The answer to this riddle lies in one central fact: the new science unleashed by Priestley’s mint experiment took two centuries to evolve. What Priestley had hit upon was not a simple element, like oxygen, or a fundamental law, like gravity. It was, instead, a system, a flow of energy and molecular change. Priestley had a hand in filling out other key parts of the system as well. He connected the metabolic flow of plant respiration with the energy needs of animals in a paper published in 1776, “Observations on Respiration and the Use of the Blood.” Priestley’s argument was, naturally, couched in the language of phlogiston, but it was the first to suggest that there was some essential transfer of energy involved in the contact between air and blood in the lungs. In 1778, Priestley noticed that some kind of “green matter” was spontaneously forming in glasses of pump water; he noted suggestively that it required sunlight to emerge, though for a time he denied that the mysterious substance was “vegetable” in nature. The Dutch biologist Jan Ingenhousz would turn Priestley’s speculations into a more rigorous proof of the energy transfer of photosynthesis, showing that a single leaf was capable of transforming sunlight into breathable air. By the end of the decade, Priestley had helped sketch out the first draft of the cycle of life on Earth: plants convert the energy of light into chemical energy, releasing oxygen into the atmosphere and absorbing carbon dioxide; animals power themselves through the energy stored in plant tissue and oxygen itself, releasing carbon dioxide as a waste product. Priestley never presented these insights as a unified system in his published work, and of course the confusion about phlogiston undermined his thinking at several key points. But in the years to come, the connective power of that system became more and more visible. In the middle of the next century, the German chemist Justus von Liebig shed important light on the nutrient cycles that drive all dynamic ecosystems. The revolutions in microbiology at the end of the nineteenth century suggested for the first time the productive role that bacteria might play in breaking down organic compounds for further reuse. The first detailed analysis of food webs—documenting the flow of energy through a population of plants, animals, and microorganisms—were sketched out by the British zoologist Charles Elton in the 1920s. Yet the word “ecosystem” wasn’t even coined until the 1930s, when an Oxford botanist named Arthur Tansley asked a colleague to come up with a name for the complex interactions between organisms and their physical environments.
Like the hunch that helped bring it about, the fundamental premise behind Priestley’s mint experiment took a long time to bloom into a mature science—almost two centuries, in fact. Part of that slow evolution can be attributed to the consilient structure of ecosystem science: it is a discipline that by nature is built out of the layered interactions between multiple fields of expertise, each operating on distinct scales. For the ecosystem to work as a practical object of study, you need microbiologists to explain the role of bacteria in decomposition; geologists and chemists to explain the chemical weathering of rocks; molecular biologists to explain the energy transactions of mitochondria; zoologists and botanists to identify the food webs that form between species; climatologists and atmospheric physicists to analyze the global weather patterns that shape every ecosystem on the planet. Priestley had laid the cornerstone for that amazing body of knowledge, but the building itself didn’t become visible for a hundred and fifty years.
And so the legend of Priestley the scientist accumulated around his troubled discovery of oxygen, because the discovery was contemporaneous with the science it helped inaugurate. By the 1780s, the “chemical revolution” was in full swing, ignited in large part by Lavoisier’s Méthode de nomenclature chimique, published in 1787, the founding text of modern chemistry, which established for the first time a standard nomenclature and classification system for the core elements, such as oxygen, nitrogen, mercury, and hydrogen. A new science needs its origin stories, and Priestley had undeniably been there at the beginning. Despite the Copley Medal and Franklin’s enthusiasm, the mint in the glass faded into the background. In time, it would mark the origin of a new science, too, but by that point Joseph Priestley had entered the pantheon as the discoverer of oxygen, albeit one with an asterisk.
IF FRANKLIN had played a deciding role in Priestley’s Leeds experiments, forces conspired to make him but a distant observer of his friend’s discovery of dephlogisticated air. In late 1772, a member of Parliament secretly passed Franklin a packet of letters written by the Massachusetts governor, Thomas Hutchinson. The letters talked openly of restricting “English liberties” in the colonies in order to suppress the growing rebellion. Franklin sent them back to his friend Thomas Cushing in Boston—apparently thinking that somehow they would help ease tensions with England. Instead, the letters were published to much outrage among the colonists. As the American protests grew in intensity, there was fierce speculation in London over who had sent the Hutchinson letters back to Boston. Franklin ultimately revealed that he had been the culprit, and in early 1774, he was denounced in front of the Privy Council, in the famous Cockpit chamber in Whitehall. The solicitor-general Alexander Wedderburn claimed Franklin’s duplicity had “forfeited all the respect of societies and men,” his mind “possessed by the idea of a Great American Republic.” There were catcalls from the audience against Franklin, but he had allies in the room as well: Priestley was in attendance, watching his fellow Honest Whig suffer perhaps the greatest public humiliation of his life. The Cockpit was so crowded that Priestley was forced to stand the entire session, next to their mutual friend Edmund Burke. (Ironically, when the political furies turned against Priestley fifteen years later, Burke would be one of Priestley’s key antagonists.) Along with Lord Shelburne and a young Jeremy Bentham, they formed a small band of Franklin supporters in an otherwise hostile crowd. They were, to a man, appalled by the ferocity of Wedderburn’s attack. Shelburne later called it “scurrilous invective.” After the session ended, Wedderburn approached Priestley in the antechamber to extend his greetings; Priestley turned his back on the solicitor-general and immediately marched out to the street in protest.
The next morning, Franklin had breakfast with Priestley and insisted that he had no regrets about his actions. Hours later, a note arrived, informing Franklin that he had been stripped of his cherished position as postmaster general for his role in inciting the colonial uprising—as well as for circulating purloined letters, something of a faux pas for the postmaster general. Fearing arrest or the wrath of an angry mob of patriots, Franklin took a boat downriver to a friend’s house in Chelsea, where he kept out of the public eye for a few weeks. He threatened to leave England for good, but ended up staying for another year, despite an almost complete end to his dealings with the ministry. During that summer, which Priestley spent in Wiltshire with Lord Shelburne, the surviving correspondence between Franklin and Priestley drops down to one or two letters, and there is no record of Priestley sharing his early investigations into pure air with his longtime collaborator.
The winter of 1774-75 gave Priestley and Franklin one last opportunity to revive their close relationship. The Priestleys moved to London for the winter, and he and Franklin dined together nearly every night, often in the company of the Honest Whigs. The shoptalk had turned almost exclusively to politics by that point, given the disintegrating bond between England and her transatlantic colonies. It is not stretching matters to suggest that Priestley’s thinking suffered from the political distractions of his old friend. Two years before, after witnessing the mint growing in the jar, Franklin had suggested in a letter that “the air is mended by taking something from it, and not by adding to it.” Taking, not adding. If only Franklin had suggested a parallel hunch to Priestley in 1774, and steered him clear of phlogiston’s magnetic pull, the chemical revolution might have played out quite differently.
By March of 1775, Franklin realized that the time had finally come to pledge his allegiance to his native land, and he booked passage on a packet ship leaving Portsmouth on March 11, bound for Phil
adelphia. He spent the entirety of his final day in London—the last he would spend there as a subject of the British crown—with his friend Joseph Priestley. They read through a package of American newspapers that had just arrived, surveying the colonial responses to the Boston Port Act, which had established a blockade against the Boston harbor as a reply to the Tea Party. “As he read the addresses to the inhabitants of Boston,” Priestley later recalled, “. . . the tears trickled down his cheeks.”
A year later, when Priestley published the first edition of his Observations and Experiments on Different Kinds of Air, the defining account of his golden years as an experimental scientist, he would invoke the loss of his great friend in the opening pages.
The greatest success in [politics] seldom extends farther than one particular country, and one particular age; whereas a successful pursuit of science makes a man the benefactor of all mankind, and every age.
Then he quoted a private letter from the Italian philosopher Francesco Beccaria:
Mi spiace che il mondo politico, ch’è pur tanto passeggero, rubi il grande Franklin al mondo della natura, che non sa né cambiare, né mancare.
[I am sorry that the political world, which is so very transitory, should take the great Franklin from the world of nature, which can never change, or fail.]
Priestley couldn’t have known it then, transcribing those words in the safety of his lab in Calne or Berkeley Square, but in time, the world of politics would take him as well.
THE CARBONIFEROUS ERA
CHAPTER THREE
Intermezzo: An Island of Coal
300 Million B.C.
Pangaea
IN 1877, THE FRENCH PALEONTOLOGIST CHARLES Brongniart began excavating fossils from the coal measures near the town of Commentry, in central France. It was a promising spot for a fossil hunt; originally a lake several miles long, the site had been bordered by a marshland where streams from the surrounding hills drained down, depositing plants and insect life in the swamp. Brongniart was only eighteen at the time he discovered the site, and he would work the Commentry quarry for almost twenty years, nearly the entire span of his adult life. (He died at the age of forty.) During that period he unearthed a spectacular array of fossils, most of them dating back 300 million years, older than the first dinosaurs.
Many of the fossils that Brongniart uncovered shared a defining characteristic: compared to their modern equivalents, they were massive. He discovered ferns the size of oak trees, and flies as big as birds. In 1880 he unearthed his most startling find: a monster dragonfly with a wingspan of 63 centimeters. Brongniart named it Meganeura in the paper he published about his discovery in 1894. The original fossil can be seen today in the Museum of Natural History in Paris. Subsequent fossils have been discovered with a wingspan of more than 75 centimeters.
Meganeura was not alone. Paleontologists worldwide soon discovered that giantism was a prevailing trend between 350 and 300 million B.C., a period now called the Carboniferous era. Like some strange Brobdingnagian natural history exhibit, the landscape of the Carboniferous was populated by foot-long spiders and millipedes, and water scorpions the size of a small boy. The plant life was even more spectacular. Club mosses growing in damp forests towered above the swampland below, reaching heights of 130 feet, five hundred times taller than their modern descendants. Horsetails and rushes that now top out at around four feet regularly reached the height of a five-story building. Early conifers sprouted leaves that were three feet long.
The planetary fad for giantism didn’t last. The dinosaurs evolved immense body plans in the coming ages, of course, but by 250 million B.C. the rest of the biosphere had largely retreated back to the scale that we now see on Earth. But that pattern was distinct enough that it presented a tantalizing mystery: just as the Cambrian explosion raised the question of why life suddenly grew so diverse, the Carboniferous age raised the question of why life suddenly grew so big, and how it managed to survive with such exaggerated proportions. Meganeura shouldn’t have been able to fly, given its size. The respiratory systems of modern insects and reptiles wouldn’t be able to generate enough energy to support a body plan that was ten times their current size. And yet somehow the giants of the Carboniferous managed to thrive in that exaggerated state for a hundred million years.
ALMOST EXACTLY two centuries after Priestley first explained the mystery of breathable air, scientists began to piece together the puzzle of Meganeura, and when they did, the process that Joseph Priestley had first observed in his Leeds laboratory turned out to be central to the story. The giants of the Carboniferous illuminate the enduring power of Priestley’s original mint experiment, the long flame of associations and insights that came out of that original spark.
Priestley and Franklin’s hunch that plant life was central to the planet’s production of breathable air first approached scientific consensus in the late 1960s, after two physicists, Lloyd Berkner and Lauriston Marshall, proposed in a seminal paper that the vast majority of atmospheric oxygen originated in photosynthesis. The “natural” level of oxygen on Earth was less than 1 percent; the 20.7 percent levels we enjoy as respiring mammals was an artificial state, engineered by the evolutionary breakthrough that began with cyanobacteria billions of years ago. The scarcity of oxygen before the evolution of plant life suggested one logical follow-up question: Why had oxygen levels stabilized at around 20 percent for so many millions of years? It is easy to imagine that number fluctuating more dramatically over evolutionary time: were it to drop to 10 percent, most of aerobic life would suffocate; were it to double, the combustion reactions of oxygen would engulf the planet in a worldwide inferno. So what mechanism allowed the atmosphere to regulate itself with such precision, like some kind of emergent global thermostat keeping the planet in its oxygen comfort zone?
That knack for self-regulation—also known as homeostasis—was the driving question that led James Lovelock and Lynn Margulis in the early 1970s to formulate their famous and endlessly debated Gaia Hypothesis, in which the two argued that “early after life began, it acquired control of the planetary environment and that this homestasis by and for the biosphere has persisted ever since.” That control system sought an “optimal physical and chemical environment for life on this planet.”
Lovelock and Margulis began the first significant paper they published on Gaia with the story of oxygen’s miraculous stability. They gave the paper the provocative title “Atmospheric Homeostasis By and For the Biosphere.” By and for: these were fighting prepositions. Not only had the planet achieved some kind of sustained atmospheric balancing act, with oxygen levels maintained at optimal levels for its present biosphere, but that biosphere had somehow collectively been responsible for it, acting in its own self-interest. We accept the premise that organisms have comparable purposes in the systems that collectively keep them at homeostatic norms: our bodies stay marvelously calibrated at 98.6 degrees for a reason, and that reason is that our particular mode of staying alive is optimized for that temperature. That is one of the defining characteristics of what it means to be an organism: a system of cells and organs that are explicitly devoted to ensuring the survival of the larger group to which they belong. Each works, in the language of the original Gaia paper, as “a contrivance specifically constituted for a set of purposes.” The cells that help pump blood through our bodies go to elaborate lengths to keep blood-pressure levels at an equilibrium, because stable blood pressure is important to the survival of the organism. Lovelock and Margulis saw the same principle at work on a planetary scale: the Earth itself could be seen as a single organism, with the collective behavior of every member of the biosphere contributing to its survival. It was a variation on Sir John Pringle’s “no vegetable grows in vain” homily, with mankind replaced by Mother Earth. The biosphere regulates O2 levels, and it does it for a reason: because stable O2 levels are good for the biosphere.
A thousand holes have been punched in the Gaia Hypothesis in the three decades since Lovelock and M
argulis first proposed it, and it remains an open question whether the strong claim—the Earth is an organism—has empirical merit, or even utility as a metaphor. (Lovelock and Margulis have backtracked from some of their more provocative assertions, while at the same time defending their central premise.) The weaker claim, that there are planetary systems that settle around stable states far from their “natural” equilibrium, and that life has a knack for evolving solutions that thrive in those conditions, is largely unchallenged. (It is the founding principle behind the Earth Systems perspective that we saw in the Bretherton diagram.) But wherever one falls on the spectrum of responses to Gaia, there is no contesting that it was one of the most electric and influential ideas of the late twentieth century.
One of the intriguing side effects of Gaia is that it helped trigger a multidisciplinary search to determine if oxygen levels had indeed been consistently locked in at 21 percent over the ages. In 1989, the geologists Robert Berner and Donald Canfield published a paper that described a “rock abundance” approach to measuring changing levels of oxygen in the atmosphere. By measuring the levels of carbon and sulfur in sedimentary rocks for each geological period—drawn largely from the extensive data compiled by oil companies seeking new deposits of fuel—Berner and Canfield were able to build a portrait of atmospheric oxygen dating back 600 million years. In general, Berner and Canfield’s model reinforced the Gaia story: oxygen levels had been relatively stable for the last 200 million years. But the most startling finding came before that long equilibrium. The data showed a dramatic spike in oxygen levels, reaching as high as 35 percent around 300 million B.C., followed by a plunge to the borderline asphyxia of 15 percent in the Triassic era, 100 million years later. The oxygen pulse overlapped exactly with Meganeura and the other giants of the Carboniferous.
The Invention of Air Page 9
