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Evolution and the Human Brain
Review by Jeffrey P. Bigham

The basic functions of the vertebrate brain are quite extensive and can be placed into six general categories.  The ability to access the senses is one of the most important functions of the brain and it drove much of the brain’s evolution.  The brain is also responsible for regulating the body's posture and movement in an organized manner.  The two categories already mentioned are closely related to the next group of functions that allow the orientation of oneself to specific stimuli in the environment.  Next are the functions dealing with instincts and reflexes that are associated with basic behaviors such as eating, reproduction, fighting, and fleeing.  The brain also contains functions related to memory and learning, which are present to varying degrees in different species.  Next are the functions related to socialization, which provide the basis for interaction with members of an animal’s own species and other animals for purposes such as communication and reproduction.  While there are undoubtedly exceptions, the brains of most vertebrates handle these six functions to some degree and have evolved under their influence  (Comparative Mammalian Brain Collection).

When a function related to a certain area of the brain has increased in relative importance in the past due to a change of environment or other factors, its selective advantage increased as well and it was more likely to be favored through evolution.  In general this caused these areas to increase in size but rarely unilaterally.  The interconnectivity of the brain caused a general increase in the overall size of the brain even if the main cause for the increase was centered in only one portion of it.  Therefore, the general trend in the evolution of the brain is for it to have become larger relative to body size and for the interconnectivity within it to increase. 

In the early, aquatic vertebrates the brain was divided into three main regions.  Functions concerned with the sense of smell were concentrated mostly in the forebrain, those dealing with vision mostly in the midbrain, and those with balance and hearing were located in the midbrain.  The forebrain of these early vertebrates was the largest section because of the crucial role the sense of smell played for them.

With the development of the terrestrial reptiles came an increased role for the information garnered by the auditory and vision systems, which resulted in an increase in the size of the midbrain and hindbrain.  This increased importance also resulted in the midbrain and hindbrain developing an increased role in coordinating these sensory activities, which made the whole brain more complex.

The early mammals had already developed a more complex brain than their reptilian ancestors.  The dinosaurs and other large reptiles had taken over the daylight niches causing the early mammals to adapt to nocturnal and twilight niches.  The major modifications that occurred at this point were the addition of an expanded cerebral cortex and the development of an acute auditory system.  Being active at night forced these early mammals to rely much more on their auditory and olfactory senses to determine distance than reptiles, and the need to organize this additional input into useable information is the main reason for the expansion of the brain in early mammals.

To make use of the information that the animal received form its auditory and olfactory senses for useful purposes such as determining distance, the brain must be able to encode such events in respect to both location in space and time received.  The earliest mammals and mammal-like reptiles already had a fairly developed vision system that could map the visual information into useful information, but, unlike the auditory and olfactory system, much of the vision processing and encoding could be handled in retina and surrounding neural tissue.  There wasn't room for such computation in the ears or nose so the most likely place for it to be located was in the brain itself, which partially explains the initial increase in brain size in the mammals.

The vision system of the early mammals had been modified for night vision from the original reptilian vision system through the addition of rod cells in the retina.  After the extinction of the dinosaurs and other large reptiles, the mammals moved in to the daylight niches that they had left behind.  As the mammals moved into these niches, their vision system adapted for use in the daytime, but it did not just simply return to a system like that of the reptiles from which it had evolved.

Unlike the vision system of their reptile ancestors, the nocturnal vision system of these early mammals was integrated into a complex auditory and olfactory system that included a temporal encoding of space.  It is reasonable that the mammalian encoding of visual information would have develop similarly and included this temporal information as well, and, if it did, then it seems reasonable that saving a copy of these images with a timestamp might not be that big of a step.  This new visual system was not centered in the midbrain, as it had been in the reptiles, but was instead centered in the forebrain.  This sequence of advances seems to be the major advancement that led to a brain capable of mammalian intelligence and learned behavior.

This increase in brain size and power brought about several other specific changes in mammalian biological complexes in regards to reproduction.  These include the development of embryological development in utero, the birth of a reduced number of totally dependent young, the development of mammary glands for postnatal nurturing, and a relatively long period of dependency during which the young mature.  These changes allow for more advanced brains and for young mammals to expand their corpus of learned behavior and adapt it to include behaviors designed to maximize their chance of survival in their environment before going it alone.

The varying importance of the functions contained in the human brain have shown their influence on the evolution of the brain in regards to both size and organization during the development from vertebrates to mammals and this adaptation has greatly influenced the type of behaviors exhibited by mammals.

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