Another take on the origin of life arrived with the new discipline of biochemistry. As the extraterrestrial-life debate was waged between the optimism of physicists and the pessimism of biologists, it made sense that new approaches should arise from the field that bridged the conceptual gap between atoms and organisms.
Proteins (organic chemical components of all living cells) were first isolated from cellular material in the first years of the twentieth century. Around that time many simple biochemical reactions were duplicated in laboratory flasks, adding to a growing sense that life is, fundamentally, chemistry. This more sophisticated version of spontaneous generation renewed hopes of finding the key to life's origins in special brews of chemicals native to the primitive Earth.
A chemical origin of life became widely accepted after 1936 when the Russian biochemist Aleksandr Ivanovich Oparin published his landmark book Origins of Life. Oparin, no doubt influenced by the dialectical materialist philosophy permeating the Moscow air, postulated an inevitable historical process in which conditions on the young Earth caused the molecules of life to rise up and organize out of nonliving matter.
The prevailing view of Earth's earliest environment at the time included an atmosphere composed mostly of carbon dioxide (CO2), similar to that which we already knew to exist on neighboring Venus. Oparin argued for a very different kind of ancient atmosphere, rich in methane (CH4) and ammonia (NH3), gases which had recently been detected in the atmospheres of Jupiter and Saturn.
This proposed change in the early atmosphere, from CO2 to CH4 and NH3, has a crucial effect on the social behavior of carbon atoms. In an environment rich in hydrogen compounds such as methane and ammonia, called a reducing environment, carbon atoms will tend to grab on to each other, forming the giant carbon conga lines and group carbon hugs we call complex organic molecules. Carbon behaves very differently in an oxidizing atmosphere richer in carbon dioxide or oxygen (O2). The carbon is seduced by oxygen's pull and ignores its own kind. Organic molecules don't stand a ghost of a chance.*
*Yes, it's true. Our precious oxygen is lethally toxic to the basic molecules of life. We'll return to this chemical irony in a later chapter.
Oparin described how, on an early Earth with a reducing environment, simple organic compounds formed and began reacting with one another. This led to "chemical evolution" in which the more stable (or "fit") molecules hang around, accumulating and evolving further. The result was a rich soup of chemicals that gradually increased in size and complexity until the organic molecules essential to forming the first living cells were abundant in the ponds and oceans of the juvenile Earth.
Origins of Life was a watershed in modern thought about life's beginnings, strongly influencing both astronomical and biological beliefs about the primitive Earth for the rest of the century. Although Oparin's book was strictly about Earth, the theory described the inexorable chemical development of life from conditions believed to exist generally on young planets. The cosmic consequences were inescapable.
In the 1930s, we knew precious little of the actual conditions on other planets in our solar system, and even less about the primitive environments on these planets way back when life on Earth began. It seemed probable that early conditions were similar on all planets, so Oparin's chemical evolution seemed like a universal life-generating theory.
In 1953, at the University of Chicago, Nobel laureate Harold Urey and his grad student Stanley Miller realized they could test Oparin's thesis experimentally. Urey, one of the fathers of modern planetary science,* created the subfield of cosmochemistry, in which we follow the chemical forms of matter through the stages of cosmic evolution. As the first to improve upon the nebular hypothesis using sophisticated chemical modeling, he cleverly deduced what the planets were made of when they condensed out of the solar nebula. He concluded that the early Earth was rich in methane, ammonia, hydrogen, and water—a picture similar to Oparin's.
Miller, then a beginning student, wanted to try simulating the natural creation of organic chemicals on the early Earth. Urey was skeptical, but he agreed to let Miller attempt a preliminary experiment to test the concept. The setup was simple: brew up some primitive air, zap it with simulated lightning, and see if anything happens. They mixed ammonia, methane, and water in a flask and sparked it up. After a few days they were both astounded to find their experimental flask full of an ugly, sticky, brown goo. The gunk turned out to be made of amino
*And my "intellectual grandfather": Urey was thesis adviser to my thesis adviser, John Lewis.
acids—the building blocks of protein, the stuff of life! This finding far exceeded the ambitions of their initial mock-up investigation, which is now inscribed in textbooks as the Miller-Urey experiment.
The astonishing result suggested that unremarkable conditions and processes on the primitive Earth would inexorably have produced the molecules of life. Of course, that was Oparin's original thesis twenty years earlier, but a bird in the lab is worth two on the page: experimental proof is more convincing than the most sophisticated theoretical conjecture. Miller and Urey had not actually created life in the lab, but by producing life's crucial building blocks from garden-variety chemicals, they removed what had seemed a fundamental barrier to the spontaneous generation of life from nonlife on Earth or elsewhere. Like Oparin, Miller and Urey did not at first discuss the extraterrestrial applications of their work. However, ammonia, methane, and water were known to be among the most abundant compounds in the universe. Embedded in the results of Miller-Urey was the clear implication that the steps to life here were the result of common cosmic processes.
The Miller-Urey experiment, by establishing that the origin-of-life question is subject to experimental inquiry, generated not only a flask of dark, promising sludge, but a cottage industry. Investigators trying to discover the essential early steps of life have endlessly varied the formula of the gaseous brew, following evolving ideas about the primitive atmosphere, and they've zapped these mixtures with all kinds of energy that might have been present on the young Earth, including ultraviolet radiation and simulated asteroid-impact explosions. To this day, the resulting brown goos are eagerly analyzed like the precious elixir of life. Just as all later theoretical discussions of the chemical origins of life on Earth or anywhere in the universe can be seen as refinements of Oparin's ideas, all experimental efforts in the field are variations on the theme begun by Miller and Urey.
These rigorous scientific results returned to the study of extraterrestrial life much of the legitimacy it had lost in the aftermath of the Lowell affair. Just as the nebular hypothesis predicted that planets were a natural byproduct of the formation of stars, Oparin's theory and the Miller-Urey experiment implied that life itself is a natural byproduct of the formation of planets.
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