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why brains?

posted 27 Feb 2012, 02:29 by John Brown   [ updated 13 Mar 2012, 19:57 ]
Natural selection offers a perspective how brains came about.  John Allman has provided a particularly clear view of why brains came about (Allman 2000). I cannot do better that quote his words directly in many cases.

Allman identified three key themes in the evolution of the human brain: “that the essential role of brains is to serve as a buffer against environmental variation; that every evolutionary advance in the nervous system has a cost; and that the development of the brain to the level of complexity that we enjoy – and that makes our lives so rich – depended on the establishment of the human family as a social and reproductive unit.”. (Allman 2000)

Allman goes on to explain that “Brains exist because the distribution of resources necessary for survival and the hazards that threaten survival vary in space and time. There would be little need for a nervous system in an immobile organism or an organism that lived in regular and predictable surroundings. In the chaotic natural world, the distribution and localization of resources and hazards become more difficult to predict for larger spaces and longer spans of time.” (Allman 2000)

In most animals the brain is located near to the entrance to the gut which suggests that the brain arose as the gut’s way of controlling its intake by accepting nutritious foods and rejecting toxins. There are several families of genes that govern both brain and gut development. There is strong evidence that the brain and the gut compete for metabolic energy in the organism and that gut size limits brain size. The main energy using organs are the heart, liver, kidney, stomach and brain so there is a forced trade off between brain and digestive organs. (Allman 2000)

“The brains of warm-blooded vertebrates, the mammals and birds, tend to be larger than the brains of cold-blooded vertebrates of the same body weight. The larger brain in mammals and birds are a crucial part of a large set of mechanisms for maintaining a constant body temperature. Since all chemical reactions are temperature dependent, a constant temperature brings about stability in chemical reactions and a capacity for precise regulation and coordination of complex chemical systems. However, maintaining a constant body temperature requires a tenfold increase in energy expenditure.” The evolutionary changes required to maintain a constant temperature have included changes in the quantity of food consumed and the way it is chewed, in breathing, in locomotion, in parenting behavior, in the senses, in memory, and in the expansion of the forebrain. (Allman 2000)

For example, primates that eat fruit tend to have larger brains than leaf eaters. Fruit has higher energy and is easier to digest than the more readily available leaves. However, fruit is more widely dispersed in space and time, and subject to greater competition. (Allman 2000)

“In a newborn human the brain absorbs nearly two thirds of all the metabolic energy used by the entire body. … and ... Nurturing a large-brained baby imposes enormous energy costs on the mother because of the burden of lactation, which is far more costly than digestion.” (Allman 2000)

“The invention of the extended family enabled humans to evolve much larger brains and avoid the constraints imposed by the extremely slow maturation and low fecundity associated with such large brain size.” (Allman 2000)  Caring for other members of the species holds reproductive advantage for genes because a very large number of genes are contained in all members of the species. The remaining genes in the population, those that are not ubiquitous, account for differences between individual members of the species. There is a particular incentive to care for immediate family members (children, siblings and parents). During sexual reproduction, each parents contributes a  randomly selected half of their genes to their child. Thus, caring for an immediate family member will benefit this even higher proportion of shared genes.  This benefit also exists, but is less pronounced for more distant relations. 

However, there is also an incentive for each individual to obtain more than their fair share of the care on offer. It pays for a child to attract a disproportionate amount of parents resource (self investment vs investment in siblings) by deception (ie screaming louder, looking weaker etc). Dawkins goes so far as to say that "...most animal signals are best seen as neither informative or deceptive, but rather as manipulative." In response, the mother has an incentive to detect the deception and thwart the cheater.

“The personal and immediate social domain is the one closest to our destiny and the one which involves the greatest uncertainty and complexity” (Damasio 1994). Brains that have developed to navigate a social environment face a very much more difficult challenge than brains that exist in environments dominated by inanimate objects. Humans are much harder to anticipate than rocks (except those rocks thrown by humans).

In sum, the development and operation of a brain is tremendously expensive in terms of the energy required. Despite the expense, brains can confer a reproductive advantage through access to even more resources and in navigating a social group. A brain achieves this advantage by both predicting resources / hazards and by physically acting on the environment accordingly. Both the evolution and the development of a brain are inextricably intertwined with the environment.
 




Allman,J. (2000), Evolving Brains. W. H. Freeman. 
Damasio,A.R (1995), Descartes' Error : Emotion, Reason, and the Human Brain. Harper Perennial. 
Dawkins, R. (1987), The Selfish Gene. Oxford University Press.