Survival of the Fattest: The Key to Human Brain Evolution
Stephen C. Cunnane World Scientific Publishing Company, June 2005.
In this book by Cunnane on how the human brain evolved, he focuses a great deal on shoreline nutrients -- especially iodine. He has an entire chapter on iodine, as well as another chapter on other key minerals.
Chapter 6. Iodine: The primary brain selective nutrient (pp 115-130)
Chapter 7. Iron, copper, zinc, and selenium: The other brain selective minerals (pp 131-150)
Here are a few brief selections from this excellent book:
Brain Selective Minerals and Lipid Synthesis (pp 146-150)
"In addition to their diverse roles in cellular biochemistry and energy metabolism, iodine, iron, zinc, copper, and selenium all have one broad metabolic effect in common that is directly relevant to brain function and evolution -- they all interact importantly in lipid metabolism. Lipids are major components of both the neuronal synapses needed to receive and integrate information leading to the development of a signal, and the myelin needed to protect the integrity of the signal being sent from the brain to the target tissue.
"Iron is a structural component of the desaturase enzymes, which are required to add double bonds to long chain fatty acids. There are several fatty acid desaturases that are structurally similar but not interchangeable because they insert double bonds in different locations in fatty acids. They all have in common a structural requirement for one iron atom per enzyme molecule. One of the desaturases, the delta-9 desaturase, makes the eighteen carbon monounsaturated fatty acid, oleic acid, from the eighteen carbon saturated fatty acid, stearic acid. The delta-5 and delta 6 desaturases are needed at separate steps in the synthesis of polyunsaturated fatty acids, i.e., docosahexaenoic acid from alpha linolenic acid and arachidonic acid from linoleic acid.
"Although there is intense discussion amongst nutritionists and pediatricians concerning how to get adequate docosahexaenoic acid into the developing human brain, there is actually a lot more oleic acid than docosahexaenoic acid in the brain. Oleic acid is used in all brain lipids, i.e., both in myelin and in synapses, whereas docosahexaenoic acid is principally found in synapses.
"The crucial point about iron-dependent desaturation of fatty acids is that although there is abundant stearic and oleic acid in the diet and in body fat stores of all mammals, the brain is largely incapable of acquiring sufficient amounts of these two fatty acids from outside itself. The brain takes up small amounts of stearic and oleic acid but the amount is too low to meet the requirements of brain growth and lipid deposition. Thus, in contrast to the brain, other organs such as the liver can get their oleic acid directly from the diet or via synthesis from stearic acid.
"Iron deficiency inhibits delta-9 desaturation in all tissues studied and hence affects tissue stearic and oleic acid levels, but organs other than the brain do not depend on synthesis to make oleic acid. However, iron deficiency reduces the oleic acid content of brain synapses and myelin. Iron deficiency also decreases the brain content of other longer chain monounsaturated fatty acids derived from oleic acid that, like oleic acid, are important components of myelin and contribute to optimal learning and memory.
"Copper is also needed for delta-9 desaturation and oleic acid synthesis. Thus, copper deficiency also reduces oleic acid levels in tissues whereas copper excess increases oleic acid. Copper's exact role in desaturation is much less clear than for iron. It is not a structural component of the desaturase protein itself but copper does have a role in electron transport at the cytochrome C oxidase transferring electrons to the terminal step at the desaturase protein itself.
"The main consequence of low brain oleic acid caused by iron or copper deficiency is hypomyelination causing weakness or errors in the electrical signal. The outcome of low desaturation caused by copper deficiency is therefore spasticity, tremor and muscle weakness and fatigue.
"Iodine is also needed for normal myelination and at several stages in ketogenesis. Ketogenesis is essential not only to provide alternate fuel to the brain (ketones) but for the synthesis of lipids such as cholesterol and long chain fatty acids destined for both neuronal synapses and myelin. Hence, the role of Iodine
in ketogenesis impacts on myelin synthesis because long chain saturated (palmitic and stearic acids) and monounsaturated fatty acids (oleic acid) used in myelin synthesis are important products of ketogenesis in the developing brain.
"The process of electron transport from the donor proteins (cytochromes) to the fatty acid desaturases is dependent on zinc. The delta-5 and delta-6 desaturases involved in converting linoleic acid and alpha-linolenic acid to the longer chain polyunsaturates seem somewhat more sensitive to zinc deficiency than does the delta-9 desaturase converting stearic acid to oleic acid.
"As a result, synthesis of the long chain polyunsaturates, especially arachidonic acid, is markedly impaired by even mild to moderate zinc deficiency. This inhibition of the desaturases by zinc deficiency is so strong in infant humans and in young animals that it causes a more rapid decline in tissue arachidonic acid and docosahexaenoic acid than does the direct dietary deficiency of all the omega 6 or omega 3 polyunsaturated fatty acids."
"Several key nutrient participate in the control of human growth, metabolism and brain function. Iodine
promotes early development but it depends on zinc and amino acids for protein and tissue building. Iodine
stimulates cellular metabolism but it depends on copper and iron for the cellular furnaces (mitochondria) to generate heat and maintain body temperature.
"Some animals can survive in environments that are quite low in these required nutrients but they are all small-brained. For several required nutrients, humans now occupy a large number of environments that no longer meet their daily requirements for normal development. In industrialized countries, this problem has been artificially addressed by iodine supplements but nutrient deficiencies, principally for iodine, iron, and zinc, remain a problem in many areas.
"The metabolic interactions within brain selective minerals and between brain selective minerals and other nutrients represent some of the many such interactions between the full spectrum of essential nutrients needed for normal human development. This complexity and interdependence on nutrients coevolved with complex, multicellular systems and is widespread in both plants and animals; it is not a function of hominid or human evolution in particular.
"Two (or even five) million years ago, some groups of hominids may have been genetically predisposed to having a brain weighing 1500 grams but it didn't happen. I believe that a hominid brain weighing 1500 grams didn't arise until 50-1000,000 years ago because, prior to that, there was insufficient nutritional support for it. Even if only one nutrient is 'limiting' and all the others are available and working in concert, permanently changing key metabolic processes or limitations on cell structure, i.e., expansion of the cerebral cortex, cannot bypass dependence on that limiting nutrient.
"Humans are now living beyond the optimal nutrient limits for intake of several nutrients. Adaptation will be necessary, either by making supplements more widely available or by moving back to shorelines, or we will conceivably face evolutionary processes that could eventually reduce cognitive capacity."
Thyroid Hormone and Hominid Evolution (pp 271-2)
For normal development in humans, no organ has a higher dependence on thyroid hormone or on iodine than the brain. It therefore seems inevitable that dietary iodine and thyroid function are closely linked to the unique changes in brain function that occurred as one branch of Australopithecines became the founder population for early Homo and eventually H. sapiens.
Improved stress resistance was probably an essential feature of exploration that occurred as H. erectus colonized temperate regions within and beyond Africa. The curiosity and fearlessness about unknown neighbouring localities that became large-scale exploration would have been strongly supported by the emergence of true bipedalism which, itself, would have arisen more easily in a habitat providing more thyroid hormone and/or iodine.
Bipedalism probably arose by heterochrony in a founder population of pre-Australopithecine primates that were normally quadrupedal. Many primates that lack the distinct and dedicated bipedal skeletal morphology found in bipedal hominids and humans still occasionally stand on two feet. As observed in the virtual domestication of foxes, only two things are required to exaggerate such a tendency from opportunistic to fully developed bipedalism: First is sufficient intrinsic variability in developmental morphology of the skeleton. Second is a slightly different rate or timing of skeletal development in the founder (bipedal prone) group. Different habitat and food sources would probably have been key stimuli, not only to opportunistic bipedalism, but also to a changing thyroid rhythm, which would have been necessary to promote the required morphological changes in the pelvis and spine….
All animal tissues including eggs contain some thyroid hormone. They also contain more iodine than do plant-based foods. Consumption of fish, shellfish, meat or eggs therefore inevitably led to intake, even if sporadic, of exogenous thyroid hormone. Small animals eaten whole (frogs, some fish, fledgling birds, etc.) would have given a larger surge of thyroid hormone because of consuming the thyroid itself.
Thyroid hormone is the only hormone that is absorbed intact during digestion. This increase in available thyroid hormone would have stimulated relatively rapid morphological changes in development of the infants of a founder population of hominids that were eating more meat, eggs and shellfish. At a minimum, the combination of higher intake of both exogenous thyroid hormone and iodine would have provided unmatched metabolic and developmental support for the skeletal changes and habits that were beginning to favour bipedalism. Bipedalism required better unconscious control by the brainstem and cerebellum. Improved manual dexterity, planning, and memory all involve improved conscious function of the forebrain or cerebrum….
Neanderthals – A High or Low Thyroid Variant? (pp 276-7)
Dobson postulates that a mutation helped convert certain Neanderthals into the earliest Cro-Magnon stock. By making the uptake of iodine by the thyroid more efficient, this mutation helped overcome the nutritional and population pressures created by advancing and retreating shorelines and allowed both streamlining of the newly human torso as well as cognitive improvement of the already large brain. In effect, Dobson suggests that Neanderthals were a diseased version of Cro-Magnon. He suggests that a genetic mutation increasing the efficiency of iodine uptake by the thyroid released the Cro-Magnon from this metabolic stranglehold, particularly on brain function.
The attractive element in such a pathological explanation is that it accounts for several unexplained features including the probable co-existence of Neanderthals and Cro-Magnon, the limited cultural and hunting repertoire of the Neanderthals, and their very rapid disappearance during less than 10,000 years. Successive waves of migrants from healthy coastal locations would have contributed to maintaining clan density in certain areas as well as skill levels for both hunting and modest cultural advancement. The higher incidence of symptoms of iodine deficiency in women supports the fossil evidence suggesting female Neanderthals were more sedentary and less mobile because of skeletal pathology and obesity. If Dobson is correct, Neanderthals are the evidence that iodine deficiency crippled and eventually killed off the last non-human branch of hominids.
One way the discrepancy might be resolved between Crockford’s high thyroid/iodine and Dobson’s low thyroid/iodine explanations of how thyroid or iodine limitations contributed to the rise and fall of the Neanderthals is that high protein intake contributes to Iodide
loss from the thyroid. Thus, as Crockford proposes, by eating more meat Neanderthals may well have had higher thyroid hormone intake. This would have contributed to better cold tolerance and morphological changes increasing their skeletal and muscle mass. However, high intake of meat may have helped deplete iodine, thereby curtailing brain development in Neanderthals and limiting their cultural and technical development. Iodine deficiency could then have contributed to the potbellied physique which occurs in present day cretins who can fully develop physically but are neurologically impaired. A variant of Neanderthals with more efficient thyroid uptake of iodine and/or better access to dietary iodine became the Cro-Magnon who noticed and sculpted the unique anatomical features of iodine deficient Neanderthals into their figurines before they became extinct.
Plausibility, Prediction, and Parsimony (p 287-8)
Thus far, other theories of human brain evolution have not shown this predictive ability: there is no evidence that low intake of alternative sources of dietary energy, such as meat or nuts, is associated with impaired brain function during either early development or during aging. Hence, diets that are not shore-based may have contained sufficient energy to meet the requirements of hominid brain expansion but they had and still have two serious inadequacies for human brain development: First, they are more likely to create nutrient deficiencies, particularly of docosahexaenoic acid, iron ad iodine. Second, plant-based diets contain anti-nutrients such as phytate and goiterogens, that exacerbate deficiencies of nutrients such as zinc and iodine, respectively.
The areas of the brain used for hearing need the most energy and are also the most vulnerable to dietary or congenital iodine deficiency. Good hearing is a key prerequisite for language, which is a defining feature of humans. Therefore, it is appropriate to think of iodine as a sentinel nutrient for human brain evolution because of its intimate link to the function of brain areas responsible for hearing and, indirectly, speech. Conversely, woodlands and grasslands occupied by non-human primates provided less iodine but these lower iodine levels were in keeping with lower demand for (and supply of) other brain selective nutrients. Lower iodine intake in fruit or vegetable-based diets wouldn’t prevent other primates from having good hearing but would prevent a combination of both good hearing and brain expansion.