An Evolutionary Perspective on Aesthetics

Abstract

The emotional responses of organisms, which strongly influence how organisms respond to their environments, have evolved because, on average, they increased the survival and reproductive success of their bearers. Survival is enhanced by making better decisions about avoiding environmental hazards, where to find and how to choose food, places in which to live, and associates for various activities, including reproduction. Experimental studies have shown that humans have stronger responses to evolutionarily relevant hazards (snakes, large predators, spiders) than to the important modern day hazards (guns, frayed electrical wires), raising challenges for education about current dangers. The use of an evolutionary perspective in the design of spaces where people live and relax, has been shown to enhance people's sense of well-being and assist psychological restorative processes. Results from existing studies clearly demonstrate the power of an evolutionary approach to aesthetics. They strongly indicate that we have just begun to tap the potential of the approach.

An evolutionary perspective on aesthetics

Humans have strong emotional responses to living organisms and to natural and human-modified environments. Depending on the circumstances and the needs of an individual, nature may evoke a wide variety of feelings, including feelings of awe, respect, fear, loathing, longing, nostalgia, excitement, challenge, or belonging. These powerful emotions, which are the foundations of aesthetics, influence how we respond to nature and how we attempt to manipulate it. They may also help us understand why we care about elements of nature. Despite some popular current notions, there is a rich array of real things in our environment. Our minds (and those of all animals) have been designed by evolutionary processes to identify them, label them with words, understand their characteristics, and make predictions about their future behavior (Pinker, 1994, p. 154).

If one accepts that life on Earth has been molded by the long-term action of natural selection, it follows that the strong emotional responses of organisms evolved because they, on average, increased fitness; that is, they improved the survival and reproductive success of the individuals that expressed and acted on them. Some examples are obvious. For example, those of our ancestors who did not enjoy food and sex certainly passed fewer genes to subsequent generations than those who did enjoy and, hence seek out, food and sexual partners. Similarly, individuals who chose ecologically inferior environments in which to live should have contributed fewer genes to subsequent generations than individuals who made better habitat choices.

The health, survival, and reproductive success of our ancestors depended on their ability to obtain and use environmental information wisely. Evaluating and responding to environmental information is difficult because the rich array of information that impinges upon us exceeds our capacity to assimilate and synthesize it. Adaptive behavior requires selective attention to components of the environment that strongly influence fitness. In fact, the complex behavior of organisms would be impossible in the absence of neural filters that emphasize, de-emphasize, or eliminate components of information emanating from the environment. These filters operate at all stages in the receipt, transmission, and processing of incoming information, including the sense organs, transmission pathways, and central nervous circuits (Marler, 1961). In effect, these filters embody evolutionarily stored knowledge that enables us to construct hypotheses capable of describing and understanding the external world. Extensive experiments on artificial intelligence have clearly shown that the rapid and efficient learning of a language performed by nearly every human child is quite impossible in the absence of preformed neural structures (Dennett, 1995; Pinker, 1994).

Aesthetic responses are fundamental to the ways in which organisms know about and adapt to the world; they probably developed early during life's evolution (Marchett, 1998). The English word aesthetic is derived from the adjectival form of the Greek aisthanomai, - to perceive. Thus, "aesthetic pleasure" means literally "pleasure associated with or deriving from perception." The central evolutionary problem in aesthetics is to explain why powerful emotions are so strongly associated with the perception and recognition of certain kinds of objects, but not with other kinds.

The filters that regulate the input of, and response to, environmental information, also strongly influence what organisms learn and how readily they learn it (Seligman, 1970). In other words, evolution has predisposed organisms to develop associations or responses to certain objects or situations. An important corollary is that even though modern societies have greatly reduced both the real danger posed by the typical objects of fears and phobias, fear and avoidance responses persist because selection against those responses is weak (Ulrich, 1993; Wilson, 1984). Conversely, fears and phobias are slow to develop to dangerous objects that are recent arrivals in our environments.

What is Beauty?

Beauty, it is often said, is in the mind of the beholder. The phrase is usually taken to imply that concepts of beauty are fundamentally personal, idiosyncratic, and determined primarily by the culture in which an individual lives. Everyone accepts that culture and learning clearly exert strong influences on the ways human perceive and respond to environmental information, or that they have important impacts on the symbols we attach to natural objects (Appleton, 1990; Schama, 1995). But these culturally influenced response norms are the result of complex interactions between objects and the structure of the minds of the beholders. Interestingly, attempts to understand and interpret human aesthetic responses to environmental features without asking why they evolved have had very limited success, primarily because beauty is not an intrinsic property of the objects that we call beautiful. Rather, it is the product of interactions between traits of objects and the human nervous system that evolved so that objects we consider beautiful have properties that result in improved performance in some aspect of living if we respond positively to them (Appleton, 1975). Conversely, we regard as ugly, objects that should be avoided, modified, or destroyed. Thus, an evolutionary perspective suggests that concepts such as beauty and ugliness should be viewed from a functional rather than a structural perspective. In other words, it is profitable to study emotional responses by asking - "how did these responses help us solve problems?"

Aesthetic emotions are a major component of how humans solve problems, They determine how much and what kind of attention we pay to elements of our environments, the value we place on them, and what we try to do when we perceive them. In other words, aesthetic emotions are major drivers of human problem-solving behavior.

What are the problems to be solved?

Computer scientists have noted that computers are good at memorizing things but are poor at making decisions, whereas people are good at making decisions but have rather poor memories (Pinker, 1994). This difference should not be surprising. People designed computers to be able to store and manipulate vast quantities of data, but the minds of animals evolved not to store information but to use it to make decisions that enhance their survival chances. Effective decision-making requires highly selective storing of information, because the information impinging upon an individual includes both elements that are highly relevant to survival and reproductive success and others that are of little or no consequence. In addition, which components of incoming information are important varies with time, location, age, and the needs of the individual (Heerwagen & Orians, in press).

Although decisions vary in myriad details, the basic problems that people (and other animals) attempt to solve fall into four major categories: 1) Safety: protecting themselves from being injured or killed by other people or dangerous animals; avoiding parasites and disease-causing organisms; and protecting themselves from adverse physical conditions. 2) Acquiring sufficient quantity and quality of food. 3) Finding a place in which to live. 4) Choosing good associates for reproduction, foraging, protection, and gaining higher social status (Heerwagen & Orians, 1993, in press).

Information can usefully be classified according to which of these problems it is most relevant, while recognizing that considerable overlap exists. Environmental information can also be classified according the time frame over which the information is relevant. For the purposes of this article, time is most usefully divided into categories that correspond to the time frames over which decisions about them are made (Table 1). Some environmental information signals events of temporary significance - spotting a prey animal, a dangerous predator, or encountering an unfamiliar conspecific individual. Adaptive responses to that information must be quick. Information about seasonal changes is associated with shifts in the types and locations of resources that will be available in the near future. Responses to such information can, and should, be made deliberately. Responses to information about features of the environment, such as status of vegetation and courses of rivers, which change slowly over periods of years or decades, are likely to be infrequent. The same is true for responses to information about objects that persist relatively unchanged over many human lifetimes.

Aesthetics and Safety

Key ecological dangers faced by people include predators, conspecific individuals, and inanimate hazards (storms and geomorphological conditions). Fear is a normal reaction to a real or imagined situation that is perceived to be a threat to one's safety or well being. Fear is a mechanism for avoiding or coping with dangers in many animal species (Archer, 1979; Russell, 1979). In contrast, fears that are judged to be out of proportion to the situation, cannot be reasoned away, and are beyond voluntary control, are called phobias. An evolutionary perspective suggests that phobias may not be as irrational as generally believed.

The set of objects that evoke fearful responses (snakes, spiders, heights, closed spaces, open spaces, blood) appears to be the same in all industrialized societies for which data are available (Ulrich, 1993). Interestingly, these objects and situations are ones that have been associated with threatening situations throughout human evolutionary history. Precipitous cliffs are dangerous if approached too closely. Closed spaces offer few escape routes; people in open spaces are vulnerable to attacks by enemies and dangerous predators. Venomous and predatory animals have been significant sources of human injury and mortality for many millennia.

In addition, there is convincing evidence, especially from studies of twins, that genetic factors play major roles in the acquisition and retention of phobias (see Ulrich, 1993 for an overview of experimental results). Environmental psychologists in Scandinavia have carried out a series of imaginative experiments on the acquisition and retention of fears to natural fear-evoking objects (snakes, spiders), culturally generated fearful objects (hand guns, frayed electrical wires), and neutral stimuli (geometric patterns). The general finding of these experiments is that conditioned responses to natural fear-evoking objects are usually, but not always, acquired more quickly than responses to neutral stimuli (McNally, 1987). Conditioned responses to modern dangerous stimuli extinguish more quickly when no longer reinforced than responses to snakes and spiders (Cook, Hades, & Lang, 1986; Hugdahl & Karker, 1981). In addition, aversive responses to fear-relevant natural stimuli, but not aversive responses to fear-irrelevant stimuli, can be acquired merely by telling a person that a shock will be administered (Hugdahl, 1978). People also acquire much more persistent defensive reactions when watching an experimenter's reactions to fear-relevant stimuli than to fear-irrelevant stimuli.
Even more striking are the results of experiments, in which slides are displayed subliminally (15-30 milliseconds) before being "masked" by a slide of another stimulus or setting. Even though the subjects are not consciously aware of having seen the stimulus slide, presentations of natural settings that contain snakes or spiders elicit strong aversive/defensive reactions in non-phobic persons. If a previous conditioning has already occurred, a masked subliminal presentation is sufficient to elicit defensive responses to the feared stimulus (Öhman & Soares, 1994).

Avoiding hostile conspecifics. Throughout human history, other people have been more important than other species as sources of human mortality. Although members of one's own social group can and do pose dangers, individuals from other groups pose greater risks. Not surprisingly, fear of strangers is widespread and occurs cross-culturally (Smith, 1979). It appears among children as soon as independent movement is possible (Bronson,1972). In addition, children fear strange males more than they do strange females (Smith & Sloboda, 1986), which is not surprising given that children are much more likely to experience harm from males than from females, and from unrelated than from related males (Daly & Wilson, 1988).

Avoiding dangerous animals. Animals have posed dangers to people throughout human history. Dangerous animals may be large (large cats, dogs, hyenas, bears, and large herbivores - elephants, rhinoceroses, etc), small (poisonous snakes, scorpions, spiders) or tiny (disease-causing bacteria, fungi, and protists). Otherwise non-dangerous animals may be vectors of disease-causing organisms. Interestingly, although the dangers posed by large and small animals have been recognized for millennia, and figure prominently in human phobias (Wilson, 1984), the existence of microorganisms was unknown during most of human history. The fact that these organisms cause many human diseases was not discovered until the 20th Century. Consequently, we tend not to fear them and often fail to adapt our behavior to reduce the risks they pose.

Fear of small animals develops in children soon after they are capable of independent movement (Agras, Sylvester, & Oliveau, 1969) and fear of larger animals is common among children more than four years old (Muris, Merckelbach & Collaris, 1997). Fears of other animals, real or fanciful, are often associated with dark and unfamiliar places. The ghosts and witches of children's stories typically inhabit deep, dark forests. These environments are ones in which the risk of being surprised and attacked by dangerous animals is high, which may account for the strong associations (Russell, 1979)
Avoiding Physical Hazards. Physical hazards that have been significant forces throughout human evolutionary history include fire, deep and fast moving water, cliffs, and storms. Weather systems, earthquakes, and fires often appear suddenly and necessitate fast responses, such as finding shelter, getting help, or moving away from the hazard. Fears of stimuli associated with storms, such as loud noises and bright lights, appear during infancy (Scarr & Salapetek, 1972), and children aged 6 to 12 fear earthquakes, fire, thunder, lightning, and deep water (King, Hamilton, & Ollendick, 1988; Marks, 1989). Accounts of both perfect and less than perfect storms command high levels of attention among adults of all ages.

Foods, Foraging, and Aesthetics

Eating is essential for survival, but to eat animals must do something that they otherwise tend to avoid, namely voluntarily insert foreign objects into their bodies. Sampling objects to assess their palatability is risky because toxic materials and pathogens may be ingested. Therefore, a major task for all animals is to distinguish edible from inedible items as they search for food. Individuals of omnivorous species eat a large number of food types, and what is available varies geographically. Therefore, such species rarely have innate food-recognition abilities. Humans learn what is edible by sampling items in their environments and by observation and instruction. Observation and instruction have obvious value because the individuals being observed and offering instruction have survived many years of ingesting those substances.

Foraging animals make two major types of decisions: Which items are acceptable as food, and which acceptable items should actually eaten during a foraging bout. Aesthetic responses are especially prominent in the first of these decisions. The strong emotional responses associated with foods are not surprising because vomiting and diarrhea are the only available post-ingestion defenses. People accept or reject foods for complex reasons. Direct sensory responses - tastes good or tastes bad - are important, but they are only one component of acceptability. People also make decisions on the basis of anticipated consequences, both physiological and social, of eating a type of food. In addition, particular foods may acquire features that limit their acceptability to particular situations or cause them to be categorically rejected. Because of the variety of reasons that influence which foods are actually acceptable, human cuisines differ more than would be expected simply from knowing what edible resources are available in the environments in which social groups live (Rozin, 1996).

Nonetheless, a few generalizations have emerged. First, most items that evoke disgust are of animal origin. Plant parts rarely evoke disgust. On the other hand, "inappropriate" items are primarily vegetable in origin (Rozin & Fallon, 1981). Disgust may be an adaptation that deterred our ancestors from eating animal tissues, such as feces, rotting meat, and soft internal parts, all of which commonly harbor large numbers of potentially harmful microorganisms. Microorganisms have the ability to multiply rapidly, so there is no safe dose for ingesting them. Thus, disgust may be evolutionarily programmed intuitive microbiology that developed long before people knew that microorganisms existed, much less that they were the causes of diseases.

Perhaps the most interesting feature of human cuisines is the stability of the major spices and sauces that characterize them. Humans are remarkably conservative in their food habits and are typically reluctant to try new foods or to abandon familiar ones. Traditional flavorings are high-priority culinary items; immigrant groups go to great lengths and expense to procure them in foreign settings. The deliberate manipulation of food by adding ingredients that reliably alter its taste is a uniquely human behavior. No other animals are known to do so. Which flavorings are used probably evolved in relation to what was available in the environment, but once established, they are remarkably persistent. They probably signal that the food is safe.

Cross-cultural similarities in which spices are used have evolved because spices inhibit or kill food-spoiling microorganisms. The most widely used spices all have strong anti-microbial properties (Billing & Sherman, 1998); mixes of them, which are common in many traditional recipes, are even more powerful. Not surprisingly, given that disease-causing organisms are more abundant in tropical than in temperate regions, the proportion of traditional recipes containing anti-microbial spices is inversely correlated with latitude.

Finding a Place to Live - Habitat Selection

Habitat selection is a vital decision in the lives of all organisms. When selecting a habitat an organism responds as if it understood the significance of objects, sounds, and odors for its future survival and reproductive success. Initial responses typically are emotional feelings that lead to rejection, exploration, or a certain use of the environment. Because the strength of these responses is a key to immediate decisions about where to settle and what to do there, the emotional states habitats evoke should be positively correlated with the expected survival and reproductive success of an organism in them. That is, good habitats should evoke strong positive responses; poor habitats should evoke weak or even negative responses.

Three functional concepts - prospect, refuge, and hazard - have guided recent approaches to the role of aesthetics in habitat selection (Appleton, 1975). Prospect refers to the ability of an individual to gather information about an environment with which to evaluate its characteristics and decide how to use it. Environments high in prospect offer rich opportunities for evaluation; environments low in prospect offer fewer opportunities. Refuge refers to the degree to which an environment provides security from negative agents while the individual is exploring and gathering information. Hazard refers to the dangers to which an individual is exposed during information-gathering activities. These concepts have been applied primarily to the initial evaluation and exploration of unfamiliar environments, but, combined with more recent developments in evaluating environmental information, they are readily applied and extended to a rich array of circumstances in which people must solve problems.

Habitat selection has been used as a perspective for a number of studies on human aesthetic responses to landscape features (Heerwagen & Orians, 1993). Habitats occupied by humans during most of our evolutionary history rarely provided resources that were reliable long enough to support permanent occupation of sites. Frequent moves through the landscape were the rule even though traditional sites were often revisited. Because relatively few generations have passed since humans started to live in mechanized and urban environments, evolutionarily based response patterns of humans to landscapes are unlikely to have been substantially modified since the rise of industrialized, urban societies.

Responses to environmental cues vary with a person's age, social status, and physiological state. Nevertheless, indicators of the presence of food, water, shelter, and protection from predators generally evoke positive responses, whereas potential hazards, such as inclement weather, fire, dangerous predators, and barriers to movement, typically evoke negative responses. Although no direct evidence yet exists for genetic influences on these responses, a number of evolutionary hypotheses have generated predictions, some of which have been tested experimentally.

One approach is based on the fact that Homo sapiens evolved in African savannas and only recently has invaded other continents and ecosystems. Therefore, landscape features and tree shapes that are characteristic of high quality African savannas are expected to be especially attractive to humans today. This hypothesis has been tested by determining responses of people to tree shapes and by examining the features of "aesthetic environments," that is those environments, such as parks and gardens, that are designed to make them attractive (Orians, 1986).

The shapes of trees that dominate savannas are good predictors of the resource-providing capacities of those environments. Therefore, people evolved to find the shapes of trees prominent in environments that provided the highest quality resources more pleasing than shapes of trees that dominated poor quality habitats. Trees that grow in the highest quality African savannas have canopies that are broader than they are tall, trunks that bifurcate close to the ground, and layered canopies. College students in Australia, Brazil, Canada, Israel, Japan, and the United States preferred trees with broad spreading crowns over conical and columnar trees (Sommer & Summit, 1996).

The changes recommended by landscape architects to their prospective customers are another source of data on human responses to landscapes. Humphrey Repton, an eighteenth century British landscape architect, presented his clients with "before" and "after" drawings of their estates (Repton, 1907). Repton changed landscapes by creating more savanna-like scenes, increasing visual access and penetrability of closed woods, opening up distant views to the horizon (that is, he increased prospect), adding refuges and cues signaling ease of movement, and adding evidence of resource availability, particularly large mammals (Heerwagen & Orians, 1993).

Although a love of flowers is a pervasive human trait, it is not obvious why we should take flowers to hospitals, bring them to dinner parties and house warmings, and annually spend billions of dollars on them. Nor is it obvious, given that flowers did not evolve their forms and colors to attract us (we are not historically pollinators), why we should find them so esthetically attractive. An evolutionary perspective suggests that flowers evoke strong positive feelings because they have long been associated with food resources. Because flowers precede fruits, flowering plants provide excellent cues to timing and locations of future resources. In addition, flowers may attract animals that are potential human prey. In species-rich environments, paying attention to flowering plants may particularly enhance resource acquisition abilities in the future. Until the 19th Century, honey was the only natural source of sugar; bee-keeping is an ancient human enterprise.

No studies have investigated which traits of flowers evoke strong positive feelings, but the obvious changes produced in many species of flowers by artificial selection - increased size and duplication of floral parts - result in flowers similar to those that historically produced large nectar rewards. Additional research on human responses to, and manipulation of, flowers should prove highly productive of new insights.

Restorative Responses

If aesthetic responses evolved because they enabled people to better solve life's problems, exposure to high quality environments should be restorative; that is, it should reduce feelings of tension and stress. Stress reduction consistently emerges as one of the key benefits reported by users of wilderness areas. Restoration from stress is also reported as a key benefit from time spent in urban parks with savanna-like vegetation and water (Schroeder, 1989). Patients recovering from surgery in hospitals with either views of natural vegetation or simulated views that depict natural scenes with water, recover more rapidly and have less post-operative anxiety than patients with no access to natural views or who are presented with simulations of abstract designs. Many studies have shown that even a brief exposure to nature, real or via photographs, leads to positive emotional feelings, reductions in stress, and better performance on demanding tasks (see Ulrich, 1995 for a review). Clearly, the positive responses people have to nature have important implications for the design of work and living spaces and healthcare facilities that are just beginning to be implemented.

Aesthetics and Biodiversity.

Although people are strongly attracted to living organisms, it is less clear that the attractiveness of an environment is consistently positively correlated with number of species in it. On the one hand, for example, the most highly evolved garden traditions - European formal gardens and Japanese gardens - are based on just a few species of woody plants. Scenes of environments that contain a jumble of plants of many species receive low scores in psychological tests. Subjects report that they are too difficult to interpret; it is difficult to determine how to enter and use them (Kaplan & Kaplan, 1982).
On the other hand, people take great pleasure in finding as many species of birds as they can on a given day and from assembling "life lists" of species they have seen. Generally, seeing more species is better than seeing fewer species. Journeys of hundreds of even thousands of miles to see a rare species not on one's life list are a regular feature in today's mobile society. People are also powerfully attracted to the unusual - rare species, individuals outside the normal range of the species, or individuals present at unusual times of the year or in unusual habitats.

Because these familiar patterns of human behavior have been subjected to remarkably little formal study, we can only speculate about why they have evolved. It makes sense that environments with intermediate levels of biological complexity should be preferred over both simpler and more complex environments because the range of resources present in an environment and the ability to find and use those resources probably peaks at intermediate levels of complexity. Simple environments have too few resources; complex ones have so many that choosing among them becomes difficult. Developing a suitable classification system to guide responses to the components of complex environments may be especially difficult.

People may have evolved to respond to rare and unusual events because they provide new information about the state of the environment. Not all novel events are associated with something important, but it may be best to pay attention to them to find out if they are rather than to ignore them. Such events may indicate that current patterns of use of the environment should be altered.

Aesthetics and Social Interactions

Concepts of beauty are clearly multidimensional. They are associated with all sensory modes and they are varied within single sensory modes. What we consider beautiful in poetry and music is not the same thing. However, in all dimensions, feelings of beauty and ugliness strongly influence responses to objects and situations. In this section I explore concepts of beauty as they relate to our intraspecific social environment. As the structure and size of departments in modern universities attests, we are more concerned with our relationships with our fellow humans than we are with our relationships with all of Earth's millions of other species combined.

Beauty in Conspecifics. People are much preoccupied with assessing the beauty of other people. A rich body of literature demonstrates that people find symmetrical objects - especially faces - but also including abstract patterns, woody plants, and human bodies, aesthetically pleasing. Other vertebrates also are sensitive to very small asymmetries (see Møller and Swaddle 1997 for a review of the human and animal literature). Conversely, asymmetries typically evoke negative emotional responses, the most extreme of which is selective infanticide of asymmetrical newborn babies in many human cultures.

Positive responses to symmetry may be adaptive because symmetrical animals perform better physically than asymmetrical animals, just as symmetrical objects, such as bows, arrows, axes, boats, autos, and aircraft function better than asymmetrical ones. Asymmetrical individuals may have been exposed to stressful environments that disrupted normal development, thereby rendering them less functionally adequate. For example, alcoholic mothers give birth to children with greater developmental asymmetries than do mothers who consumed less alcohol. Thus, it is not surprising that which objects to use, the design of objects, and choices of social partners have evolved to favor use of, and association with, symmetrical objects.

In addition, asymmetrical individuals may have genetic defects that could be passed on to offspring. For example, humans suffering from trisomy-21 (Down's syndrome) have noticeable asymmetries due to skeletal abnormalities. Greater asymmetry in dental traits is associated with genetic disorders. Therefore, by avoiding asymmetrical individuals as mates people would be likely to improve the quality of their offspring.

Finally, because diseases can cause asymmetrical development, asymmetrical individuals may be avoided because they may currently harbor communicable disease-causing microorganisms. Good symmetry signals a long history of good health! Again, a positive aesthetic response to symmetry developed long before humans knew about microorganisms or the causes of diseases. This fact emphasizes the potential fitness value of aesthetic responses.

Beauty in Language. Language is a remarkable and unique human capability whose origins are uncertain. Many theories vie for primacy, but one thing is certain. Language evolved because it increased survival and reproductive success, and it continues to serve those functions today (Pinker, 1994). As many anthropologists have noted, tribal chiefs are often both gifted orators and highly polygynous. Verbally skilled individuals are better able to manipulate the behavior of their associates to their own advantage than are less skilled individuals. Why we find some utterances more beautiful than others, what constitutes beauty in poetry, and what makes some prose more beautiful than others are questions that should stimulate much future research.

Music. Every known human culture has music, and musical instruments have been recovered from deposits at least 50,000 years of age. Human music-making varies dramatically among cultures, but its universal presence suggests that there is a deep human need to create, perform, and listen to music. In addition, the ability to memorize, recognize and create musical patterns also characterizes the music-making of whales and birds. As in human society, musical traditions in these animals may be passed from parents to offspring, from adults to unrelated younger individuals, and between individuals of the same generation (Gray et al., 2001).

Similarities among species in the nature of music may have deep roots in the sounds that characterize most natural habitats. The ambient sounds in most environments resemble a modern-day orchestra in which the sound produced by each animal has its own frequency, amplitude, timbre, and duration. Many of the auditory pathways that we use to perceive music evolved in animals for communication, sound source identification, and locating objects. Rats and starlings distinguish between chords deemed to be consonant and dissonant by Western musical standards (Hulse, et al. 1995). Responses to musical form appear remarkably early in human infants. Thus, by 4 months of age, babies prefer consonant musical intervals (major and minor thirds) to dissonant musical intervals (minor seconds).

The prevalence of octaves and fifths in music from many different cultures is probably a consequence of the way that our ears and brains are built. The first three notes of any major triad correspond to the fourth, fifth, and sixth harmonics of any harmonic series. The basilar membrane of the human inner ear behaves like guitar strings of varying thickness. As a result, groups of sensory receptors along its length become activated in response to sounds of specific frequencies. The pattern of hair cell excitation is arranged as equal steps along the chromatic scale mapped out as equal distances along the basilar membrane. In the auditory nerve, which transmits information in the form of action potentials from the inner ear to the brain stem, the neural excitation map encodes the octave the triad is played in; the timing of neural activity may indicate the pitch of each note.

Whatever the neural mechanisms, the brain becomes highly active when we listen to music, and music stimulates powerful emotional and aesthetic responses in nearly all people. Thus, future investigations into relationship among the neurobiological foundations of the aesthetics of music have the potential provide insights into various aspects of the neural basis of perception, emotions and learning.

Closing Remarks

The central theme of this essay is that the evolution of aesthetic experiences has been molded by the positive role of emotional responses in decision-making and solving problems. Evolutionary biologists assume that both positive and negative responses to looking at particular sights or hearing particular sounds evolved because the consequences of having those emotions have been, on average, beneficial over evolutionary time. This concept is actually rather ancient. In 1785, seventy years before Darwin published the Origin of Species, the Scottish philosopher Thomas Reid wrote: "By a careful examination of the objects to which Nature hath given this amiable quality [of Beauty] we may perhaps discover some real excellence in the object, or at least some valuable purpose that is served by the effect which it produces upon us. This instinctive sense of beauty, in different species of animals, may differ as much as the external sense of taste, and in each species be adapted to its manner of life."
Thomas Reid knew nothing about geological time or the theory of evolution by means of natural selection, but he nonetheless clearly grasped the role of aesthetics in human life. Since then, we have made relatively slow progress in exploring the rich implications of this perspective. The slowness of progress may be owing to our resistance to investigations into the foundations of our emotions. Evolutionary theory suggests that such resistance also may be adaptive, but that topic is beyond the scope of this essay. Nevertheless, it is abundantly clear that when an evolutionary approach to aesthetics has been adopted, the results have been informative, interesting, and sometimes counter-intuitive (Orians, 1998). Rich harvests await us as we continue to explore the power of an evolutionary approach to both deepen and broaden our understanding of aesthetics.

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Table 1. Classification of Information in Terms of the Time frame of its Relevance (after Orians 1998)