Evolution Of Intelligence Research Paper

1. Defining Intelligence

Intelligence is notoriously difficult to define in any brief and simple way. One reason for this is that whilst intelligence gives rise to adaptive behaviors, adaptive behavior is a ubiquitous phenomenon, most of which is not a consequence of intelligence. The first requirement, then, is to distinguish between behaviors that are a consequence of the action of intelligence and those that are not. A second reason for the difficulty in defining intelligence is that it has generic meaning in all animals that have the trait of intelligence; but it also has specific meaning in that intelligence may, and often does, manifest itself differently in different species. The problem is made more complicated by the commonly held view on intelligence in our own species, which is partly a manifestation of the problem of extending the generic definition to its speciesspecific manifestation in Homo sapiens. Given the fundamental tenet of evolutionary theory that living forms have a common origin based upon descent with modification, at least some aspects of human intelligence must be related to intelligence in non-human species.

1.1 A Generic Definition

Unlike plants that can incorporate and utilize the energy of the sun to fuel their needs, animals must indirectly gain access to the sun’s energy by consuming plants, or other living forms, including other animals that eat plants. In order to do this they must move about and act on the world, that is they must behave. Most behavior is the result of inherited patterns of central nervous system (CNS) functions that have evolved as efficient ways of exploiting such energy resources and avoiding damage. Such adaptive behaviors may be described as innate or instinctive in that they are solely caused by genes expressed by appropriate and necessary conditions of development to result in relatively invariant species-typical behavior patterns. Such behavior neither is generated by, nor results in, long-term changes in CNS states. However, in some animals, adaptive behavior is the result of enduring changes to the state of their CNS that arise from experience. This capacity to generate adaptive behavior resulting from within-lifetime experiences leading to changes in CNS states is the generic definition of intelligence. Thus defined, intelligence is identified with psychological mechanisms and processes such as learning, memory, and forms of internally manipulated CNS state changes referred to as thought and problem solving. Most intelligent animals are confined to relatively simple mechanisms of learning and memory.

1.2 Specific Definitions

Intelligence is often defined in terms of specific, often species-specific, manifestations. The reduction of response to irrelevant events (habituation) and associative learning (classical conditioning and instrumental learning) are widespread and common means of generating adaptive behavior in most intelligent species. There are also a host of more restricted forms of intelligence which include the ability of honey bees to learn about the position of food resources from other members of their hive, the ability of some bird species to acquire species-typical bird song, and tool use in chimpanzees. The human capacity for acquiring language, doing arithmetic, and forming abstract concepts are other examples. Very little is yet known about the psychological and neurological mechanisms that are the basis of any forms of intelligence. They may be similar if not identical in some cases, but are clearly different in others. For example, most psychologists agree that there has been a singular failure to identify language acquisition with associative learning. Nonetheless, all are forms of intelligence conforming to the generic definition given above.

2. The Origins Of Intelligence

One of the few things that Darwin wrote about with uncharacteristic lack of clarity is the origin of intelligence and its relationship to other forms of behavioral adaptations. It is now believed that intelligence originates in the limitations of instincts in dealing with changes in the world that occur at certain rates. However, it must be recognized that the difficulties faced by Darwin and later writers is that evolution is an historical process. The traits that are seen in living forms now are the result of selection forces acting on variations in the past, often millions, or hundreds of millions of years before the present. This applies to all adaptations including intelligence. Beyond the reach of empirical verification, one can never be certain about the reasons why adaptations originally evolved. The strength of any analysis rests upon the extent to which conjectured past selection forces are still present.

2.1 The Uncertain Futures Problem

Nothing in the world is constant. Change and flux are pervasive, giving rise to what C. H. Waddington called the uncertain futures problem. All problems of survival concern coping with change, and all extinctions are caused by the inability of species to cope with uncertain futures. One of Darwin’s great contributions to modern thought was his recognition that explaining the dynamic, not the static, is the aim of science. Change, however, takes different forms and occurs at different rates. Intelligence is one of a family of processes that make up biological responses to dynamically changing circumstances (Plotkin 1994).

All adaptations have three characteristics. The first is they increase individual fitness (that is, the chances of survival and reproduction). This is why they are selected and become widespread in a population or species. The second is they are heritable (that is, they are part-caused by genes and hence inherited from parents by offspring). The third is they constitute a relationship between parts of organismic organization (for instance, the existence of an enzyme, or a limb of specific form) with a particular feature of the environment (for example, a nutrient, or the form of terrain). Behavioral adaptations possess all these characteristics. They increase fitness; they are genetically part-caused, finding expression in behavior through development in an appropriate environment; and they have matching relationships to specific features of the environment.

2.2 Adaptations To Different Rates Of Change And The Evolution Of Intelligence

Adaptations constitute matching relationships to relatively enduring features of the world, that is, whose change is so slow that they appear to be enduring. This is why they are present in most members of a species and endure as species-specific traits over long periods. The information for the construction of the trait is present in the gene pool of that population or species. However, features of the world that change more rapidly than can be tracked by the main evolutionary program have also to be adapted to if they constitute significant features of the environment. Some behavioral adaptations show developmental flexibility such that different ranges of environmental experience will result in different forms of behavior (for example, living as solitaries or swarming in locusts). Such developmental adjustments to adaptations, however, are bound to the temporal restrictions of the developmental process. Other fluctuations in the environment are so fleeting and pervasive, like non-seasonal temperature changes, that flexible development cannot provide appropriate adaptations. In such cases, short-term responses like shivering or sweating provide a means of tracking such changes.

An important and different class of change is those which occur rapidly and possibly repeatedly throughout a lifespan, but which may constitute features of the world that endure for significant periods. Neither developmental plasticity nor short-term physiological adjustments can furnish adequate adaptations to such forms of change. Examples are the position of a food resource or the identity of an ally or enemy in a social species. Such short-term stabilities concern the relationship between organisms and their environments that are central to the social and behavioral sciences. It is the need to adapt to such short-term stabilities that constitute the evolutionary origins of intelligence. Hence, the definition of intelligence as the capacity to alter CNS states because of experience now extended to the notion that such changes in short-term stabilities be tracked and matched by neural network states that generate adaptive behaviors to such short-term stabilities.

One important feature of the environment is cause-effect relationships. Some of these are so constant that adaptations in the form of innate or instinctive behaviors of approach or avoidance will evolve. Others are transient. For example, the characteristics that identify food that causes illness may and do change and such changes must be tracked. A sensitivity to such changing cause–effect relationships and the ability to conserve these in the form of changes in CNS states (that is memories) that direct appropriate behavior were likely to have been amongst the earliest forms of intelligence to have evolved.

3. The Phylogenetic Distribution Of Intelligence

There are no reliable reports of intelligence in nonanimal forms such as plants or fungi. Depending upon classificatory system, the animal kingdom is made up of about 25 major divisions (phyla), with reports of intelligence being present in species belonging to seven of these.

3.1 Intelligence In Vertebrates

Vertebrates comprise much the greater part of the phylum Chordata, the comparative evidence indicating that intelligence in the form of associative and other relatively simple forms of learning is widespread throughout this subphylum, which ranges from cartilaginous and bony fish, through amphibia, reptiles, and birds, to mammals. The comparative study of intelligence, in any phylum, has never been undertaken in any systematic fashion. Most laboratory-based animal learning studies have used just two species of animal, the domesticated laboratory rat and the pigeon. The poor status of this work as comparative scholarship is attested to by few texts on animal learning carrying an index of species. That said, and noting that negative studies are seldom published, it is probably the case that positive results on simple learning have been reported in all studies undertaken in different vertebrate species.

In addition to associative learning capacity, intelligence in the form of the ability to acquire knowledge of complex spatial arrays, complicated sound patterns being incorporated into vocal signaling, problem solving, transitive reasoning, and tool use have been demonstrated in a variety of species of bird and nonhuman mammals. The existence of imitation learning and the ability of apes, especially chimpanzees, to attribute mental states to others and to be able to learn and use language are highly contentious issues.

3.2 Intelligence In Other Phyla

The most complete review of invertebrate intelligence is to be found in three volumes edited by Corning et al. (1973a, 1973b, 1975). In general, the evidence is either weak or disputed. The unicellular Protista (for example, amoeba) may show habituation, but lacking a nervous system, the mechanisms underlying the ability to cease to respond to non-significant events will be entirely different from multicellular animals that have evolved nervous systems. Claims for the existence of habituation in Coelenterate species (e.g. hydra, medusa) are also disputed, though such animals do have nerve-net nervous systems. Platyhelminthes (flatworms) have received considerable study. These animals have a CNS, occupy a pivotal position in the evolution of multicellular animals, and may have been demonstrated to have simple associative learning. The evidence for associative learning in members of the phylum Annelida (earthworms, leeches) is stronger. The great majority of animals are members of the phylum Arthropoda, which includes crustaceans, spiders, and insects. The behavior of honeybees is the most extensively investigated of all invertebrates. There is no doubt that they have associative learning, which shows remarkable similarities to the learning of the laboratory rat. Gastropod molluscs (like snails) can habituate, and possibly condition. There are extensive reports of Cephalapod molluscs (octopods and squids) demonstrating associative, instrumental, and discriminative learning. Given that these are active predators with highly developed sensory and nervous systems, this might be expected. However, it should be noted that claims for learning in these species have been vigorously contested.

In general, the evidence for habituation in a number of non-chordate animals is strong. Honeybees apart, the status of work on other forms of learning is poor. There is much to be done on invertebrate intelligence in the future, an area of research that for some years now has been moribund. It should be noted that centralized nervous systems with neural networks evolved around 500 million years ago. The selective advantages of being able to detect conserved cause– effect relationships and other short-term stabilities must always have been very strong. It would be surprising if high quality research into non-chordate learning in the future did not prove positive.

3.3 Intelligence, Ecology And Life-History Strategies

The nature of the short-term stabilities that are significant for the survival and reproduction of animals will vary with the life-history strategies and ecologies of different species. This view has led to a conceptually powerful approach in recent years which seeks to place intelligence within the context of the natural history of any intelligent species, where intelligence is seen to be interwoven with instincts (Gould and Marler 1987). It is likely that much of the ambiguity of findings on animal intelligence, especially that of invertebrates, will be resolved by research carried out within this framework.

4. Human Intelligence

Intelligence has been a central area of human psychological research throughout the twentieth century. Much of it has revolved around the seeming dichotomy between general intelligence (g) and specific intelligences (s) such as verbal and numerical skills. In recent years, s has come to be identified with computationally specialized cognitive modules (Fodor 1983). There is, however, no reason to think of g and s as being exclusive properties of human intelligence (Plotkin 2002). It is entirely conceivable either that some property of the brain’s information processing capacity is common to all computational modules, or that the outputs of individual modules are subject to further common processing which is reflected in g.

4.1 Current And Future Research

Since the early 1980s there has been a remarkable advance in understanding the emergence of intelligence in the child. The findings are relatively uniform in supporting the existence of discrete intelligences (s) that appear to be largely innate cognitive systems (Hirschfeld and Gelman 1994). The notion of human intelligence as a tabula rasa has been shown to be wrong. Furthermore, there is a move towards relating such individual cognitive skills to more basic mechanisms, for example showing the connections between language, gesture, and manual skills (Wilson 1998). This conceptual marriage between viewing intelligence as a collective of specific, innate, cognitive devices, and yet relating it to more basic mechanisms like information processing and memory mechanisms (Deacon 1997), is likely to be the dominant form of research into human intelligence in the coming decades.

It should be noted that lineages maintain many of the features of ancestral species. For this reason, human intelligence certainly comprises evolutionarily older forms of intelligence, such as associative learning because of the continuing need to be sensitive to conserved cause–effect relations, as well as more newly evolved forms of intelligence such as numerical and linguistic competence.

4.2 Intelligence And Culture

Culture is at once an extraordinary manifestation of, and an extension of, human intelligence. The appearance of culture is one of the major evolutionary transitions in the history of life (Maynard Smith and Szathmary 1995). An important focus of research into intelligence in this century will concern the essential psychological mechanisms of culture, especially those features of intelligence that allow humans to enter into culture (Plotkin 1998), as well as the interrelationship between culture and human cognition (Donald 1991) and between culture and biological evolution (Laland et al, 1995). Intelligence will be seen as an evolved adaptation with specific manifestations in Homo sapiens that make it central to understanding human culture. Understanding the relationships between biological evolution, individual intelligence, and human culture delineates one of the most important developments in the social sciences in the decades to come.

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