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Cognition is the product of brain mechanisms that have been shaped by the evolutionary process. Understanding the form and function these mechanisms take, how they develop, and how they are distributed across species requires situating these mechanisms in an evolutionary context, and studying them using the diversity of methods available to the evolutionary sciences. Here we review how the mechanisms underlying cognition can be studied from an evolutionary point of view.
- Psychological Adaptations
- Methods for Investigating Psychological Adaptations
- Laboratory and Naturalistic Experiments
- Neuropsychological Dissociations
- Developmental Studies
- Cross-Cultural Studies
- Cross-Species Comparisons
- Genes, Fossils, and Other Sources of Evidence
- A Case Study: Theory of Mind
Evolutionary theory is widely agreed to be the best explanation for biological complexity and organization yet conceived, but the power of this theoretical approach has only in the last few decades been fully realized in explanations of human and nonhuman behavior. The behavior of humans and other animals is generated by functionally specialized nervous systems that develop and operate within the specific ecology of the organism (Tooby and Cosmides, 1992). Since the work of Niko Tinbergen, biologists have recognized that in order to fully understand an organism’s behavior, explanations at several complementary (i.e., nonmutually exclusive) causal levels are required (Tinbergen, 1958).
On the proximate level, behaviors can be explained mechanistically as the product of neural activity, which itself responds to both external states, such as temperature, and internal ones, such as goals, emotion states, and physiological conditions. Neurobiology, cellular biology, and related disciplines have produced explanations for behaviors at this direct, causal level that are critical to understanding how behavior is proximally caused. Also at the proximate level are developmental explanations. An organism’s psychological and physical traits are the result of developmental processes, initiated at conception, that shape the growing organism as a function of its interaction with the environment. Developmental biology and psychology have produced explanations of behavior that consider the timing of the appearance of adaptations over a single lifetime and the roles of learning and plasticity in shaping them.
Evolutionary or ultimate explanations are of two kinds. Phylogenetic explanations take into account the evolutionary history of descent with modification that has led a given organism to possess some traits and not others. This history can shape and constrain adaptations underlying behavior in powerful ways. Paleontologists, comparative anatomists, and geneticists have reconstructed the lineages of many species, and these phylogenies allow us to understand the evolution of brains and behavior in comparative and historical perspective. Another kind of ultimate explanation is functional or adaptive explanation, in which the traits of organisms are analyzed in terms of the roles they play in promoting survival and reproduction (fitness).
Functionalist or adaptationist approaches to cognition have made significant contributions to our understanding of how particular forms of behaviors and neural mechanisms reflect their adaptive functions. Until recently, researchers studying behavior have tended to focus exclusively on one level of analysis. More recent approaches to the evolution of cognition, such as evolutionary psychology, have sought to incorporate multiple levels of causation by explaining behavior not merely in adaptive terms (i.e., in terms of their immediate effects on fitness in current environments), but as the product of underlying proximate mechanisms, shaped by evolution, that might or might not produce adaptive behavior depending on the conditions under which they develop and operate.
An adaptationist approach to the mind begins with a simple premise: just like lungs, livers and flagella, brains are evolved organs with functional parts. Each of these parts, which are sometimes called psychological adaptations or modules, has evolved for something. They exist because they have been selected for, meaning they had positive effects on survival and reproduction over evolutionary time. They are ‘for’ something, in that they have consistently solved some adaptive problem, whether it was to discern light from darkness, to discriminate loud noises from soft ones, or to identify an object as food or as inedible. Any process in the mind that has evolved through natural selection to carry out an information-processing function can be regarded as a psychological adaptation (Barrett and Kurzban, 2006).
Formal descriptions of psychological adaptations include descriptions of their inputs, information-processing procedures, and outputs. The set of inputs that a psychological adaptation processes is called its domain, and adaptations vary in the kinds of information they have evolved to use. Similarly, the information-processing features of mechanisms can vary substantially, from the time series analyses performed by classical conditioning mechanisms, to Bayesian inference performed by systems underlying word learning and categorization, to if-then rules characterizing reasoning systems. Finally, the outputs of psychological adaptations can exhibit special design; for example, different emotion systems cause a variety of diverse behavioral and physiological responses in organisms, from sexual attraction to nausea.
At the outset of the study of psychological adaptations, many researchers held that they should possess a common set of features, such as narrow information domains, encapsulation (inaccessibility to conscious processes), lack of environmental influences on development, and automaticity (Fodor, 1983). While some researchers continue to view psychological adaptations as strictly ‘modular’ in this sense, others argue that the hallmark of adaptation is likely to be functional diversity rather than functional uniformity (Barrett and Kurzban, 2006). This ‘functional diversity’ view of psychological adaptations is able to accommodate many types of mechanisms under the rubric of specialized adaptation, including mechanisms of learning and cultural transmission.
All the problems mentioned above are solved via the processing of information, but not all psychological processes use the same information. The information that a psychological mechanism uses is called its domain. For example, the visual system processes information about light energy transduced by photoreceptors in the retina, whereas the auditory system processes information about sound waves transduced by the cochlea. Within vision, some mechanisms respond to color, whereas others respond to shape, and even more domain-specific mechanisms can exist, such as mechanisms specialized to process the configuration of facial features in humans. In each case, the mechanism was shaped by the evolutionary process because of the effects it had on the organism’s survival and reproduction. Visual mechanisms, for example, evolved because of their benefits for detecting predators and mates, for spatial navigation, and so on.
The behavior of complex organisms such as humans is not likely to be carried out by a single, general-purpose mechanism. In brains, information processing is made more efficient by cognitive division of labor: delegating tasks among smaller subsystems, whose operations can be coordinated through hierarchical organization and feedback loops. The nervous system is therefore a mosaic of many evolved mechanisms, often with different evolutionary histories. However, the fact that brains are complex networks of neural tissue means that investigators often cannot distinguish one subsystem from the next using anatomy alone.
Methods for Investigating Psychological Adaptations
Laboratory and Naturalistic Experiments
The signature of natural selection is functional design, characterized by a fit between form and function. In the case of cognition, this means that the brain mechanisms that perform specific information-processing functions will exhibit processing features designed to carry out those functions. To investigate adaptive design, rigorous empirical methods from a variety of fields including cognitive psychology, developmental psychology, anthropology, and neuroscience are in order, because these methods allow some measure of control over the multitude of variables interacting within a thinking mind. Laboratory experiments are tightly constrained and often artificial, which has advantages and disadvantages. It is informative to complement laboratory studies of cognition with studies in the field, and across diverse settings. An advantage of field studies is increased ecological validity, i.e., resemblance of the task to the natural setting in which it evolved.
As an example, standard experimental methods from cognitive psychology have been used in ecologically valid contexts in work investigating sex differences in human spatial cognition. Silverman and Eals (1992) proposed that sex differences in human spatial cognition may reflect an ancestral division of labor between hunting and foraging. Prior work has shown that women are better than men at certain abstract spatial tasks involving memory for object location, but since most of the data were gleaned from laboratory experiments, ecological validity was lacking. New et al. (2007) studied spatial cognition in an outdoor setting in a farmers market in Southern California, and showed that women are more accurate than men when pointing in the direction of newly learned locations for food items, but that both men and women are equally good in locating calorically rich food items in the same market. This finding is consistent with the existence of two distinct adaptive mechanisms: one to rank the value of food resources and one to remember the locations of those resources. By approximating foraging in a simple, natural design with modest controls, New and colleagues were able to offer a stronger argument for functional design. While sex differences may exist in spatial foraging ability, they are attenuated when their caloric value is higher, consistent with a mechanism that prioritizes the location of high-value food resources above low-value ones. This example shows how the use of multiple methods, within and outside the lab, can be used to provide converging evidence for or against evolutionary hypotheses.
Just as multiple functionally specific adaptations can independently solve problems that confront the organism, so too can they independently malfunction, to varying degrees. Much of our knowledge of neurology and neuroanatomy has come from studies that examine impairments associated with localized brain tissue damage. The fact that small amounts of damage can result in specific deficits or dissociations in cognitive ability is powerful evidence for specialized design in the brain. Prosopagnosia, for instance, is a specific dissociation that impairs the facial recognition system, rendering patients unable to recognize people they know from facial features alone, while leaving relatively intact social knowledge of those individuals and the ability to identify them through other cues such as voice, hair, and gait. However, there is debate regarding the exact nature of the mechanisms impaired in prosopagnosia, with some arguing that they are evolutionarily specialized for face recognition, and others arguing that they are specialized for developing expertise in object recognition, with faces as a special developmental case (Gauthier and Nelson, 2001; Kanwisher, 2010). This debate illustrates an important feature of domain specificity. Mechanisms are not either ‘domain specific’ or ‘domain general,’ but rather, can be described in terms of the nature of their domain: faces and objects are both domains, albeit different ones.
One mistaken, though widely held, belief about psychological adaptations is that they cannot be shaped by developmental input: that they must be present at birth and develop identically across individuals (i.e., ‘innate’ in a narrow sense). Instead, virtually all adaptations exhibit some degree of plasticity, though they vary in the nature and degree of that plasticity. Thus, hypotheses about psychological adaptations should include descriptions of how the mechanism in question is proposed to develop. This would include age trajectories of development, as well as hypotheses about how the mechanism in question might vary across developmental environments. For example, experiments with infants suggest that some mechanisms, such as mechanisms underlying the perception of objects, are thought to develop early in childhood, and similarly across children (Spelke, 1990). Other mechanisms, such as mechanisms shaping notions of fairness and moral judgment, appear to develop later, and can vary significantly depending on the culture in which the child is raised (House et al., 2013). This suggests that controlled studies across ages and populations are necessary to test hypotheses about the adaptive nature of cognition.
Cultural variation in cognition does not necessarily imply that adaptations are not at work. Even physiological and morphological traits that have been clearly shaped by natural selection, such as the immune system and the heart, vary across individuals. In the case of cognitive mechanisms, learning and other forms of external input are likely to play a role, in addition to – especially in the case of humans – culture. Thus, the existence or lack of variation does not by itself rule in or rule out the possibility that an aspect of cognition has been shaped by natural selection. Instead, theories of cognitive adaptations should include hypotheses about how and whether variation in the trait is expected to be manifest.
For example, there seems little doubt that spatial cognition in humans and other animals has been shaped by natural selection. In some cases, highly specialized spatial navigation systems are known to exist, such as dead reckoning in ants (Gallistel, 1990) and celestial navigation in indigo buntings (Emlen, 1975). Even in these cases, however, environmental input and learning play a key role: for example, the bunting celestial navigation system is calibrated through repeated exposure to the night sky during the course of development. In the case of humans, cross-cultural studies have shown that language is likely to play an important role in shaping some aspects of spatial cognition. For example, speakers of languages that customarily use absolute instead of body-centered frames of reference (equivalent to using, e.g., ‘north’ versus ‘south’ instead of ‘left’ and ‘right’) show greater awareness of their orientation relative to environmental landmarks and frames of reference (Levinson, 2003). This cross-cultural variation does not suggest that human spatial cognition is not the product of adaptations; it does, however, suggest that these adaptations are shaped by linguistic and/or other experiential input. More broadly, dimensions of cross-cultural variation, or lack thereof, are important sources of evidence for testing specific hypotheses about the features of psychological adaptations.
As Darwin recognized, adaptations evolve through descent with modification, and can therefore exhibit homologies, or similarities across species as a result of descent from common ancestors. On the other hand, specific taxa can have derived traits or traits that are unique to the taxon (and therefore do not have homologs in other taxa). And finally, distantly related taxa may share analogous versions of traits – traits that are similar – not due to common ancestry, but due to convergent evolution to solve similar adaptive problems. Because of this, the comparative method – comparing traits across species – is an important source of evidence for testing hypotheses about psychological adaptations, including hypotheses about their functions and evolutionary origins. Among the earliest cross-species comparisons were Darwin’s analyses of similarities and differences in emotional expression across species.
An example of recent comparative work on psychological mechanisms is research examining the nature of prosociality, or cooperation, in humans and other great apes. Key to this work are methods that can be compared across taxa, such as simple choices of how to allocate resources between oneself and a social partner (Silk et al., 2005). This work provides evidence for both similarities and differences in prosocial decision-making between humans and our closest relatives, suggesting some homologies and other derived features in humans. For example, humans and other primates respond prosocially to some signs of need, but only humans perform altruistic, prosocial acts in the absence of explicit signals of need from another party, suggesting a difference in the mechanisms underlying cooperation (Jaeggi et al., 2010).
Genes, Fossils, and Other Sources of Evidence
Finally, new technologies are beginning to provide data about cognitive evolution from novel sources. Genetic studies, for example, are beginning to provide evidence about genes that have diverged in humans and chimpanzees, and genes that have been under recent selection in our lineage. These include genes potentially related to language, motor control, brain connectivity, and other aspects of brain development. Studies of gene expression in humans and other animals also potentially provide a window into the nature and development of cognitive mechanisms, with a variety of studies showing substantial divergence in how genes are expressed in the brains of humans and other apes (Enard et al., 2002). Finally, paleoanthropological and paleoarchaeological evidence potentially bear on hypotheses about human cognition. For example, anatomical evidence has been used to examine hypotheses about the origins of language in humans, and data from stone tools have been used to evaluate hypotheses about the cognitive mechanisms of early toolmakers, such as their working memory and spatial cognition (Wynn and Coolidge, 2004). Given the speed of technological change, it is likely that even more previously unknown sources of evidence will be brought to bear on our understanding of cognitive evolution.
A Case Study: Theory of Mind
Few cognitive adaptations have been investigated using all or most of the potential sources of evidence described above. For example, there are many claims about human universals that are supported by evidence from only a small and perhaps nonrepresentative sample of human societies. Ideally, multiple and converging sources of evidence are necessary in order to be confident about the nature of the evolved mechanisms underlying any particular aspect of cognition.
A useful case study for how diverse sources of evidence can be brought to bear in understanding the evolution of a cognitive ability is the case of ‘ Theory of Mind,’ also variously known as mentalizing, mind reading, folk psychology, and psychological reasoning. Broadly, these terms refer to the ability to represent and make inferences about, and decisions based on, the mental states of others, such as their goals, desires, beliefs, intentions, attitudes, and motivations. While some theorists regard Theory of Mind as an ‘all or none’ capacity – you either have it or you don’t – many researchers regard Theory of Mind as a continuum of abilities, distributed in different forms and combinations across species. Many agree that humans have particularly well-developed and elaborate Theory of Mind abilities, which develop early in childhood and are deployed across a range of contexts, from cooperation, to communication, to social learning.
A variety of sources of evidence have been used to examine how Theory of Mind is distributed across species, how it develops, how it operates on the neural level, and what happens when it is impaired. In humans, hundreds of studies have been conducted using the false belief task, a task designed to measure the ability to represent another’s belief that is different from one’s own (i.e., a false belief). The majority of studies have used so-called elicited false belief tasks in which children are asked to report about another’s beliefs or behavior, and these generally show an age of development around four years, with substantial cross-cultural variation (Wellman et al., 2001). More recently, ‘spontaneous’ response tasks have shown that tracking of others’ beliefs develops much earlier, and shows a similar developmental trajectory in early childhood across cultures (Barrett et al., 2013). Across species, no other species has been shown to track false beliefs, but our closest relatives, chimpanzees, represent the knowledge or ignorance of others and use it to predict their behavior (Call and Tomasello, 2008). Moreover, other mind-reading skills, such as the ability to track gaze, to understand when one has been seen, and to understand communicative intent, have been found in species ranging from scrub jays to domesticated dogs (Dally et al., 2006; Hare et al., 2002). Importantly, this work shows that the mind-reading skills of a species tend to be correlated with the adaptive problems facing the species, such as avoiding food theft (Dally et al., 2006). In humans, brain mapping studies have begun to reveal the neural substrates underlying Theory of Mind (Saxe and Powell, 2006). Impairment of mind-reading abilities, for example in autism, have been used to test hypotheses about the specificity of the underlying mechanisms (Baron-Cohen, 1995). In all, these studies converge on the conclusion that Theory of Mind is a specialized ability, produced by psychological adaptations that exhibit characteristic developmental timing and that are distributed across species in ways that make sense, given the social ecology of the species. While Theory of Mind is perhaps atypical in the diversity and breadth of techniques that have been used to study it, it provides a good example of how converging work across many fields can be used to understand the evolution of cognitive mechanisms.
There can be little doubt that the cognitive abilities of all species are products of the evolutionary process. They are the products of neural systems that have evolved through descent with modification, shaped by the success or failure of their information-processing properties over evolutionary time. The question, then, is not whether cognition is the product of evolution. Instead, we should ask how the mechanisms giving rise to a species’ cognitive abilities have been shaped by the evolutionary process, how those mechanisms develop, and how they interact with the species’ environment to produce the behaviors we observe.
In many ways, the study of cognitive evolution is still in its infancy. Contemporary psychology is only a little more than a century old, and evolutionary theory is only slightly older than that. The blending of the two, although it began in the work of Darwin, has only recently begun to take shape as a mainstream form of scientific inquiry. As a result, there is as yet no standard toolkit of methods for studying the evolution of cognitive mechanisms, but rather a diverse assortment of approaches drawn from the many branches of the biological and behavioral sciences. However, with the rise in popularity of evolutionary psychology and related fields, the possibility of a mature science of cognitive evolution is now in sight. Our map of the brain’s evolved mechanisms still contains many blank spots, but the development of new and innovative ways of testing hypotheses about psychological adaptations suggests that much more will be known about the evolutionary basis of human cognition and that of other species, in the coming decades.
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