Before scientists discovered that comets and meteors carry the elemental building blocks of life – carbon-based amino acids – the best guess for the origin of life on Earth was the ‘primordial soup’ theory. Its progenitor was none other than Charles Darwin, who in an 1871 letter to his close friend and fellow scientist Joseph Hooker speculated that life may have begun in a “warm little pond, with all sorts of ammonia and phosphoric salts, lights, heat, electricity, etc. present, so that a protein compound was chemically formed ready to undergo still more complex changes.”
Darwin’s idea lay dormant until the 1920s when two researchers, Alexander Oparin and J.B. Haldane, working independently of one another, asserted that the Earth’s original oceans were a vast “primeval soup” of non-organic molecules that bubbled and stewed for millennia, absorbing the energy of sunlight, until it “grew” organic molecules that could survive and reproduce on their own – as some molecules (bacteria) do today in hot springs and volcanic vents. Oparin argued it only had to happen once; life, once started, could take care of itself. The first living organism, they said, would be little more than a few chemical reactions encased in a thin membrane to keep them from being destroyed. These organisms would grow by absorbing organic molecules around them, grow, divide, and grow again. Eventually, photosynthesis would arise and the oxygen it created would change the Earth’s atmosphere, making it amenable to further life. Haldane called this process biopoiesis – a name that didn’t catch on. His description of the oceans as a “hot dilute soup” did, however.
In 1952, this powerful metaphor received a significant jolt, literally. University of Chicago graduate student Stanley Miller and his professor, Harold Urey, decided to test the Oparin-Haldane hypothesis in what became one of the classic experiments of post-war science. Their goal was to recreate the prebiotic conditions of Earth’s early oceans and atmosphere in the laboratory to see if they could generate organic compounds from inorganic ones. Speculating that volcanic activity would have released methane, hydrogen, and ammonia into the Earth’s proto-atmosphere, they sealed these gasses in glass piping, built in a closed loop. On one end of the loop was a flask filled with water, which was boiled to create water vapor, on the other side were two electrodes – representing lightning. After sparking the vaporous mixture with the electrodes, the gasses were cooled and allowed to “stew” for a few weeks before being analyzed.
What they discovered made headlines around the world – and still forms the foundation of most scientific inquires into the origin of life on Earth.
They discovered that as much as 10–15% of the carbon in the system had formed simple organic compounds, and 2% had actually become amino acids – essential to life. In an interview at the time, Stanley Miller said: “Just turning on the spark in a basic pre-biotic experiment will yield 11 out of 20 amino acids.” More remarkably, in 2007 scientists reanalyzed the sealed vials from the original experiment, discovering that there over twenty different amino acids in the mixture. In this way, the experiment strongly supported the Oparin-Haldane “primordial soup” theory, showing that simple organic compounds could be formed from gases with the addition of energy. Lightning, their experiment suggested, had provided the original spark of life on Earth.
Recent research has challenged parts of their conclusion, however. Investigations into the actual composition of the Earth’s atmosphere during its proto-development phase, called the Hadean Period (after the Greek god of the Underworld), reveal that its chemical composition was more complicated than Millar and Urey envisioned, including the presence of oxygen, which would have hostile to the formation of organic compounds. While complicated, the picture emerging is one of an extremely turbulent, mostly liquid planet subjected to intense ultraviolet radiation, massive undersea volcanic eruptions, and frequent bombardment by rocky debris from outer space. These impacts would have kicked up large amounts of steam which eventually blanketed the entire planet with hot, smelly clouds. Rain – and lightning – followed. It is quite possible, under this scenario, that additional amino acids arrived on Earth hitched to meteorites and comets – tossed into the bubbling primordial soup like cosmic potatoes or carrots. Directions for Life: add carbon and let stew for a few hundred thousand years!
Here’s a diagram of the famous Miller-Urey “soup” apparatus:
This raises a question: is life possible without carbon?
Yes, said a group of scientists in a report published by the National Research Council in 2007. They call it “weird life” – life with an alternate biochemistry than what’s found on Earth. According to the report’s authors, the fundamental requirements for life as we know it – water-based biosolvents, a carbon-based molecular system capable of evolution, and the ability to exchange energy with the environment – are not the only ways to support phenomena recognized as life. “Our investigation made clear that life is possible in forms different than those on Earth,” said lead author John Baross, professor of oceanography at the University of Washington. But we’ll never recognize it, he continued, if we’re only searching for Earth-like life in outer space.
“No discovery that we can make in our exploration of the solar system would have greater impact on our view of our position in the cosmos, or be more inspiring, than the discovery of an alien life form, even a primitive one,” wrote the report’s authors. “At the same time, it is clear that nothing would be more tragic in the American exploration of space than to encounter alien life without recognizing it.”
The astronomer Carl Sagan once referred to this situation as “carbon chauvinism,” arguing that life could alternatively be based on silicon or germanium. This may have been the inspiration for a famous Star Trek episode where Captain Kirk and Co. explore a planet dominated by aggressive and gooey silicon-based life forms called Horta (an encounter with one prompts a memorable mind-meld with Mr. Spock). The trouble with silicon, however, is its powerful attraction to oxygen. Life, as we define it, requires a respiratory process, which removes waste. In carbon-based life forms, the waste product is a gas, carbon dioxide, which is easily dispatched. The waste product of silicon, however, is sand – a solid. This means, according to biochemists, that it would be very difficult for silicon to provide a basis for viable life, even “weird” life.
As for the report’s authors, they point to ammonia and formaldehyde as possible biosolvents that could support a home for “weird” life. They also noted that recent experiments demonstrate that DNA could be constructed from nucleotides based on sodium hydroxide and hydrochloric acid – meaning that an organism could have an entirely non-carbon-based metabolism. Weird life might even exist on Earth, they argued. We’ve just not tuned our minds to the possibility. Field researchers should therefore seek out organisms with novel biochemistries, they said, to better understand how life on Earth truly operates. This improved understanding will help us in our restless search for life beyond the confines of our blue-green planet.
It’s all food for thought, whether carbon or silicon-based. Here’s a humorous take on the soup theory: