At the other end of the scale the Bumblebee Bat, found today in Thailand, has a wingspan about of 15cm (six inches) and weighs about 2 grams: as much as a penny. The world’s smallest bird, the Bee Hummingbird, weighs about 1.6 grams (0.056 ounces), and has a wingspan of just 3 cm (an inch and a quarter). This tiny being, known as zunzuncito or “little buzzing one” in its native Cuba, is extreme in other ways. When trying to impress a potential mate, a male will increase the frequency of his wing beats from about 80 per second in normal flight to over 200 per second – the fastest of any known bird. This effort drives the Bee Hummingbird to the highest body temperature of any known bird – about 40 °C (104 °F) – and it has to drink eight times its weight in water every day to keep cool.
The smallest known winged creature is a kind of parasitic wasp a little over a tenth of a millimeter, or 0.004 inches, long. For most beings the size of this tiny wasp or smaller, however, wings are an irrelevance because the fluid properties of air allow them to swim or float through it pretty much as we do through water. If, to borrow a thought experiment suggested by Richard Dawkins in one of his more psychedelic moments, you were to shrink a hippopotamus to a thousandth of its normal size it would float effortlessly on the breeze even without wings because its surface area would be so greatly increased relative to its mass. This is the best argument I have yet heard for keeping a constant watch for tiny flying hippos.
What else lives, unseen, in the skies? “We live submerged in the ocean of the air,” wrote Evangelista Torricelli, one of the first people to understand that air is a substance not an absence. It remains a visionary statement, but Toricelli, a student of Galileo and the inventor in 1643 of the barometer, would not have known that the air is full of tiny wanderers much smaller even than the tiny spiders that ride the wind on silken threads. Neither he nor his contemporaries would have begun to imagine a whole world of aerial beings too small to see: spores, algae, fungus and bacteria.
Antonie van Leeuwenhoek, the “father of microbiology”, who first looked through a microscope in 1648, the year after Torricelli died, only discovered microorganisms in 1674. (His first encounter was with “infusoria”, or what we would now call protists: basically, pond life.) But van Leeuwenhoek did not think of looking to see what might be floating in the skies. And while Robert Hooke had speculated, in the preface to his Micrographia of 1665, on air “impregnated with the steams and effluvia of several odorous Bodies”, he did not dwell on the idea. Only in the 19th century did scientists begin to observe that very small organisms travel a long way on the winds. In 1832 Charles Darwin noticed dust landing on the Beagle and deduced that it must have come from the African coast more than 500 kilometres (300 miles) away. He collected some, and found it contained several species of African freshwater algae. (Later, 100km off the coast of Argentina, he saw tiny red spiders suddenly appearing in the rigging of the Beagle.) It was not until the second half of the 20th century that the study of airborne microorganisms, or “aeroplankton”, became a systematic study.
A few bacteria and fungi venture as high as the mesosphere, some 70 kilometres (40 miles) up, where the atmosphere is about a billionth as dense as it is at ground level. But most aerial organisms are found much lower down, and recently it’s even been suggested that some bacteria not only survive but thrive and reproduce in the harsh, beautiful environment of clouds, “eating” the organic acids, alcohols, nitrogen and sulphur that are found there in trace amounts. It may even be that some bacteria help “seed” the clouds they inhabit. Clouds form when water droplets cling to tiny particles in the air such as dust and ash, and maybe water droplets cling to microbes too. The evidence is intriguing but not conclusive. It has been shown, however, that in heavily forested places like France and Montana (but not in treeless landscapes like Antarctica) snowflakes are mostly “seeded” by bacteria. In the Amazon rainforest, spores released by fungi may play a key role in cloud formation.
Microorganisms not only travel long distances in the air; some of them also help to make it, contributing a range of simple and complex molecules. In an old-growth forest, for example, the air is impregnated with more than a hundred different complex compounds created by the trees, which themselves would not thrive without microbial symbionts. The good, sweet smell of forest air may be one of the first things you experience in the woods, but few – especially in Western countries – have thought to pay close attention to everything that is in it. They do a little better in Japan, where shinrin-yoku, or “wood-air bathing”, is a well-established practice. Japanese researchers have found that the blood-sugar levels of diabetic patients walking through a forest fall to healthier levels, and the health benefits may extend much wider than that.
Other molecules made by living things are simpler and smaller than many of those made by the trees, but they are abundant on a planetary scale. Dimethyl sulphide, for example – a gas produced in the decomposition of marine algae, and said to give the sea its smell – is an important nucleus for cloud condensation. These clouds play a major role in weather and climate and so influence the conditions for life itself. At an even grander scale, photosynthesis by algae and plants keeps atmospheric oxygen concentrations at 21 per cent. If this stopped, the oxygen in the atmosphere would react with other elements until there was almost none left, and we would all suffocate while the planet rusted.
Every breath we take links us to the biogeochemical cycle. This is something to meditate on, and sometimes from strange angles. The science writer Oliver Morton suggests, for example, that every now and then we should bend down and look at a large tree upside down. Seen that way, he says, something essential of the tree’s true nature is more readily apparent: it is at least as much a being rooted in the sky – with which, powered by sunlight, it exchanges carbon dioxide and oxygen – as it is one rooted in the earth.
We are not so different from trees. Unlike the ancestors of pterosaurs such as the Quetzalcoatlus, we are most unlikely ever to evolve physical bodies into forms suitable for true flight, but we can breathe deeply of the ocean of the air: a physical link to its wonders that is more fundamental to our wellbeing than anything we do with our machines or even with our imaginations.
Caspar Henderson's The Book of Barely Imagined Beasts is published by Granta.