Primate Evolution free essay sample

Many theories have been posed as selective mechanisms for the trends toward increased intelligence in primate evolution. Some emphasize complex ecological pressures, mainly foraging strategies as the primary forces driving an evolutionary increases in cognitive abilities, and other suggest that increased social complexity favored the evolution of primate intelligence (Tomasello and Call, 1997). Across the animal kingdom, brain size increases with increasing body size, however, despite the common scaling principle, brain size to body weight ratios differ from one taxonomic group to another (Jerison, 1973; Gould, 1975). In primates, for example, the brains of apes are generally larger relative to body weight than the brains of monkeys, whereas the brains of monkeys are larger than those of prosimians (Jerison, 1973). Structural differences are also apparent. In chimpanzees, a larger proportion of the brain is devoted to neocortex than in monkeys, who in turn have proportionately more neocortex than prosimians (Martin, 1990; Passingham, 1982). Despite the fact that it is metabolically costly, there has been increase in primate brain size (Harvey et al, 1987). We will write a custom essay sample on Primate Evolution or any similar topic specifically for you Do Not WasteYour Time HIRE WRITER Only 13.90 / page What selective pressures have overcome these costs? When the question is applied to humans, answers typically refer to the adaptive advantages of technology – initially stone tools and language. But monkeys and apes use only rudimentary tools and lack language entirely, yet their brains are significantly larger than those of similar-sized mammals (Jerison, 1973). Thus it can be concluded that, some other selective pressures must be at work. This paper will discuss how the possible selective mechanisms such as complex ecological pressures and increased social complexity have favored the primate cognitive evolution and increased he brain size in the primate order. As the large brain correlates with greater intelligence, we have to take in consideration that the primates would not have been able to evolve larger brains if there was no selective advantage in doing so (Finlay et. al. , 2007). Thus in this paper it will discuss how both social and ecological pressures simultaneously trigger the bigger brain in primates therefore evol ving the complex behavior and intelligence in them throughout their course of evolution. Life History Strategies Primates are relatively intelligent compared to other animals, as seen earlier, primates tend to have larger brains relative to their body size, humans are an extreme case, even among primates. Among primates, relative brain size is greater in species with larger home ranges and greater in species that are frugivorous or omnivorous than in species that eat leaves (Clutton-Brock and Harvey, 1980). Species that feed on fruit may face special problems in learning and memory because they depend on widely spaced food that is temporary in both space and time (Milton, 1988; Tomasello and Call, 1997). There have always been speculations as to why are primates have larger brain to body size and are intelligent than other mammals. Defining intelligence is a very slippery issue as it is more than just complex behavior. Many complex behavior among animals do not impress us as particularly intelligent for example, we don’t think that homing pigeons are particularly smart in spite of their ability to find their way home from distant, unknown places (Wright et al. 988; Wasserman et al. 1995). Probably it can be said that intelligence has something to do with flexible behavior, the ability to solve novel problems. Researcher point out that we don’t seek a single explanation for primate intelligence, but rather a series of explanation, each related to one portion of the sequence of evolutionary changes that led to larger brains which eventually led to higher cognitive capabilities in primates (Finlay et al, 2001; Tomasello and Call, 1997). Primates did not just evolve large brain and intelligence out of nowhere; there were series of evolutionary sequence that led these changes. First we have to answer, why did earliest, strepsirrhine-like primates develop brains larger than those of other mammals? Why evolving larger brain and intelligence have improved their reproductive success more than other animals? It is answered by the life history strategies, which refers to the general plan of an organism’s life, including its rate of maturation, body size, lifespan, effort spent on reproducing vs. arenting vs. surviving, that is viewed in terms of how this plan affects the organism’s reproductive success (Passingham, 1982; Deacon, 1992). Animals that face a high risk of predation maximize reproductive success by having a life history strategy that produces lots of offspring as quickly as possible, this maximizes the number of offspring the individual produces before a predator kills it (Deacon, 1992). The first strepsirrhine-like primates, evolved from small, rodent-like mammals that apparently had shorter life history strategy (Martin, 1990). They had grasping hand and feet which allowed them to cling to small outer branches of bushes and trees, this would have enabled them to evade many of the predators; with less predation, the need to mature rapidly and have large quick litters was reduced, the balance of selection pressures therefore shifted more towards growing large bodies and brains that matured slowly, enabling the strepsirrhines to live longer and produce offspring at longer intervals but with more time for each to develop and become better at surviving (Martin, 1990). It is believed that earliest haplorrhine have relatively larger and more complexly organized brains than their strepsirrhines ancestor, as they were diurnal and they tend to have larger bodies, slower maturation and longer lifespan more towards a long life history strategy. They had relatively larger brains than do strepsirrhines, and have brains with larger neocortex, which is the part of the brain that is most involved in learning, problem solving and planning (Martin, 1990). Theories and Hypothesis In primates, mainly for haplorrhines, there has been many theories and hypothesis about why selection favored relatively larger brains with more neocortex. Two popular hypotheses that have been favored are Ecological intelligence hypothesis and Social intelligence hypothesis. According to ecological intelligence hypothesis, intelligence was favored ecause it helped individuals better exploit their ecological circumstances, especially regarding getting food, which were more complex due to changing from strepsirrhine insectivores and gumnivore diets to more varied haplorrhine diets involving fruit and maybe foliage, as new ecological niche rewarded more flexible, complex behavior (Reader and Laland, 2002; Tomasello and Call, 1997). Resource patchiness (frugivory) which is a variant of ecological intelligence hypothesis states that intelligence was favored among animals that had to find resources that were distributed in patches that changed seasonally, especially fruit, available at the same tree every year, but only for limited time. Thus individual with better memory, greater ability to learn about their environment, ability to plan ahead might be more efficient at finding food rather than wasting less time on travel to barren trees or random, fruitless foraging (Tomasello and Call, 1997). Another variant of ecological intelligence hypothesis; resource extraction states that intelligence was favored among animals that benefitted from performing complex tasks to extract their food, as in recognizing where to dig up roots, breaking open hard-shelled nuts or fruits, finding insects under tree bark etc (Tomasello and Call, 1997). According to social intelligence hypothesis, intelligence was favored because it helped individuals better take advantage of the more complex social interactions of larger social groups, the larger the groups, the more relationships to track and manage (Dunbar, 1998). Living in groups also in turn was a response to increase in predation, which was caused by shifting strepsirrhine nocturnal activity patterns to haplorrhine diurnal activity pattern (Dunbar, 2003). Social intelligence helped individuals better solve more complex social problems like establishing and keeping track of dominance relations at a minimum cost, without loosing more resources than necessary to less dominant individuals or getting injured by more dominant ones by forming a coalitions that successfully benefit the individual without costing too much (Dunbar, 2003; Byrne and Whiten, 1988). Also it helped by keeping track of who is more and less related so that the individuals performs altruistic acts only for kin, individuals who handled these problems better might have higher reproductive success (Byrne and Whiten, 1988; Harcourt, 1992). If the ecological intelligence hypothesis were correct, then primates should show unusually great knowledge of their environment and unusually great ability to exploit it and the result is they do (Tomasello and Call, 1997). Some species act as if they know where hundreds of different foods can be found at different times of the year, for example squirrel monkeys track when different foods are ripening and they move from one food patch to the next and evidently create and remember a ‘cognitive map’ of the locations of food types and recent status of food patches, as shown by their ability to move directly from one to another appropriate location on a long distance away (Tomasello and Call, 1997). But are these abilities any greater than similar ones in other animals? It is hard to measure and is debated. If the resource extraction hypothesis were correct, then the species that have to figure out the most varied and complex means of extracting resources should have experiences the strongest selection for an expanded neocortex. But neocortex ratios do not correlate with complex resource extraction behavior so this does not support the claim that having more neocortex helps with complex resource extraction (Tomasello and Call, 1997). If the resource patchiness hypothesis were correct then the species that have to deal with patchiest resources should have experiences the strongest selection for an expanded neocortex. Fruits have patchier distribution in space and time than do leaves, insects or gum, frugivores do tend to have higher neocortex ratios. Using larger home ranges means using more dispersed resource patches and probably more of them, species with larger home ranges tend to have higher neocortex ratios. Both of these findings suggest that the resource patchiness hypothesis may be correct (Passingham, 1982; Tomasello and Call, 1997). If the social intelligence hypothesis were correct, then the species that have to deal with the most complex and frequent social interactions should have experienced the strongest selection for expanded neocortex and they should be unusually good at handling complex problems that have to do with social interactions (Dunbar, 2003; Reader and Laland, 2002). The larger the social group, the more complex the social intelligence is because the larger the group, the more possible combination of individuals for interactions, alliances etc (Heyes 1994; Thompson 1995). So if intelligence evolved because it helped with solving social problems then species that live in larger groups should have higher neocortex ratios (Harcourt, 1992). Sure enough, neocortex ratios do correlates well with group size, the bigger the group, the more larger the neocortex; this supports the social intelligence hypothesis. So it looks like intelligence probably evolved because it helped with exploiting patchy resources and with living in large groups but not so much because it helped with complex processing of resources (Harocourt, 1992; Cheney, 1983). It is hard to compare abilities for solving different social problems in different species but we can at least show that many primates understand a lot of complex features of their social lives, which supports the idea that primate intelligence developed specifically to handle these kinds of social problems (Harcourt, 1992). For example, many primates ‘know’ a lot about the kin relationships in their social groups, such as Japanese macaques/ Gelada baboons, they know who their own relatives are, usually only know from their mother’s side from growing up with them (Cheney, 1983). This is not surprising, since kin selection would favor individuals who can most effectively direct their altruistic behavior towards close kin but many also know how other individuals are related to each other (Harocurt, 1992; Cheney, 1983). These are third party relationships; between two other individuals, not directly involving the individuals who knows about them. There is no obvious kin selection explanation for this, primates don’t know who is related to who automatically: they have to learn it, in many primates, all groups members study new infants, apparently learning to recognize them and associating them with their mothers (Cheney, 1983). This requires a lot of learning, and a lot of memory. The fact that they can do this suggests that natural selection for these social abilities must have been very strong (Cheney, 1983). There have been experiments done to say that primates recognize kin. Experiment done by Cheney and Seyfarth (1995), where they played recorded calls of a specific infant to a group of vervet monkey, not surprisingly, the mother looked toward the call for longer than the other monkeys did. She obviously recognized her own infant’s call, what was interesting was that other monkeys tended to look not at the source of the call, but at the mother. That is, they not only recognized the infant but they also knew who its mother was and that she was likely to do something worth seeing. So these monkeys don’t just know their own kin, they know kin relationships between others who are not their own close relatives that presumably take some intelligence to learn and keep track of more. Cheney and Seyfarth (1995) also documented redirected aggression, where they found out that they know the relationship of others, as vervet redirected aggression to the relative (kin) of former opponent. The monkeys may not be aware of kinship per se, they may just know which individuals spend more time with which others, but the effect is the same. They are still observing, learning and keeping track of the social relationships between many individuals, including many relationships that do not directly involve them. For instance, Baboons know about dominance hierarchy (rank) relationships among third parties, they responded to dominance grunt or submission bark, which shows that they know the relative ranks of other individuals (Cheney and Seyfarth, 1995). There have been problems with social intelligence hypothesis for hominines, on what led the earliest hominines (great apes) to develop even longer life histories, larger brains and relatively more neocortex (Gould, 1975; Tomasello and Call, 1997). Some of the hominines do not live in large groups as suggested by social intelligence hypothesis, such as orangutans are solitary and gorillas live in small groups (Tomasello and Call, 1997). But many hominines use complex sequence of actions to process food, gorillas break and peel stalks of wild celery, then pick out the edible bits in the center, chimpanzee and orangutans use sticks to fish for insects and break into fruits, chimps break open hard nuts using hammers and anvils of stone or large branches and roots (Harvey et al, 1987; Tomasello and Call, 1997). All the great apes are able to use tools in captivity so maybe greater intelligence helped with resource extraction, which is a variant of ecological intelligence hypothesis, but on other hand chimpanzee and bonobos do live in large and very socially complex groups. They have better ability to solve complex social problems (mating strategies, dominance hierarchies etc), which definitely improve their reproductive success and/or inclusive fitness (Tomasello and Call, 1997). So which is the ancestral condition for hominines, that is, the situation in which hominines intelligence evolved? If living in small groups is the ancestral condition then the first hominines evolved greater intelligence in small groups, probably for ecological, not social reasons, this intelligence later allowed chimps and bonobos to evolve large, socially complex groups (Jerison, 1973; Gould, 1975). If living in large groups is the ancestral condition then the first hominines evolved greater intelligence in large groups, probably for social, not ecological, reasons. This intelligence later allowed gorillas and orangutans to develop complex methods of resource extraction (Jerison, 1973; Gould, 1975) There has been evidence against social and ecological intelligence hypothesis. For example, other animals also depend on patchy resources but they have not developed notable intelligence, like fruit-eating birds (Milton, 1988; Tomasello and Call, 1997). There are animals that depend on complex extraction behaviors but are not particularly intelligent, like sea otters that float on their backs with a rock on their chest and bang sea urchin on it to open them (Tomasello and Call, 1997). There are some animals that live in large groups such as birds, bison, deer etc but are not particularly categorized as intelligent. But there are methods to test these hypotheses by measuring the neocortex (Martin, 1990). Higher neocortex ratio means that relatively more of the brain is neocortex, which means that brain is ‘smarter’, measure of smartness of the brain is not affected by brain’s size, but rather by which portions are more emphasized within it. Now if the conditions are associated with having ‘smart’ brain with a high neocortex ratio, this evidence is based on measuring the neocortex index of many primate species (Martin, 1990; Reader and Laland, 2002). Conclusion Testing the hypothesis, it is concluded that social complexity does result in larger brains better than resource patchiness or extraction (Tomasello and Call, 1997). But social intelligence hypothesis does not explain why apes are smarter than monkeys. Apes all live in relatively small groups. It cannot clearly be said that what favored the evolution of enhanced cognitive abilities in apes, as they do use lot of extractive foods and some foraging skills take a long time to learn. Big brain may be linked to foraging challenges, but complete explanation may require multiple selective pressures. Big brains may be linked to particular types of social challenges; it could be the result of selection for flexible behavior in both ecological and social domains. The link between brain size and ecological or social intelligence however is entirely hypothetical. It can be assumed that having spatial memory of location of ripe fruit or remembering the kin relations of one’s, demand considerable cognitive or brainpower, but this hypothesis is neither supported nor refuted by any evidence (Dunbar, 1998 and 2003). The intelligence of different species is despicably difficult to compare, different species manifest their intelligence in different ways, making it impossible to find an objective measures of intelligent performances that can be used across taxa (Hodos and Cambell, 1969). Natural selection may have favored an increase in brain size because of benefits derived from innovation or social learning that are independent of a species’ typical group size. But ecological and social intelligence are difficult to distinguish in present-day species and unlikely to have played entirely separate roles during evolution (Tomasello and Call, 1997). Social learning often helps individual to acquire food, whereas tool use can have social as well as ecological benefits. It is true that there is strong support for the social intelligence hypothesis but the social domain of primates is much harder to define than domains of expertise like the tern’s skills in navigation or the bee’s skill in communicating about food (Byrne and Whiten, 1998; Finlay et al, 2001). Answer is not clear on what ecological or social intelligence that drove primate brain growth. It is unlikely to be one or other; both of the factors could likely be operating at the same time or in alternation over evolutionary history. Reference: Byrne, R. W. and Whiten, A. ds (1988) Machiavellian Intelligence: Social Expertise and the Evolution of Intellect in Monkeys, Apes, and Humans (Clarendon, Oxford) Cheney, D. L. (1983). Primate Social Relationships: An Integrated Approach, ed Hinde R A. (Blackwell, Oxford), pp 278–285. Cheney, D. L. , Seyfarth, R. M. , and Silk, J. B. 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