other foods are eaten regularly. Our nearest relative, the bonobo, eats between 60 percent and 95 percent fruit, depending on the fruit productivity of its specific habitat. The rest of its diet comprises mostly shoots and herbs and a small amount of insects, eggs, and the occasional small mammal. Fallback foods like bark may also be eaten in times of fruit scarcity.

What humans in the forest ate is, of course, unknown, but it is likely that they would have eaten a similar balance of foodstuffs. They would not have been purely vegetarians. Even figs (perhaps the most preferred food) contain a small amount of insect matter as their pollination mechanism results in eggs and larvae of small wasp species remaining in the fruit. These insects may have served as an important source of essential micronutrients such as vitamin B12 and provided a little extra protein.

As they were the most highly intelligent animals in the forest and fruit was the best food, it is likely that humans developed strategies to maintain a high percentage of fruit all year round. Being efficient bipeds would have given them the potential to travel easily between widely separated fruit sources. The quest for distant fruit trees may have even honed their bipedal adaptation. The larger arboreal primates are known to travel on the ground between distant fruit trees, as it is more efficient than traveling in the trees. Archaic humans, being better-adapted bipeds than apes, would have found this way of life much easier.

Humans Are by Nature Frugivorous

There has been much study and even more speculation about what sort of diet our teeth and guts are best designed for. From the type of ­dentition, gut length, and toxicity of foods like meat, a very strong case can be built for Homo sapiens being designed to eat and process a largely fruit-based diet. The brain’s requirements for food and the gut’s requirements for energy, the optimal acid/alkali balance, and the structure of the intestines all point to a frugivorous diet. A shift to fruit specialization answers all the problems and anomalies that have spawned countless conflicting theories.

Katherine Milton, professor of anthropology at the University of California, Berkeley, has carried out important work on diet and primate evolution. Her research has led her to believe that “the strategies early primates adopted to cope with the dietary challenges of the arboreal environment profoundly influenced their evolutionary trajectory” (Milton 1993). This has a great significance for us today for the foods eaten by humans now bear little resemblance to the plant-based diets anthropoids have favored since their emergence. She believes these findings shed light on many of the health problems that are common, especially in our industrially advanced nations. Could they be, at least in part, due to a mismatch between the diets we now eat and those to which our bodies became adapted over millions of years?

The plant-based food available in the forest canopy comprises fruit and leaves, but subsisting on this diet poses some challenges for any animal living there. For a start it is high in fiber that is not only difficult to break down and hence digest but also takes up space in the gut that may otherwise be filled with more nutritious foods. Many plant foods also lack one or more essential nutrients such as amino acids, so animals that depend on plants for meeting their daily nutritional requirements must seek out a variety of complementary food sources. Fruit is usually the food of preference, for it is rich in easily digested forms of carbohydrate and relatively low in fiber, but its protein content is low (the seeds may be protein rich, however). Leaves offer higher protein content, but they are lower in nutrients and contain much more fiber. Balancing these constraints has led to different strategies that are reflected in behavior and physiology. Colobine monkeys have compartmentalized stomachs (a system analogous to that of ruminants) that allow fiber to be fermented and hence processed very efficiently, but humans and most other primates pass fiber largely unchanged through their digestive systems. Some fiber can be broken down in the hind gut of these latter species, but the process is not as efficient as that in the Colobus genus.

Milton’s research focused on two contrasting species of South American primates: howler and spider monkeys. These two species are about the same size and weight as each other and live in the same environment, eating plant-based foods, yet they are very different. Howler monkeys have a large colon, and food passes through their digestive system slowly, whereas spider monkeys have a small colon through which food passes more quickly. These physiological differences relate to dietary specialization. The foundation of the howlers’ diet is young leaves: 48 percent of their diet is leaves, with 42 percent fruit and 10 percent flowers. The spider monkeys’ diet comprises 72 percent fruit, 22 percent leaves, and 6 percent flowers. Another fundamental difference is that although these animals are the same size, the brains of spider monkeys are twice the size of howler brains. Very significantly, Milton comments, “The spider monkeys in Panama seemed ‘smarter’ than the howlers—almost human” (Milton 1993).

This is something we have commented on before: big brains and a diet high in fruit appear to go together. Why should this be so? Could this brain enlargement result from the need to memorize the location of productive fruit trees, as some have suggested, or did elements within the fruit itself fuel this change more directly, as we propose? Animals such as squirrels, and even birds like jays, memorize the locations of stored food most efficiently without an overlarge brain, thus it seems that something else must be responsible.

Although Milton has concluded that it is quite difficult for primates to obtain adequate nutrition in the canopy, she observed that spider monkeys consume ripe fruits for most of the year, eating only a small amount of leaves. Bonobos also appear to find enough food to eat easily, for much of their time is spent in other “social” activities. Thus being a fruit-eating forest primate appears a very viable option, but one question remains: If fruit is so low in protein, how do these fruit specialists obtain an adequate supply of these essential nutrients? Milton found that spider monkeys pass food through their colons more quickly than leaf-eaters such as howler monkeys. This speed of transit means that spider monkeys have a less efficient extraction process, but as much more food can be processed, it more than makes up. By choosing fruits that are highly digestible and rich in energy, they attain all the calories they need and some of the protein. They then supplement their basic fruit-pulp diet with a very few select young leaves that supply the rest of the protein they require, without an excess of fiber. Of course, by processing so much fruit, a large quantity of chemicals that naturally occur in fruit will also be absorbed. It should also be noted that wild fruit contains a higher percentage of protein than the cultivated fruit that is available to us humans today. It is clear that many wild primates are able to satisfy their daily protein and energy requirements on a diet largely or entirely derived from plants. It is likely that our ancestors in the forest did, too.

As stated, the wild fruit that we propose was the mainstay of our ancestral diet for the longest and most significant part of our evolutionary history contains more fiber than the fruit we buy today in our shops. Chimpanzees take in about 100 grams (3.52 ounces) of fiber a day compared with about 10 grams (0.35 ounces) that the average Western human consumes. At one time it was believed that humans did not possess microbes capable of breaking down fiber. Studies on the digestion of fiber by twenty-four male college students at Cornell University, however, found that bacteria in their colons proved quite efficient at fermenting the fiber of fruit and vegetables. The microbial populations fermented some three-quarters of the cell wall material, and about 90 percent of the volatile fatty acids that resulted were delivered to the bloodstream (Wrick et al 1983). It has been estimated that some present-day human populations with a high intake of dietary fiber may derive 10 percent or more of their required daily energy from volatile fatty acids produced in fermentation.

Furthermore, experimental work on human fiber digestion has shown that our gut microfloras are very sensitive to different types of dietary fiber. We are very efficient at processing vegetable fiber from dicotyledonous sources (flowering plants like fig trees, carrots, and lettuces) but are less so from monocotyledons (grasses and cereals). This provides yet another pointer to the archaic diet of humans as being largely fruit based and indicates that the grass seed that we eat so much of today in cereals, cookies, and much else is a poor substitute.

The chimpanzee gut is strikingly similar to the human gut in the way it processes fiber. As the percentage of fiber in the diet increases, both humans and chimpanzees increase the rate at which they pass food through the gut. These similarities indicate that when food quality declines both these primates are evolutionarily programmed to respond to this decrease by increasing the rate at which food passes through the digestive tract. And this compensates for the reduced quality of the food available.

It appears that the human system then, like those of the chimps and bonobos, is designed for a plant-rich fibrous diet. We are not designed for a diet high in refined carbohydrates and low in fiber or one that includes significant quantities of animal protein. Meat eating in man has been, on an evolutionary timescale, a very recent development. It certainly couldn’t have influenced the development of our physiology. Though the passage of food through the guts of spider monkeys, chimps, and humans is faster than in leaf specialists like howlers, it is much slower than in carnivores. Meat hanging around in the digestive system is bad news because of its inherent toxicity. The transit time for the passage of food through a carnivore’s gut is between seven and twenty-six hours, while for humans it is between forty and sixty hours.

Though we do have a shorter colon and a longer small intestine than the great apes (and this has led one camp of researchers to speculate that our intestines are more similar to those of carnivores), these differences are more appropriately explained by a specialist fruit diet, not a carnivorous or grain-based one. Fruit is easier to digest than leaves, tubers, and stems, and has a lower fiber content. Thus a specialist fruit-eater would not need such a long colon as other apes that have more fibrous bulk to deal with.

Another feature of humans that is strongly indicative of our vegetarian origins is our inability to synthesize our own internal vitamin C. This trait is very rare, but where it occurs, the animals concerned (such as guinea pigs) eat a plant-based diet. In these cases ample supplies of the vitamin are available within the food. Vitamin C plays many extremely important roles within the human body. Research seems to be always finding more functions for this “miracle chemical.” These have been summarized by Ross Pelton, clinical nutritionist and cancer researcher at the University of California, in his book Mind Foods and Smart Pills: Vitamin C stimulates the immune system, enabling one to better resist diseases. Terminal cancer patients taking megadoses of vitamin C have been found to live longer. It promotes faster wound healing and reduces the amount of cholesterol in the blood. It is a powerful detoxifier and protects against the destructive power of many pollutants. In addition, it protects the body against heart disease, reduces anxiety, and is a natural antihistamine. A severe deficiency causes scurvy and eventually death. Increasing its intake has been found to increase mental alertness and brain functioning in a variety of ways. Vitamin C is the main antioxidant that circulates in the blood. When available in sufficient quantity, blood carries it around the body, washing over the cells to create a bath of protection. Whenever a free radical turns up, a molecule of vitamin C gives up one of its own electrons to render the free radical ineffective. According to Pelton this process may take place somewhere between one hundred thousand and one million times a second, depending on the body’s level of metabolism and the amount of vitamin C available. Unfortunately, with each free radical decimated, a molecule of vitamin C is lost, so the body rapidly loses its supply of vitamin C (Pelton 1989).

Vitamin C is a key player in keeping our neural system healthy. The body has a system that operates like a kind of a pump to concentrate vitamin C around our nerve and brain tissues. These tissues have more unsaturated fats than any other organs in the body, making them more vulnerable to attack by free radicals and oxidation. The vitamin C pump removes vitamin C from the blood as it circulates to increase the amount of vitamin C in the cerebrospinal fluid by a factor of ten. The pump then takes the concentrated vitamin C from the ­cerebrospinal fluid and concentrates it tenfold again in the nerve cells around the brain and spinal cord. Thus our brain and spinal cord cells are protected against free radical damage by more than a hundred times as much vitamin C as our other body cells.

For such an important chemical, it is extremely odd that we are dependent on vitamin C from outside sources. But how much of it does the body need? Research carried out by the Committee on Animal Nutrition demonstrated that monkeys needed around 55 milligrams of vitamin C per kilogram (2.2 pounds) of body weight. When this measure is extrapolated to humans, a 150-pound person would need a daily intake of 3,850 milligrams. Nutritional science recommends that a human needs 45 milligrams each day. This is just enough to prevent scurvy but not enough to keep the body functioning at an optimal level. We would not, and indeed do not, obtain the sort of levels our bodies really need from a diet high in meat and low in vegetables and fruit, but we would from one high in fruit, shoots, and leaves. Analysis of wild plant foods eaten by primates shows that many of these foods contain notable amounts of vitamin C. The young leaves and unripe fruit of one species of wild fig were found to contain some of the highest levels ever reported. Our closest living relatives, the great apes, eat a diet that contains between 2 and 6 grams (0.07 to 0.21 ounces) of vitamin C every day. When our ancestors were living in the forest they would have consumed similar amounts.

In contrast, we can and do produce our own vitamin D. This vitamin cannot be obtained from a leaf- and fruit-based diet, but it can from a carnivorous one, thus if we were designed to eat meat we would have less need to synthesize our own. Being able to synthesize vitamin D and not vitamin C is then a strong indication of our true ancestral diet and the one we are really adapted to. Accumulating evidence for meat being an unhealthy food option further strengthens this case. One study at the Cancer Epidemiology Unit in Oxford showed that vegetarians were 24 percent less likely than nonvegetarians to die of ischemic heart disease (Key 1999).

Carbohydrates also appear to be problematical when eaten in large amounts. A diet high in carbohydrates, especially refined carbohydrates (cakes, cookies, pasta, etc.), dumps large amounts of glucose rapidly into our bloodstream. This can cause insulin resistance in which the absorption of glucose from the bloodstream is disrupted. This in turn can lead to obesity, adult onset diabetes, hypertension, heart attacks, and strokes. It can also lead to an excess of male hormones, which, among other effects (e.g., aggression), encourages pores in the skin to ooze large amounts of sebum. Acne-promoting bacteria thrive on sebum. Up to 60 percent of twelve-year-olds and 95 percent of eighteen-year-olds in modern society suffer from acne, yet it is almost unknown in subsistence societies such as the Kitava Islanders in Papua, New Guinea, and the Ache of the Amazon. The Inuit people of Alaska also used to be free of acne, but they began to be affected by these skin complaints after they started to eat processed foods.

The problem with eating highly processed carbohydrates may be further reaching still. If refined cereal consumption results in an excess of male hormones it could have a ripple effect on the immune system for we know that the thymus gland starts to shrink in response to these hormones at the time of puberty. (More carbohydrates lead to more testosterone, which shrinks the thymus gland, which is seat of much of our immune response.) Grain products have also been associated with celiac disease, an autoimmune condition of the gut, and some researchers suspect they trigger rheumatoid arthritis, too.

It is highly significant that these foods have the ability to alter the quantity or at least the activity of our hormones. It is another example of the way our diet can affect the way our bodies work. It is possible, probable even, that they also affect the way we act, and thus how we moderate our sense of self. If we compare refined carbohydrates with fruit, we can see that fruit has a much lower glycemic index, which means it is digested more slowly, thus avoiding the problems of the “glucose rush.” The chemicals within fruit also reduce the activity of sex hormones. They thus have the diametrically opposite effect to that of refined cereals.

There is a view held by some that meat, and particularly the high protein content of meat, was somehow responsible for the enlargement of our brains. The assumed “higher-quality” meat diet theoretically allowed more energy to fuel the brain with a shorter small intestine. This reasoning is flawed on several fronts. First, meat is supposed to be easy to digest and to be a high-energy food, but fruit is much more easily digested and provides more readily available energy, too. Second, if there were sufficient external pressure to bring about such a change as a shortening of the gut, we would expect other adaptations and changes toward a carnivorous diet as well. Certainly we would not expect adaptations to be heading in the opposite direction. Our teeth, for instance, are nothing like the teeth of a carnivore. The teeth of our nearest relative, the bonobo, are much better adapted to eating meat than human teeth are, and bonobos hardly eat any meat. In fact, it is known that bonobos are, if anything, more intelligent than chimpanzees, and it is chimps that eat at least some meat. So, if bringing meat into the diet of an ancestral human was enough to shorten the gut and expand the brain (both major changes), where are the parallel changes in areas that would be needed to cope with a meat diet?

If we look at areas such as dentition, the physiology to digest meat, and the ability to catch it, we find nothing that looks even vaguely carnivorous. If we lined up the three most evolved species of primates—chimps, bonobos, and humans—we would have to conclude that humans are, in fact, the least adapted to eat meat. Humans have much smaller teeth, and they cannot chase the meat nearly so well. Also there is a structural distinction between carnivore guts and those of frugivores or vegetarians. Our guts are like those of the noncarnivores; they are folded, smooth, and still significantly longer than a carnivore gut. There is a difference in saliva as well. Carnivore saliva is acidic, but the saliva of humans is alkaline, which provides the right functional environment for digestive enzymes, such as amylase, to break down starch.

Now, if we ask what sort of food really fits these human adaptations, we have to conclude it is fruit. Fruit fits the brain-gut energy equation: the shorter gut, the more ease of digestion, the lower the toxicity, and the smaller the teeth. Fruit is easy to assimilate, and the nutrition it provides is in a form that needs very little conversion to the real requirement of the brain—glucose. (The sugar in wild fruit tends to be rich in glucose and fructose compared with cultivated fruit that has been bred for its sweeter-tasting sucrose content.) Humans thus have a proportionately shorter small intestine than chimps and bonobos, not because of increased levels of meat in our diet but because of an increased specialization on sugar-rich fruit. High-quality fruit is low in toxicity and provides all the fuel the brain needs. Meat, conversely, is more difficult to digest, particularly without cooking, and then to turn protein into sugar requires yet more energy. So meat as an energy food doesn’t make as much sense as fruit that is full of fruit sugars that are easily assimilated and take little conversion.

The anatomy and physiology of our digestive system support the case for the biochemical role of tropical fruit in human development. However, the case could be stronger still if we could show that the human brain in archaic times actually worked the digestive system in a way that extracted the nutritive elements within the plant-based diets more efficiently. More research needs to be done in this area, but preliminary indications (from T. W.’s private research) hint that a digestive system run without interference from the left hemisphere may do just that.

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