In this sense, Oparin was continuing a line of dawning awareness of life's elemental nature begun a half century earlier. In 1869, when, in retrospect, one might be surprised that even the chambermaids and coachmen weren't debating the origin of life, Darwin's British bulldog, Thomas H. Huxley, president of the Geological Society of London, professor of natural history in the Royal College of Surgeons, and renowned orator, published an influential article that made just the above point. Titled “On the Physical Basis of Life,” it argued that the nature of every living being is in fact the expression of what could be called the four great elements of life—carbon, nitrogen, oxygen, and hydrogen. What we call “life” isn't some mysterious force vital, Huxley said, but rather is the synergistic result of a myriad of combinations of these and other less abundant elements. Huxley made the point that when oxygen and hydrogen combine to form water, “we do not hesitate to believe that, in some way or another, [water's properties] result from the properties of the component element of water. We do not assume that something called ‘aquosity’ entered into it.”
Though the popular notion is often that life is something fundamentally apart from the rest of the universe, when taken to its atomic essence, we, and all other life, from E. coli to elephants, are not a random, ad hoc, or odd assemblage of atoms but rather standard issue. It's a realization that triggered intellectual alarms for the late nineteenth-century astronomer William Huggins, the first person to document stars’ elemental compositions. When he looked at his spectra, identifying what the stars are made of, he saw not foreign worlds but his own. “It is remarkable that the elements most widely diffused through the host of stars are some of the most closely connected with the constitution of the living organisms of our globe, including hydrogen, sodium, magnesium, and iron,” he reflected. Parsed to our atomic essence, we look strikingly similar to the astronomer's periodic table, dominated by the same handful of elements that comprise the universe. Indeed, the four most common atoms that make up you and me—oxygen, carbon, nitrogen, and hydrogen—are also four of the five most common elements in the cosmos. (The fifth is helium, the second-most common element in the cosmos. It probably wasn't used as a life building block because it's an elemental loner—it doesn't readily bond with other atoms.) Take yourself apart atom by atom, so to speak, and you'd find that by mass four simple atoms—elements 1, 6, 7, and 8—compose, remarkably, approximately 96.2 percent of you. You'd end up with a pile of oxygen atoms that weighed in at about 65 percent of your total weight, followed by 18.5 percent carbon, 9.5 percent hydrogen, and, finally, 3.2 percent nitrogen. Of course, you're wondering about the other 3.8 percent of you, of which about 1 percent is phosphorous and sulfur, and 2 percent is salts such as sodium, calcium, and potassium. All the other twenty or so elements that make us who we are, from iron to copper and every other essential element in our morning vitamin, make up less than 0.03 percent of our body mass.
MOLECULAR EVOLUTION
Alexander Oparin took the reflection on life's elemental nature one critical step further. He turned to astronomy to understand biology. In the midst of his day job—isolating and characterizing enzymes at the University of Moscow—Oparin contemplated the origin of these complex molecules as the result of a process of planetary evolution. As a result, his office desk was strewn with papers offering the latest results not only in biochemistry but also in geology and most notably in astronomy. He pulled various concepts from these disciplines together to create a cornerstone of the Stardust Revolution and, arguably, became the first astrobiologist.
Oparin's great contribution to the Stardust Revolution was the realization that the magnificent story of the origin of life on Earth only becomes accessible when viewed in a cosmic context. In this perspective, life isn't magic; it's elemental, chemical, and cosmic. Oparin situated life in a universal, physical context in the same way that Newton's description of the universal laws of gravity joined heaven and Earth. He saw life as an evolutionary process embedded in cosmic evolution.
“The carbon atom in the Sun's atmosphere does not represent organic matter,” Oparin wrote, “but the exceptional capacity of this element to form long atomic chains and to unite with other elements, such as hydrogen, oxygen, and nitrogen, is the hidden spring which under proper conditions of existence has furnished the impetus for the formation of organic compounds.”
At this point you might think, “Isn't this just another version of the spontaneous generation of life, and hasn't that idea been trashed?” It's a good point, and one that Oparin was keen to clarify. The difference in his proposal is that he was not suggesting the holus-bolus appearance of single-celled organisms such as bacteria. Prior to the development of biochemistry (and even then), small was conflated with simple. Thus, it appeared plausible that a single-celled beast could self-organize from, for example, a sterilized broth of chicken soup. Oparin, a lab biochemist, was acutely aware of the chemical complexity of even the smallest life. “It must be understood,” he pointed out, “that no matter how minute an organism may be or how elementary it may appear at first glance, it is nevertheless infinitely more complex than any simple solution of organic substances.” On the same grounds, Oparin rejected the long-dominant idea that the first life was green. He knew that the chemical pathways and molecules involved in photosynthesis are far too complex to have been life's starting point.
Oparin set the framework for modern thinking about the origin of life on Earth by arguing that life didn't emerge spontaneously but rather that it came about through a long, gradual process of molecular evolution. Whereas Darwin thought about the gradual transformation of species of animals, Oparin, the biochemist, pondered the gradual evolution of species of molecules. “All these difficulties [related to the appearance of cellular complexity] disappear,” Oparin argued, if we “take the standpoint that the simplest living organisms originated gradually by a long evolutionary process of organic substance and that they represent merely definite mileposts along the general historic road of evolution of matter.” In this context, Oparin argued, the key question is: What was the Earth like in the distant past, when life first evolved from nonliving matter? The short answer is: very different. That, said Oparin, makes all the difference. Life on Earth didn't evolve on the planet as we now know it but on a primitive orb that we wouldn't recognize as home.
First of all, there was no life. On the face of it, this might seem facile, but it turns out to be crucial. Ironically, Oparin conjectured, the first protean life—whatever it might have been—could get a foothold precisely because there was nothing else around to eat or chemically degrade it. “However strange this may seem at first sight, a sterile, lifeless period in the existence of our planet was a necessary condition for the primary origin of life.”
It was this line of thinking about the Earth's cosmic origins, about the Earth as a planet with its own history that marked the watershed in Oparin's stardust science. He analyzed hot-off-the-presses results from stellar spectroscopy, meteorite analysis, and the study of the atmospheres of other Solar System planets to come to a paradigm-shifting conclusion: Earth's atmosphere, and thus life, started without molecular oxygen. Today we think of oxygen as essential to life, and with good reason: spend more than a minute or two without oxygen, and we'd be worried not about the origin of life but rather about our imminent demise. Oxygen is the breath of life, both for humanity and for much of current life on Earth. But much of life on Earth is also anaerobic—from the bacteria in your gut to fermenting yeast. For these creatures, oxygen—a voracious electron stealer, or oxidizer—is deadly. We now know that oxygen in the atmosphere didn't fuel life but rather that it is in fact a result of life.
In planetary atmospheric chemistry there are two dominant types of atmospheres: those in which oxygen abounds and readily oxidizes all other atoms; and those dominated by hydrogen, which, rather than taking electrons prefers to share its sole electron. Oparin argued that rather than being oxidative, the early Earth's atmosphere was reducing, or rich, in hy
drogen. This protean atmosphere was dominated by molecules bound with hydrogen. The evidence lay in the stars, meteorites, and other Solar System planets. Oparin noted that stellar spectroscopy revealed simple carbon-carbon, carbon-nitrogen, and carbon-hydrogen molecules in the atmospheres of stars. Similarly, the minerals in meteorites revealed that many were formed in environments without oxygen. Finally, the recent spectroscopic analysis of Jupiter's atmosphere revealed vast amounts of methane—a carbon atom with four hydrogen atoms. The skies over the newborn Earth, Oparin said, weren't dominated by oxygen and nitrogen, as they are today—ours is a planet on which life has profoundly shaped the composition of the atmosphere—rather, the Earth's natal atmosphere consisted of methane, ammonia, water, and hydrogen. Here was the potential for a completely different kind of planetary chemistry than that which occurs on Earth today. It was from these fundamental molecular building blocks, Oparin argued, that life evolved.
Oparin suggested a long and detailed series of chemical reactions that, from these atmospheric molecular building blocks, could lead to increasingly more complex molecules, the ones we are familiar with in our daily diets: fats, sugars, and proteins. For Oparin, life emerged as an energetic or metabolic system that evolved through a series of increasingly complex chemical reactions. Chemical reactions in the atmosphere would have created a hazy rain of organic molecules that embedded the seeds of life in the Earth's Archean oceans, and in this chemical cocktail, Oparin imagined, the first protocells evolved.
What made Oparin's ideas revolutionary is that through detailed exposition he'd given other scientists the sine qua non of a respectable theory: it could be tested. They couldn't re-create the early Earth, but they could try to re-create its conditions.
THE EARTH IN GLASS
On a muggy August morning in 1957, more than three decades after Oparin's pamphlet first hit the streets of Moscow, and just as B2FH—the famous paper describing the origin of the elements in stars—was going to print, Stanley Miller stood outside the Soviet Academy of Sciences, wondering at his good fortune to be a young American in the Soviet capital at the height of the Cold War. The summer heat only added to his mixture of excitement and apprehension. He knew that in the Red-baiting atmosphere at home, his trip to the Soviet Union could come back to shred his career. Yet any fear of the government men who'd visited him before his departure, urging him to keep his eyes and ears open, was ultimately fodder for discreet mockery rather than concern. After all, he was a chemist, interested in molecules, not a nuclear physicist with an eye for missiles. He wasn't concerned, as were the government agents, that the Soviets might be on the brink of creating artificial life. At the moment, what was more intimidating for this clean-shaven young chemist was that he spoke only English—he was out of his league amid a European polyglot ensemble of the world's leading biochemists, a group in which conversations switched rapidly from German to French to Russian. And he'd been studying biochemistry for only several years. But he wouldn't have missed being here for the world. Oparin himself had invited the young man to this historic event, the first International Symposium on the Origin of Life on Earth. Though they'd never met, Oparin and Miller already shared a special scientific bond, the bond between the theoretician and the experimentalist who proves him right.
Miller knew that his path to Moscow was a serendipitous one. In the spring of 1951, he'd arrived at the University of Chicago as a graduate student attracted by the university's “can-do” spirit. A decade before, in a basement lab on the university's village-like campus, physicist Enrico Fermi had sparked the first controlled nuclear reaction. Miller had quickly landed a plum position with Edward Teller, the scientist who'd be remembered as the father of the hydrogen bomb, to work on big-bang nucleosynthesis—how the elements might have formed at the birth of the universe. However, this astrophysics research took a U-turn toward biology during the fall of Miller's first year, when he attended one of planetary-science pioneer Harold Urey's famously inspiring lectures.
Urey was part of a generation of quantum physicists and atomic-bomb makers who, either inspired by the relationship between the quantum atom and life or disenchanted by the atomic bomb and its impact on lives, had turned from physics to questions of biology. He was perfectly positioned for the transition. He'd begun his career as a physical chemist working with Niels Bohr in Copenhagen, exploring the relationship between atoms, their isotopes, and the resulting chemistry of their interactions. The work led to his discovery of deuterium, a heavy isotope of hydrogen, for which he won the 1934 Nobel Prize in Chemistry. During World War II, as part of the Manhattan Project, Urey developed the critical technique for separating the isotopes of uranium to produce bomb-grade material. Based on his knowledge of differences in isotope ratios, he began thinking about what these revealed about the story of the Solar System, particularly about the Earth's origins—the new field of cosmochemistry. He became one of the first to view the Earth as an evolving planetary geochemical system, rather than as a static, isolated orb.
Urey came to the same conclusion that Alexander Oparin had come to thirty years before: the newborn Earth, and the environment in which life first evolved, was nothing like it is today. Urey asserted that if researchers wanted to understand first life, they needed to get the primordial planetary chemistry right. Based on the planetary evidence, the newborn Earth's atmosphere, still steaming from its formation from a stellar cloud, was dominated by methane, ammonia, water, and hydrogen—the simplest forms of carbon, nitrogen, and oxygen fully bonded with hydrogen.
Looking out at a University of Chicago lecture hall of young students one morning in 1951, Urey suggested “that experimentation on the production of organic compounds from water and methane…and the possible effects of electric discharges on the reactions [simulating] electric storms…would be most profitable.” For Stanley Miller, the words were electrifying, sparking a creative vision that caught even Urey off guard. At first, Urey had balked at accepting Miller as his graduate student. But Miller persisted, and Urey's abundant curiosity won out over any administrative reserve.
Now, six years later, it was Miller who stood before an audience at the Soviet Academy of Sciences. Everyone in the Moscow audience knew the details of what Miller would say—it was a chance for the crowd to celebrate the victory and linger on what had been achieved. Oparin himself had learned of the results in 1953, when a colleague had run up to him waving a newspaper, showing the Russian that what he'd proposed thirty years before had come to pass. “I don't believe it!” Oparin exclaimed. In fact, one of the most important biochemistry experiments of the twentieth century had been quite simple. What was more remarkable was that no one had tried it before, as it used glassware and equipment available in any university chemistry lab. Standing before his distinguished audience of scientists, Miller described how he'd created a bare-bones equivalent of the primordial Earth's atmosphere and oceans in laboratory glassware. He noted that others thought they had tried this, using various mixtures of water, carbon dioxide, and oxygen—molecules common in Earth's atmosphere today—but that they'd had disappointing results. What made Miller's experiment transformational was that he'd fully embraced Urey's and Oparin's Stardust Revolution thinking and had re-created not some version of today's atmosphere but rather what was thought to be the Earth's primordial atmosphere.
Miller's experimental apparatus consisted of two globe-shaped flasks, one slightly to the side and above the other, like two party balloons connected by a glass tube. The slightly larger and higher one stood in for the Earth's early atmosphere; the lower, smaller one for the planet's primordial sea. After sterilizing the entire system for a day, Miller voided the top flask of Chicago's oxygen-rich air and filled it with Urey's proposed mixture of methane, ammonia, and hydrogen. He partially filled the lower flask with water, which, when gently heated, fed water vapor into the mix of primordial gases in the higher flask. Then Miller flipped the electricity switch, and sparks jumped between two tungsten electrodes inserted i
nto the gas mix. Here in glassware was a vision of an Earth lost to time: water, heated by the planet's fiery birth, evaporating from steamy early seas and mixing in the hazy atmosphere where one could imagine nighttime bolts of lightning illuminating a strange, wet, nascent world. Within days, Miller knew he'd done something special, and within a week, he knew it was dramatic. His Earth-in-glass had been transformed. The stand-in sea had gone brown; the walls of the upper chamber were coated with an oily sludge. Miller got to work analyzing the results, and what he found astounded him. His brown sea was a primordial soup of life. Amid the brown water and oily sludge, he separated out five amino acids—the building blocks of proteins. Among them were glycine, aspartic acid, and alanine, three amino acids that Miller knew were in him and in most living things.
He didn't even know how well he'd done. Months after Miller's death in May 2007, Jeffrey Bada, his former graduate student turned colleague at the University of California–San Diego, opened a dusty cardboard box he'd inherited from Miller. It contained hundreds of vials with the dried amino-acid residues from Miller's classic primitive-Earth-in-glass experiments. Bada arranged for these samples to be reanalyzed using modern, and far more sensitive, chemical-analysis techniques than the liquid-paper chromatography that Miller had used more than half a century before. With this keener analysis, nine additional amino acids emerged amid the residue of Miller's original experiment, and even more were discovered in later modified versions of the experiment.
Stanley Miller's results were the twentieth-century version of Friedrich Wöhler's creation of laboratory urea. Miller and Urey had added an enormous plank to the bridge between the chemistry of what scientists viewed as life and nonlife. As the New York Times reported, Miller “made chemical history by taking the first step that may lead a century hence to the creation of something chemically like beefsteak or white of egg.” For those pondering the origins of life, however, these amino acids didn't set their sights on the future but on the distant molecular past.
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