The pressures of group living were the primary selective pressures driving human evolution. Discuss.

(Author: Gabrielle)

One of the most important elements in human evolution is the expansion overtime of the brain size. Different hypotheses explain why human beings developed such an energy-eating feature. This essay provides an explanation as to why humans developed a large brain, and to what extent this was due to social pressures and to group living. This essay is based on previous studies by scientists, using their data and analyses of non-human and human primate brains, behaviour and relationship to the environment. This essay, in agreement with the “social brain hypothesis”, concludes that the pressures of group living have driven human evolution, and that social skills were an extremely important factor.

The social brain hypothesis is not the only one, other hypothesis have given alternative explanations to why primate brains are large and why human evolved. For example, one of these theories says that our brain size can be explained as consequence of our body size. The main problem with those theories is that they don’t take in account the fact that natural selection will not privilege such a costly feature if it isn’t key to the survival of the species. They understand the important physical changes that humans went through to carry a large brain, but they do not answer the question of why a large brain became compulsory to the survival of our specie (Dunbar: 178-179). Another theory, based on ecological factors, exists and gives an explanation that links the evolution of the brain to environmental factors. For example, if a primate is a frugivorous, then it will use it’s brain more often than a leaf-eater because it will need a higher memory capacity to remember where fruits, which are not as common as leaves, are located. To test this theory one can, for example, find the relation between the amount of fruits in a diet and the size of the brain (Dunbar: 179). However, it has been proven that a link exists between the size of a group and the size of primate brains, or more precisely of primate neo-cortex . An obvious interpretation of this study would be that some social constraints, related to group size, drove the evolution of the brain. The first assumption one could make is that a good memory and therefore a good brain is important to remember who the members of the group are and what the relationship with them is. However, memory is not the only useful tool in human relations for which a large brain is necessary and it is not located in the social area of the brain. (Dunbar:184) To process the information received from social interactions one needs a brain capable of dealing with complex social information. It is this last argument, called the social brain hypothesis, which will be discussed in the next paragraphs. An evidence can be found in the length of time spent by parents – human and non-human primates – to rear their children before they attain adulthood. This length of time, which is one of the longest amongst mammals, happens to be proportional to the size of the neo-cortex. During its youth, a juvenile acquires social skills, such as the understanding of rank and dominance, indispensible to group living. Furthermore, it is believed that it is because those social skills are crucial that primates dedicate such a big amount of time for their teaching and not to something else. Here, natural selection has privileged a long youth because it is necessary to acquire complex social skills, which are trivial for the survival of a specie living in large groups. This can also be interpreted as an indirect proof that the size of the neo-cortex is due to the complexity of social relations amongst primates, which are made more complex as the groups get bigger (Tracey 1997). Another evidence for this hypothesis is the importance of language and grooming in primate lives. If a group is big, the members of the group will need a large neo-cortex to maintain and process a certain number of relationships. It seems that those relationships, which are sustained by grooming, act as a shield against the pestering of other members of the group. Their effectiveness may be measured by the time spent grooming. Therefore, the larger the group is, the more amount of time individuals will spend grooming and building relationships, needing a large neo-cortex to process all the information. Humans don’t spend an important amount of time grooming compared to the size of their groups. This is probably because they have found a more effective way of creating relationships that still enables them to spend time finding food and doing other important activities. Indeed, language not only enables humans to communicate with more than one person, but also enables them to share information about individuals that are not in the conversation. The content of most conversations in the human world appears to prove that the primary use of language is not to exchange information about the environment, as was previously thought, but to socialize. As Dunbar says, “The acquisition and exchange of information about social relationships is clearly a fundamental part of human conversation. I suggest that it implies that this was the function for which it evolved. ” This means that social pressures once again acted as a constraints that enabled the neo-cortex to develop. It was “made” capable of using tools, such as grooming and language, to help primates in their social relations. Language permits humans to build greater social networks but requires a more complex, and therefore larger, brain (Dunbar 1993). Group living has without doubt acted as a selective pressure in the evolution of primates and was probably one of the primary constraints driving human evolution and human brain evolution. Living in groups was necessary but would not have been possible without developing social skills. This can be proved by the relation between the size of primate neo-cortex and group. This means that the neo-cortex evolved because it is used to deal with relationships, and that this was one of the basic necessities to keep a group together. The length of the youth amongst primates, time during which social skills are acquired, may also be a useful argument to prove how important the capacity of learning social skills is for an individual to survive and a group to be solid. The time spent grooming, i.e. building relationships, or conversing about random things is also a key argument to show that good social skills are extremely important for the survival of the specie. Social pressures and more precisely the size of a group, has been a leading factor in human (brain). This does not, however, mean that other factors, such as the ecological factor, have had no importance at all in human evolution.

Bibliography • Dunbar, R. I. M. (1993) “Coevolution of neocortical size, group size and language in humans,” Behavioral and Brain Sciences 16 (4): 681-735 in http://www.bbsonline.org/documents/a/00/00/05/65/bbs00000565-00/bbs.dunbar.html?rel=nofollow • Joffe, T.H. (1997) “Social pressures have selected for an extended juvenile period in primates” Journal of Human Evolution 32: 593–605 • Dunbar, R.I.M. (1998) “The Social Brain Hypothesis” Evolutionary Anthropology 6(5): 178-190

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