A great blog entry by Dr. McCleary
Becoming a Brain
During recent human evolutionary history (about 1.5 million years ago), the human brain increased in size by 33% in less than a million years. For this extraordinary expansion to occur required exceptionally favorable circumstances. By any measure, the modern human brain is large. Brains generally increase in size as body size increases. However, the human brain is about 3.5 times larger than that of chimpanzees which, as adults, have a lean body weight not very different from ours.
This massive expansion is even more remarkable when the nutritional demands it places on the body are considered. In adult humans the brain weighs 1400 grams, or approximately 2.3% of the body weight. However, it consumes almost one quarter of the body's daily energy requirement. In infants this disparity is even greater. At birth, the brain weighs 400 grams and constitutes 11% of the body weight yet consumes almost three quarters of the energy intake! Thus, any theory of human brain evolution must account for the environmental circumstances that would allow our ancestors to commit a large, disproportionate and continuous nutrient supply to the brain, especially early in life.
Most theories are plagued by the 'chicken or the egg' dilemma. This refers to the fact that hunting, tool use and other more cerebral pursuits have been considered as being forces that drove this rapid brain expansion. However, for these to exist in the first place, a large brain would have been a prerequisite. So how did improvements in brain anatomy and wiring develop before a larger brain existed that would have facilitated the formulation of higher brain functions? And how could this have happened over such a brief period of evolutionary time? Moreover, the main period of brain expansion was during late fetal and early post-natal development. At this stage a bigger brain would not have conferred any survival value.
Recent clues have suggested the role of a nutrient dense diet. This meant a diet higher in meat and fat with a lower plant content. As a possible means of procuring such a diet, hunting would have exceeded the mental capacity at this stage of development and brain size. A likely solution for attaining a high quality diet without the necessity for a sophisticated procurement system appears to be a shore-based diet. This meant exploiting the abundant, sustained, and easily accessible food supply at the waters edge. It probably involved harvesting bird eggs, mollusks, and crustaceans which were nutrient and energy rich. This diet could have been easily harvested by humans of all ages, both genders and required no special skills or bodily strength.
Because of the incessant demands for a nutrient and calorie dense diet, especially during the most rapid period of brain growth perinatally, during a time of absolute dependency, it was vital to prevent periods of nutrient deprivation. This is of interest in light of the fact that although the brains of newborn humans and chimps are similar in size, human infants have about one pound of body fat. Body fat is almost nonexistent in chimp newborns. This fat store provided calories that could last for three weeks and thus constituted a formidable buffer against environmental variability. Fat deposition in the human fetus accounts for 90% of the weight gain leading up to delivery. This represents an exceptional metabolic commitment and suggests that it somehow facilitates the survival of the infant.
From a caloric perspective, the brain is able to utilize glucose but can't tap into fat or protein sources to any degree. It does posses the metabolic machinery to generate energy from ketone bodies, which are compounds that occur due to partial fat combustion. These are efficiently burned in the brain and can provide 30% of its caloric demands during the newborn period. Ketones are also a source of carbon for the synthesis of brain lipids and cholesterol that comprise a large fraction of neuronal cell membranes. Neurons work by being active electrically. Most cells manifesting high electrical activity (such as neurons, photoreceptor cells and cardiac cells) have a high concentration of DHA (docosahexanoic acid-a long chain omega 3 fatty acid that provides membrane flexibility and function). The DHA concentration in the fat stores at birth is higher than it is at any other time. A shore-based diet provides a continuous supply of this vital nutrient for the synthesis and recycling of neuronal membranes and synaptic connections.
These dietary demands are vital at birth, but are no less important for the aging brain. High energy requirements and the demand for a nutrient dense diet with abundant long-chain omega 3 fatty acids are critical for optimal brain function and act to slow brain aging. The efficiency with which the brain metabolizes glucose declines as we age. If energetic demands are to be successfully met as we age, ketone bodies are an ideal fuel source because they generate energy with lower oxygen use and are delivered to the brain via a different transporter than glucose uses. For these reasons current dietary choices should more closely mirror the shore-based diet responsible for the rapid evolution of the flexible thinking machine we call our brain.
Ketones and the brain
Seems like my "yeast" must have taken acting lessons from Gloria Swanson, the death scene is going on way too long.
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