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evolution of brains

posted 4 Mar 2012, 17:20 by John Brown   [ updated 18 Apr 2012, 22:12 ]
A. Butler and W. Hodos identified a variety of mechanisms involved in the evolution of vertebrate central nervous systems (Butler & Hodos 2005). Induction is one of the most basic mechanisms where an altered chemical signal induces part of an existing embryonic structure to develop in a different manner. A second mechanism, of critical importance to vertebrate brain evolution, involves the homeobox genes. The homeobox genes specify regions in the early developing nervous system that will develop into identifiable brain structures. These two basic mechanisms are common to all species with developed brains (rather than very simple nervous systems).

A number of additional evolutionary mechanisms are apparent in the vertebrates. Invasion describes a mechanism where a population of neurons evolves new connections to an existing area of the brain and comes to dominate the function of that area. There are also examples involving the loss of connections per sae to a particular neuron population. Another evolutionary mechanism has seen a uniform population of neurons evolving into multiple sub-populations of neurons with different characteristics or connectivity (referred to as differentiation and parcellation respectively).  Duplication of whole neural regions has occurred, enabling one of the regions to evolve a new role while leaving the other region to perform its original function (Allman 2000). A particularly frequent mechanism has been changes in the proliferation of neurons in a particular region, usually in association with one of the other mechanisms.

... great diversity and great complexity have arisen because they are 
both merely the result of a few, simple random mutation events 
that affect the behaviour of particular morphogenetic fields
the phenotypes of which have been favoured highly by natural selection.” 
Butler & Hodos 2005
However, the number of mutations that have not been favored by natural selection is a very large number.

There is more than one way to build a brain. 
A great diversity in brain organization has been achieved independently 
at least four separate times within four separate radiations of vertebrates. ... 
the development of a more complex brain has been accomplished, 
not just once for the ascent of man, but multiple times.” 
Butler & Hodos 2005

For example, the cognitive abilities of birds are on par with those of most mammals, certainly including primates. The general circuitry of the avian brain routing the information from the sense organs to the “higher” areas of the brain and looping around the higher levels are strikingly similar. However, the architecture of the higher levels of the avian brain are conspicuously different (Butler & Hodos 2005).  Much has been attributed to the architecture of the neocortex in mammals. The neocortex is the outermost layer of the cerebral hemispheres with the familiar deep fissures in human brains, but with a  smooth surface in other mammals such as rat brains. The neocortex comprises six fairly uniform layers, each characterized by neuron type and connectivity. The majority of the cells in the neocortex are pyramidal neurons, for their characteristic shape. In human, the neocortex is involved in higher functions such as sensory perception, generation of motor commands, spatial reasoning, conscious thought and language. However, the avian brain has evolved into a different solution to a largely similar set of environmental challenges. Two areas of the avian brain, the wulst and the dorsal ventricular ridge, support many higher cognitive functions. Most of the neurons in these areas have a star shape, and the layering is orientated radially out from the center of the brain. Diverse neural architecture can support highly complex cognitive abilities. There is more than one way to build a brain.

The avian brain is very considerably smaller and necessarily lighter than primate brains. Smaller again, is the brain of an araneophagic jumping spider Portia fimbriata. These predatory spiders invade the web of other spiders and employ deception to attack them. If the resident spider is small then P. fimbriata mimic the vibrations of an ensnared insect in an attempt to lure the resident spider. When the resident spider is large then the P. fimbriata mimic the vibrations of an insect brushing the periphery of the web keeping the resident out of the web and avoiding a full scale attack. When P. fimbriata moves across the web they disguise their movements by simulating a large-scale disturbance of the web such as from wind (Tarsitano, Jackson & Kirchner 2001). This is an example of a minute brain that is capable of deception – the very same capacity that Turing chose as a keen test of intelligence.

In sum, brain evolution has involved an array of mechanisms which reuse the existing structures in complicated and intertwined ways. There is more than one way to build a brain. Remarkable things can be achieved with a very small brain.

Allman,J. (2000), Evolving Brains. W. H. Freeman. 
Butler, A.B. & Hodos, W. (2005), Comparative Vertebrate Neuroanatomy - Evolution and Adaption. Wiley. 
Tarsitano, M., Jackson, R.R., & Kirchner, W.H. (2001), Signals and Signal Choices made by the Araneophagic Jumping Spider Portia fimbriata while Hunting the Orb-Weaving Web spiders Zygiella x-notata and Zosis geniculatus, Ethology vol.106 issue.7, pages.595-615
Turing, A.M. (1950) Computing Machinery and Intelligence. Mind 49: 433-460.