The primate order is composed of a group of species that differs very little in its morphological structure but varies quite dramatically in its behavioral patterns. Researchers in the fields of chemistry, biology, neurology, psychology, and medicine have found that behavior is a cognitive mechanism that is processed in the brain. It has been found that larger, more complex and highly organized brains have a more elaborate behavioral repertoire than smaller, more primitive brains. Therefore, in order to have a greater appreciation for these differences, it is imperative to investigate topics such as relative brain size, expansion of the brain, social relationships, evolution of language, and ecology.
Relative Brain Size
Primates tend to have relatively larger brains than most other terrestrial mammals. This would lead one to assume that mammals with larger brains are more intelligent. However, this is not always the case. For instance, if one believed that absolute brain size, used alone, is a good predictor of intelligence, then one would find themselves less intelligent than the elephant, who has a brain 4 times larger than humans.
Another way to investigate the size of the brain is by applying a simple ratio: brain weight over body weight. As body weight increases, so does the weight of other organs in the body, such as the heart, liver, intestines, and, of course, the brain. This happens because larger bodies require larger organs to meet their everyday energy demands. Although this simple ratio seems like a good way to measure potential cognitive abilities, it also has its pitfalls. Take, for instance, the squirrel monkey, which has a brain/ body weight ratio of 0.032, and compare it with the human brain/body weight ratio of 0.020. If this ratio were used as a measure of potential intelligence, then squirrel monkeys would defeat modern humans in intellectual capacity. The main problem with this ratio is that the size of the brain increases at a slower rate than one’s body weight. Therefore, the larger the mammal, the smaller the brain/body weight ratio.
The way around this impasse is to use a regression line graph. It was in 1885 that Charles Darwin’s cousin Francis Galton invented this idea of a regression line to illustrate the average tendency within a population. This technique enables researchers to overcome the problem of comparing the human brain with that of a smaller primate. On this graph, you would find the weight of the body on the X-axis and the weight of the brain on the Y-axis. Once these points have been illustrated on the graph, a “regression” line can be drawn through these points to represent the trend among the primate species. This line allows researchers to compare the brain of a human with that of a theoretical primate of the same weight. This method along with comparisons of anatomical structures has been very helpful in comparing the brain of a human with that of other primates. These methods of analysis have enabled researchers to determine that the brain of a human is 3 times larger than the brain of a chimpanzee, whose body weight is approximately the same.
Expansion of the Brain
Now that relative brain size and absolute brain size have been examined, it is important to take a look at the relative proportions of certain structures found within the primate brain. Is the human brain simply an expanded version of another primate’s brain, or is the human brain equipped with different proportions of structures? To answer this question, one must return to the regression line method; that is, one would have to take a part of the human brain and predict how large it would be in a primate of the same body weight as the human animal. We would then compare the predicted value of this brain structure (regression line) to the actual value of this structure. The difference between the two values would tell us how much larger our brain structure is than that which would be expected of a primate of the same body weight as us. When examining the results, it has been found that both the cerebellum and neocortex are larger in humans than in a theoretical primate of the same body weight.
Now that the proportional weight of the cerebellum and neocortex have been found to be heavier in humans than in a theoretical primate of the same body weight, it is important to take a look at what these proportions would look like in a theoretical primate with roughly the same brain weight as ours. Therefore, in order to carry out this analysis, one would have to plot the brain volumes of each primate on the X-axis and then plot the neocortex volume on the Y-axis of the graph. The regression line would then be drawn through the data, which would give the predicted volume of the neocortex based on the volumes of each primate brain. The results are surprising, because they indicate that the neocortex of the human brain is no larger than would be expected of a hypothetical primate with the same brain volume. Therefore, these results suggest that the human brain differs in its relative proportions from the brain of other primates but that these differences occur in a predictable fashion.
One of the most obvious characteristics of primates is their remarkable ability to engage in ongoing social relationships. Robin Dunbar and Sawaguchi and Kudo have found that mean group size is directly related to the relative neocortical volume in nonhuman primates. Dunbar stated that studies illustrate that primates are unable to maintain the unity of groups larger than a size set by the cognitive capacity of their neocortex. Sawaguchi and Kudo also found that social prosimians have relatively larger neocortex sizes than solitary prosimians. This finding lends additional support to the notion that mean group size can be directly related to the relative neocortical volume in nonhuman primates.
It is through the use of socializing techniques such as grooming and language that nonhuman and human primates, respectively, can maintain social relationships with a maximum number of individuals based on the relative size of their neocortex. Nonhuman primates maintain relationships through the use of social grooming. This method of socialization is utilized by primates to establish and maintain friendships and coalitions within a given group. Most primate groups rely heavily on social grooming mechanisms, and thus they dedicate countless amounts of time to this tedious, yet potentially rewarding task. At times, however, despite the fact that primates can maintain a certain number of social relationships based on their cognitive capacity, they are restricted to a limited number of social relationships due to the lengthy time requirements of social grooming. Therefore, with the evolution of the primate species and the expansion of the neocortex, it may be said that “higher” (later evolved) primates are capable of maintaining a larger number of social relationships within a group. Dunbar stated that another form of social grooming evolved in modern humans, that is, “vocal” grooming that is more commonly referred to as “language.” This other form of socialization enables humans to engage in social relationships with a larger number of individuals at once. He suggested that language evolved as a “cheap” form of social grooming, so enabling the ancestral humans to maintain the cohesion of the unusually large groups demanded by the particular conditions they faced at the time.
Evolution of Language
Language is a function that is processed in the cerebral cortex in areas known as Broca’s and Wernicki’s areas. Paul Broca first discovered this area in 1861. Broca’s area is located in the left hemisphere, on the inferior surface of the frontal lobe, and is responsible for the articulation of speech. It has been suggested that Broca’s area may have evolved and expanded among the hominoids. For instance, Ralph Halloway’s research found that the surface impression left by the brains of Homo habilis indicated an enlargement of the inferior surface of the frontal lobe corresponding to the adjacent Broca’s area. Later, in 1874, Carl Wernicke discovered another area associated with speech that is located in the left temporal lobe. Wernicke discovered this speech center, which is now known as Wernicke’s area, while performing autopsies on language-impaired patients. Wernicke’s area is mainly responsible for the analysis and understanding of sounds and words. There has been evidence that similar regions have been identified in the brains of rhesus monkeys and chimpanzees. Furthermore, it has been found that apes are indeed capable of understanding some forms of speech; therefore, this suggests that their brains are equipped with structure that enables them to analyze auditory information.
Tomasello and Call have suggested that dietary regiments are closely related to brain size and cognitive potential. They reported Clutton-Brock and Harvey’s study, which argues that frugivorous (fruit-eating) primates have, on average, larger brains relative to body size than do folivorous (leaf-eating) primates.
The reasoning behind this hypothesis is that fruits are much more patchily distributed in an environment and are more difficult to find and retrieve, thereby requiring primates to possess greater cognitive abilities in order to locate their food. Another study by Milton supports this claim. Milton studied two species of New World monkeys, the howler monkey and the spider monkey. These monkeys are of comparable size, live in social groups of similar size, and inhabit the same forests of Central and South America. Upon examination of the anatomical structure of these two types of monkeys, Milton found that the frugivorous spider monkeys had relatively larger brains than did the folivorous howler monkeys. Therefore, as demonstrated by these studies, there is a strong link between foraging practices and relative brain size among the primate species.
The primate species have undergone a series of changes throughout their evolution, most notably to the brain. The primates have maintained their overall body structure. However, expansions of the neocortex and cerebellum have led to various changes in cognitive abilities, which can explain the various differences in behavioral patterns. This evolution of the brain has enabled the higher primates to have better spatial recognition and an enhanced memory, which is key to locating fruits and shelter and is necessary for their survival. Furthermore, the expansion of the neocortex has enabled the primate species to socialize within a larger group size, which in essence has enhanced their ability to exchange useful information. It remains to be seen what will become of this trend. However, we are now experiencing an evolution of socialization methods with the advent of the Internet and computers in the human species. We no longer have to communicate verbally, or face-to-face, in order to maintain or gain social relationships. We are now able to communicate globally and exchange information with thousands upon thousands of people almost instantaneously. This form of socialization or communication can play a key role in the future of our species. Only time will tell where it will lead us.
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- Rumbaugh, D., & Washburn, D. (2003). Intelligence of apes and other rational beings. New Haven, CT: Yale University Press.