Life on Earth May Have Been Jump-Started by ‘Microlightning’
Charged water droplets generate sparks that can forge organic compounds
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Earth, in its infancy, swirled with all the gases needed to construct life. But they couldn’t just assemble themselves into the building blocks of biology. That process, called “prebiotic synthesis,” required a jolt from the outside. Lightning was an obvious suspect. So in 1952 a young chemist named Stanley Miller filled a flask halfway with water, topped it with methane, ammonia and hydrogen to mimic the planet’s early atmosphere and then flung a miniature lightning bolt into that fertile soup.
In this landmark experiment, Miller produced several amino acids out of inorganic molecules. (Amino acids combine to form proteins, which in turn combine to form living organisms.) He thus showed how life could have found its first foothold. But real lightning would have struck infrequently—and mostly in open ocean, where organic compounds would have quickly dispersed.
Seven decades later, new research points to a more realistic catalyst: water itself. Today in Science Advances, Stanford University chemist Richard Zare and his colleagues report that organic molecules with carbon-nitrogen bonds can be formed by simply spraying water into a mix of atmospheric gases. The researchers basically replicated the chemical reactions from Miller’s experiment, but this time those reactions were achieved with a reliable energy source. “Unlike lightning,” Zare says, “water sprays are everywhere.” Each waterfall and wave, he suggests, brought a spark of opportunity for life to emerge.
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It’s all because of the difference in electrical charge between water droplets. When small, negatively charged droplets come near large, positively charged ones, they sometimes discharge, producing a flash of luminescence the researchers call “microlightning.” And it turns out that these interactions, like Miller’s electricity, create organic by-products: in its watery, gaseous stew, Zare’s team detected the amino acid glycine, as well as the nucleobase uracil—a key component of RNA.
Study co-author Yifan Meng, a postdoctoral scholar at Stanford, ran the physical experiment. At first, Meng recalls, he and his colleagues were more primarily interested in microlightning itself. “But then we saw the clear evidence of carbon-nitrogen bond formation,” he says. “This is something fundamental to biological molecules. It was really incredibly exciting.”
To get life going, however, it wouldn’t have been enough for these compounds to form once; that’s why random lightning strikes were likely a nonstarter. Single molecules, called monomers, would have needed a repetitive process to give them time to link up in long molecular chains, called polymers: it takes many amino acids to make a protein and many nucleobases to make a strand of RNA. “We need the building blocks to get concentrated somewhere,” Zare says.
The ideal environment for that, he argues, would have been rock crevices near water sprays. The wet-dry cycles that come with such terrain are known to foster polymerization, potentially giving rise to the complex structures that became the first single-celled organisms. David Deamer, a biochemist at the University of California, Santa Cruz, who was not involved with the study, found Zare’s conclusions compelling. Whether in a pond, a lake or a geyser, Deamer says, “these molecules would have accumulated wherever there was wave action or waterfalls.”
This initial test did not generate all of life’s prerequisites, but Meng notes that other important compounds might have been present at undetectable levels. “If we can run the experiment for longer,” he says, “we should be able to detect more.” Just as later elaborations on Miller’s work produced a wider range of molecules, future research could confirm that microlightning supports full-blown prebiotic synthesis.
There are competing hypotheses as to how organic molecules first formed. Some experts believe they originated around deep-sea hydrothermal vents, while others think they caught a ride to Earth from somewhere else in our galaxy. NASA scientists announced in January that 14 amino acids, along with all five nucleotide bases in RNA and DNA, had been found in the asteroid Bennu. Given that extraterrestrial objects routinely pummeled our planet in the early days, Deamer says, “literally, the compounds necessary for life were falling out of the sky.”
No one knows what really happened when life emerged around four billion years ago. But these findings lend evidence to what Miller proposed back in the 1950s—albeit with a twist. As he told an interviewer in 1996, “nobody questioned the chemistry of the original experiment…. The chemistry was very solid.” Perhaps now the spark that set that chemistry in motion is, too.