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Through the study of evolution, we have been able to find out more about the human brain. The phylogeny of humans, which is the evolutionary history of a particular species, places the interaction between the brain and behavior in context. Much of an organism's behavior, such as methods for satisfying its needs for survival, has shaped its brain. As a result of varying behaviors among different species, brain function may vary somewhat and evolution helps us to understand brain function. Gerald Edelman used Neural Darwinism (discussed in module 4) to explain differences and similarities found when comparing at a squirrel monkey brain and a human brain. He found that basically, the squirrel monkey brain does not have many convolutions compared to the human brain. But, both are found to have virtually the same structures due to common ancestors. The human brain seem to be just a more complicated model than the squirrel monkey brain. Therefore, he concluded that evolution is a process that has occurred over a long amount of time. This represents the scientific view, although controversy exists over how the mechanisms of evolution work.
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Through evolution, we have come to better understand emergent properties, which is a qualitative process that arises from a state of events (i.e. sleep-wake cycle as part of circadian rhythm). It has been found that body build and ecology affect the overall sleep time of a species. Animals that tend to be predators sleep more than animals that are subject to predation. Also where an animal sleeps affects how much it will sleep, for example, if the place is secure the animal tends to sleep more. It has also been found that larger animals sleep more on average than smaller animals.
There are different ways to determine the evolutionary relationships among animals. When animals that are only distantly related have similarities in behavior or structure due to their similar response to their environment is called convergent evolution. The three out of twenty-three orders of birds (perching birds, hummingbirds, and parrots) that learn their songs display an example of convergent evolution. This similar response to their environments enables these birds to defend their territories or to attract their mates. As a result of these advantages, these species have developed innate mechanisms of vocalization. Analogy is a resemblance due to convergence rather than to common ancestry, homology on the other hand, explains a resemblance by common ancestry.
Once one decides to study the evolutionary relationships among animals, it is important to look at the classifications, or taxonomy. to make There are three ways to look at these models. The first: mosaic/adaptation, which is changes in the relative size of different areas with no changes in overall size (quantitative change). The second: organismal/developmental constraint, which is a change in the overall size of the brain while retaining relative proportions of different regions (qualitative change). The third way of looking at the two models is a combination of both mosaic/adaptation and organismal/developmental constraint.
The Evolution of Cognition (Why are we smart?)
Therefore strategies developed:
What makes us different? Language, self-consciousness, pedagogy, theory of mind
(attributing beliefs to others)
Lessons of evolution:
Humans have a high degree of encephalization, which is the ratio of brain size to body size. This means that we have proportionally more of our brains available for things other than maintaining our bodies. There are several things which are believed to have contributed to encephalization: the development of night vision in early mammals, frontally placed eyes for stereoscopic vision, higher precision and control of musculature, a complex social environment, and changes in diet.
None of these things alone can account for variations in brain size. For example, one explanation for why larger brains develop is the expanding social group. As relations between members of the social group become more complicated, animals need larger brains to keep straight and sort out these relationships. However, monkeys and apes have similar complex social environments but apes tend to be smarter than monkeys. This leads us to believe that another factor, namely diet, influences brain size.
Indeed, the type of food which a particular species eats seem to be equally important. To be more specific, more difficulty in obtaining nutrition from food seems to require larger brains than eating easily extractable food. Thus, species which must eat low nutrient food, extract nutrition from encased food (such as coconuts), or avoid toxic or foul-tasting chemicals will exhibit neurological or physiological adaptations, such as tool use, to deal with these tasks.
One strategy to solve the problem of obtaining nutrition is to develop a digestive system that can process a great bulk of low nutrient foods, such as grasses or leaves. Some monkeys exhibit this adaptation, as do grazing animals such as cows and horses. The other strategy is to develop a larger brain which can handle the greater demands which finding high nutrient, easily digestible foods places on memory, reasoning, and other cognitive functions. Different strategies which require this increased capacity include the use of tools, increased memory, better recognition, and verbal signals.
The effects of food-finding strategies on brain size are illustrated in Milton's study of two types of monkeys, the Colobus guereza and the Cercopithecus pygerythrus. The C. guereza has a larger brain and a human-like digestive system; it has a richer diet. The C. pygerythrus, on the other hand, which eats mainly grasses, has a much smaller brain and a cow-like digestive system (see Figure 3.1). This led Milton to hypothesize that the massive energy requirements of the digestive system and the brain require that one develop at the expense of the other; the body cannot support the demands of both a digestive system specialized for low nutrient foods and a large brain.
Another comparison, this one between the spider monkey and the howler monkey, gives another example. Although the two species of monkeys have roughly the same body weight, the spider monkey's brain weighs over twice as much (107g) as the howler monkey's (50g). The spider monkey eats much more fruit and less leaves than the howler monkey; it also has a much larger range over which it travels to find food. Note the differences in digestive systems: the spider monkey's system is optimized for processing fruit, whereas the howler monkey's is optimized for processing the low nutrient leaves.
The perspective of evolution can provide many valuable insights into the brain. After all, the brain did not spring fully formed like Athena into the skulls of humans. Instead, it arose as a series of responses to immediate demands of the environment. No master plan coordinated the development of the various neural systems based on specific purposes or unifying principles of its design.
The systems which arose thus are somewhat haphazard, a motley collection of patches thrown together "on the fly." The brain could not be taken off-line after a few million years of evolution and optimized! Instead, evolution worked with what was there, and produced solutions which, though not perfect, work well enough to get the job done.