When I was growing up in New York City, a high point of my calendar was the annual arrival of the Ringling Brothers and Barnum & Bailey Circus — ‘the greatest show on earth’. My parents endured the green-haired clowns, sequinned acrobats and festooned elephants as a kind of garish pageantry. For me, though, it was a spectacular interruption of humdrum reality – a world of wonder, in that trite but telling phrase.

Wonder is sometimes said to be a childish emotion, one that we grow out of. But that is surely wrong. As adults, we might experience it when gaping at grand vistas. I was dumbstruck when I first saw a sunset over the Serengeti. We also experience wonder when we discover extraordinary facts. I was enthralled to learn that, when arranged in a line, the neurons in a human brain would stretch the 700 miles from London to Berlin. But why? What purpose could this wide-eyed, slack-jawed feeling serve? It’s difficult to determine the biological function of any affect, but whatever it evolved for (and I’ll come to that), wonder might be humanity’s most important emotion.

First, let’s be clear what we’re talking about. My favourite definition of wonder comes from the 18th-century Scottish moral philosopher Adam Smith, better known for first articulating the tenets of capitalism. He wrote that wonder arises ‘when something quite new and singular is presented… [and] memory cannot, from all its stores, cast up any image that nearly resembles this strange appearance’. Smith associated this quality of experience with a distinctive bodily feeling — ‘that staring, and sometimes that rolling of the eyes, that suspension of the breath, and that swelling of the heart’.

These bodily symptoms point to three dimensions that might in fact be essential components of wonder. The first is sensory: wondrous things engage our senses — we stare and widen our eyes. The second is cognitive: such things are perplexing because we cannot rely on past experience to comprehend them. This leads to a suspension of breath, akin to the freezing response that kicks in when we are startled: we gasp and say ‘Wow!’ Finally, wonder has a dimension that can be described as spiritual: we look upwards in veneration; hence Smith’s invocation of the swelling heart.

English contains many words related to this multifarious emotion. At the mild end of the spectrum, we talk about things being marvellous. More intense episodes might be described as stunning or astonishing. At the extreme, we find experiences of awe and the sublime. These terms seem to refer to the same affect at different levels of intensity, just as anger progresses from mild irritation to violent fury, and sadness ranges from wistfulness to abject despair.

Smith’s analysis appears in his History of Astronomy (1795). In that underappreciated work, he proposed that wonder is crucial for science. Astronomers, for instance, are moved by it to investigate the night sky. He might have picked up this idea from the French philosopher René Descartes, who in his Discourse on the Method (1637) described wonder as the emotion that motivates scientists to investigate rainbows and other strange phenomena. In a similar spirit, Socrates said that philosophy begins in wonder: that wonder is what leads us to try to understand our world. In our own time, Richard Dawkins has portrayed wonder as a wellspring from which scientific inquiry begins. Animals simply act, seeking satiation, safety and sex. Humans reflect, seeking comprehension.

For a less flattering view, we turn to the 17th-century English philosopher Francis Bacon, the father of the scientific method. He called wonder ‘broken knowledge’ — a mystified incomprehension that science alone could cure. But this mischaracterises science and wonder alike. Scientists are spurred on by wonder, and they also produce wondrous theories. The paradoxes of quantum theory, the efficiency of the genome: these are spectacular. Knowledge does not abolish wonder; indeed, scientific discoveries are often more wondrous than the mysteries they unravel. Without science, we are stuck with the drab world of appearances. With it, we discover endless depths, more astounding that we could have imagined.

In this respect, science shares much with religion. Gods and monsters are wondrous things, recruited to explain life’s unknowns. Also, like science, religion has a striking capacity to make us feel simultaneously insignificant and elevated. Dacher Keltner, professor of psychology at the University of California, Berkeley, has found that awe, an intense form of wonder, makes people feel physically smaller than they are. It is no accident that places of worship often exaggerate these feelings. Temples have grand, looming columns, dazzling stained glass windows, vaulting ceilings, and intricately decorated surfaces. Rituals use song, dance, smell, and elaborate costumes to engage our senses in ways that are bewildering, overwhelming, and transcendent.

Wonder, then, unites science and religion, two of the greatest human institutions. Let’s bring in a third. Religion is the first context in which we find art. The Venus of Willendorf appears to be an idol, and animals on the walls of the Chauvet, Altamira and Lascaux caves are thought to have been used in shamanic rites, with participants travelling to imaginative netherworlds in trance-like states under the hypnotic flicker of torchlight. Up through the Renaissance, art primarily appeared in churches. When in the Middle Ages Giotto broke free from the constraints of Gothic painting, he did not produce secular art but a deeply spiritual vision, rendering divine personages more accessible by showing them in fleshy verisimilitude. His Scrovegni Chapel in Padua is like a jewel-box, exploding with figures who breathe, battle, weep, writhe, and rise from the dead to meet their God beneath an ethereal cobalt canopy. It is, in short, a wonder.

When art officially parted company from religion in the 18th century, some links remained. Artists began to be described as ‘creative’ individuals, whereas the power of creation had formerly been reserved for God alone. With the rise of the signature, artists could obtain cultlike status. A signature showed that this was no longer the product of an anonymous craftsman, and drew attention to the occult powers of the maker, who converted humble oils and pigments into objects of captivating beauty, and brought imaginary worlds to life. The cult of the signature is a recent phenomenon and yet, by promoting reverence for artists, it preserves an old link between beauty and sanctity.

Art, science and religion are all forms of excess; they transcend the practical ends of daily life

Art museums are a recent invention, too. During the Middle Ages, artworks appeared almost exclusively in religious contexts. After that, they began cropping up in private collections, called cabinets of curiosity (Wunderkammern, in German). These collections intermingled paintings and sculptures with other items deemed marvellous or miraculous: animal specimens, fossils, shells, feathers, exotic weapons, decorative books. Art was continuous with science — a human practice whose products could be compared to oddities found in the natural world.

This spirit dominated into the 19th century. The early acquisitions of the British Museum included everything from animal bones to Italian paintings. In a compendious book called The World of Wonders: A Record of Things Wonderful in Nature, Science, and Art (1883) we find entries on electric eels, luminous plants, volcanic eruptions, comets, salt mines, the Dead Sea, and dinosaur bones, casually interspersed with entries on Venetian glass, New Zealand wood carvings, and the tomb of Mausolus. The founder of the circus that I used to attend was the showman and charlatan P T Barnum, who took over the American Museum in New York in 1841. There he displayed portraits of famous personages, wax statues, and a scale model of Niagara Falls, at the same time introducing enthralled crowds to the ‘Siamese’ twins Chang and Eng Bunker, and a little person dubbed General Tom Thumb. The museum was advertised on luminous posters proclaiming ‘the greatest show on Earth’ — the same show that he would eventually take on the road with his travelling circus. Today, the link between circuses and museums might be hard to fathom, but at the time the connection would have seemed quite natural. As temples of wonder, museums were showcases for oddities: a fine portrait, a waxwork tableau and a biological aberration all had their place.

By the end of the century, however, science and art had parted company. Major cities began opening dedicated art museums, places where people could come to view paintings without the distraction of butterfly wings, bearded ladies and deformed animal foetuses in jars. Nowadays, we don’t think of museums as houses of curiosity, but they remain places of wonder. They are shrines for art, where we go to be amazed.

Atheist that I am, it took some time for me to realise that I am a spiritual person. I regularly go to museums to stand in mute reverence before the artworks that I admire. Recently, I have been conducting psychological studies with Angelika Seidel, my collaborator at the City University of New York (CUNY), to explore this kind of emotional spell.

We told test subjects to imagine that the Mona Lisa was destroyed in a fire, but that there happened to be a perfect copy that even experts couldn’t tell from the original. If they could see just one or the other, would they rather see the ashes of the original Mona Lisa or a perfect duplicate? Eighty per cent of our respondents chose the ashes: apparently we disvalue copies and attribute almost magical significance to originals. In another study, we hung reproductions of paintings on a wall and told test subjects either that they were works by famous artists or that they were forgeries. The very same paintings appeared physically larger when attributed to famous artists. We also found that pictures look better and more wondrous when they are placed high on a wall: when we have to look up at an artwork, it impresses us more.

In the mid-18th century, the philosopher Edmund Burke hypothesised a connection between aesthetics and fear. In a similar vein, the poet Rainer Maria Rilke proclaimed: ‘beauty is nothing but the beginning of terror’. To put this association to the test, I, together with Kendall Eskine and Natalie Kacinik, psychologists at CUNY, recently conducted another experiment. First, we scared a subset of our respondents by showing them a startling film in which a zombie jumps out on a seemingly peaceful country road. Then we asked all of our subjects to evaluate some abstract, geometric paintings by El Lissitzky. Those subjects who had been startled found the paintings more stirring, inspiring, interesting, and moving. This link between art and fear relates to the spiritual dimension of wonder. Just as people report fear of God, great art can be overwhelming. It stops us in our tracks and demands worshipful attention.

Bringing these threads together, we can see that science, religion and art are unified in wonder. Each engages our senses, elicits curiosity and instils reverence. Without wonder, it is hard to believe that we would engage in these distinctively human pursuits. Robert Fuller, professor of religious studies at Bradley University in Illinois, contends that it is ‘one of the principal human experiences that lead to belief in an unseen order’. In science, that invisible order might include microorganisms and the invisible laws of nature. In religion, we find supernatural powers and divine agents. Artists invent new ways of seeing that give us a fresh perspective on the world we inhabit.

Art, science and religion appear to be uniquely human institutions. This suggests that wonder has a bearing on human uniqueness as such, which in turn raises questions about its origins. Did wonder evolve? Are we the only creatures who experience it?

Descartes claimed that it was innate in human beings; in fact, he called it our most fundamental emotion. The pioneering environmentalist Rachel Carson also posited an inborn sense of wonder, one especially prevalent in children. An alternative possibility is that wonder is a natural by-product of more basic capacities, such as sensory attention, curiosity and respect, the last of which is crucial in social status hierarchies. Extraordinary things trigger all three of these responses at once, evoking the state we call wonder.

Other animals can experience it, too. The primatologist Jane Goodall was observing her chimpanzees in Gombe when she noticed a male chimp gesturing excitedly at a beautiful waterfall. He perched on a nearby rock and gaped at the flowing torrents of water for a good 10 minutes. Goodall and her team saw such responses on several occasions. She concluded that chimps have a sense of wonder, even speculating about a nascent form of spirituality in our simian cousins.

This leaves us with a puzzle. If wonder is found in all human beings and higher primates, why do science, art and religion appear to be recent developments in the history of our species? Anatomically modern humans have been around for 200,000 years, yet the earliest evidence for religious rituals appears about 70,000 years ago, in the Kalahari Desert, and the oldest cave paintings (at El Castillo in Spain) are only 40,000 years old. Science as we know it is much younger than that — perhaps only a few hundred years old. It is also noteworthy that these endeavours are not essential for survival, which means they probably aren’t direct products of natural selection. Art, science and religion are all forms of excess; they transcend the practical ends of daily life. Perhaps evolution never selected for wonder itself.

wonder is the accidental impetus behind our greatest achievements

And if wonder is shared beyond our own species, why don’t we find apes carpooling to church each Sunday? The answer is that the emotion alone is not sufficient. It imbues us with the sense of the extraordinary, but it takes considerable intellectual prowess and creativity to cope with extraordinary things by devising origin myths, conducting experiments and crafting artistic representations. Apes rarely innovate; their wonder is a dead-end street. So it was for our ancestors. For most of our history, humans travelled in small groups in constant search for subsistence, which left little opportunity to devise theories or create artworks. As we gained more control over our environment, resources increased, leading to larger group sizes, more permanent dwellings, leisure time, and a division of labour. Only then could wonder bear its fruit.

Art, science and religion reflect the cultural maturation of our species. Children at the circus are content to ogle at a spectacle. Adults might tire of it, craving wonders that are more profound, fertile, illuminating. For the mature mind, wondrous experience can be used to inspire a painting, a myth or a scientific hypothesis. These things take patience, and an audience equally eager to move beyond the initial state of bewilderment. The late arrival of the most human institutions suggests that our species took some time to reach this stage. We needed to master our environment enough to exceed the basic necessities of survival before we could make use of wonder.

If this story is right, wonder did not evolve for any purpose. It is, rather, a by-product of natural inclinations, and its great human derivatives are not inevitable. But wonder is the accidental impetus behind our greatest achievements. Art, science and religion are inventions for feeding the appetite that wonder excites in us. They also become sources of wonder in their own right, generating epicycles of boundless creativity and enduring inquiry. Each of these institutions allows us to transcend our animality by transporting us to hidden worlds. In harvesting the fruits of wonder, we came into our own as a species.