Gisolfi Carl & Mora Francisco:
THE HOT BRAIN (MIT Press, 2000)

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The Spanish neurologist Francisco Mora and the American biophysicist Carl Girolfi have an interesting theory on what brains do for us: they regulate our temperature.
Their reconstruction of how life was born on Earth, and how brains were born on Earth, is biased towards showing that, from the beginning, life was capable of reacting to temperature: even the earliest unicellular organisms must have been capable of sensing heat and cold.
Heat, after all, was the primeval source of energy for living organisms. These organisms required heat to survive and the main source of heat came from the environment. The progenitors of the living cell, the "protocells", were probably units of energy conversion, converting heat into motion, just like a heat engine. The Dutch chemist Anthonie Muller, the proponent of "thermosynthesis", has shown in 1995 that such systems could form spontaneously in the primordial conditions of the Earth.
If they survived, these organisms must have developed a way to react to positive and negative stimuli, such as correct or excessive amount of heat. The early nervous system were assemblies made of cells already capable of sensing and reacting to temperature. Prove is that all known organisms, including unicellular ones, are capable of avoiding adverse environmental temperatures. In other words, throughout evolution all organisms were capable of sensing external temperature.
The authors speculate that during the transition from water to land (from stable temperature to wildly variable temperature) the nervous system must have learned to control body temperature. Nocturnal animals must have developed also a means to overcome the loss of environmental heat and produce heat internally. And the autonomic control of temperature was born.
This feature allowed the evolution from cold-blooded animals (animals whose temperature fluctuates with the temperature of the environment, whose only sources of heat are external sources) to warm-blooded animals (animals that maintain constant body temperature by producing heat internally). Cold-blooded animals are dependent on environmental heat: when ambient temperature rises, they are active and seek food; when ambient temperature decreases, their motor activity slows down. Warm-blooded animals overcame this limitation thanks to that self-regulating feature, thanks to the ability of producing heat internally when heat from outside is not enough. And they freed themselves from their habitat: they were capable of changing habitat because they were capable of maintaining their body temperature regardless of changes in the external supply of heat.
A very efficient self-regulating heat engine that maintains temperature constant opens up new opportunities for evolution: one organ that benefited was the brain, that could grow to its actual size and complexity. If it didn't have ad adequate supply of energy, the brain would not be capable of performing the tasks it performs. A hot organ is required for thinking.
Modern mammals, who have the highest demand for internal production of heat, regulate temperature through a whole system of thermostats, not just one. Experiments have proved that mammals have not one but many centers of control of body temperature: in the spinal cord, in the brainstem, in the limbic system and mainly in the hypothalamus. Rather than one point of control, this is more like a complex system, that peaks in the hypothalamus.
Ultimately, the brain is responsible for maintaining a constant temperature, the very constant temperature that allows the brain to function.
That said, the authors turn to some puzzling aspects of body temperature. First of all, humans can survive only in a narrow temperature range: a few degrees below or over 37 degrees a human body becomes a dead body. Why regulate at 37 degrees instead of, say, 20? It turns out that 37 degrees is the ideal temperature for balancing heat production and heat loss.
Fever sounds like a paradox: why would the body increase heat production to the point of threatening its own survival? Does fever enhance survival or is it an evolutionary mistake? It turns out that fever does enhance survival, but not the kind a selfish person likes. Fever is a mechanism to preserve the species rather than the individual: either it kills the parasite or it kills the individual who is carrying the parasite and who could infect the others.
Tha authors finally deal with sweating, which they prove is a cooling system (and not surprisingly prominent in humans), with the circadian cycle of body temperature and iits relation to sleep (sleep could be a way to cool off), and finally with physical exercise, which also causes an increase in body temperature like fever but of a benign kind.
Although the style is too technical for a general audience, the book contains a wealth of observation that could benefit ordinary people as well as scholars.

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