Small charges in water spray can trigger the formation of key biochemicals

We know Earth formed roughly 4.54 billion years ago and that the first single cell lifeforms were present roughly 1 billion years after that. What we don’t know is what triggered the process that turned our planet from a barren ball of rock into a world hosting amazing abundance of lifeforms. “We’re trying to understand how do you go from non-life to life. Now I think we have made a real contribution to solving this mystery,” says Richard Zare, a Stanford University chemistry professor. Zare is the senior author of the recent study into a previously unknown electrochemical process that might have helped supply the raw materials needed for life.

Zare’s team demonstrated the existence of micro-lightning, very small electricity discharges that occur between tiny droplets of water spray. When triggered in a mixture of gases made to replicate the atmosphere on early Earth, these micro-lightnings produced chemical compounds used by present-day life, like glycine, uracil, and urea, along with chemical precursors like cyanoacetylene, and hydrogen cyanide. “I’m not saying it’s the only way this could happen—I wasn’t there,” Zare acknowledged. “But it’s a new plausible mechanism that gives us building blocks of life.”

Lightning in the bulb

Scientific research into the beginnings of life on Earth started in the 1920s with Aleksander Oparin and J.B.S. Haldane, scientists who independently proposed that life on Earth could have arisen through a process of gradual chemical evolution. In their view, inorganic molecules might have reacted due to energy from the Sun or lightning strikes to form life’s building blocks, like amino acids. Those building blocks, Oparin and Haldane argued, could have accumulated in the oceans, making a “primordial soup.”

This Oparin-Haldane concept was tested by Stanley Miller and Harold Urey in a groundbreaking experiment conducted at the University of Chicago in 1953. “They took a glass bulb, and a spark plug like in a car,” Zare said. “They loaded the bulb with gases like methane, ammonia, water vapor, and they made a spark go through the bulb.” Miller and Urey found that this triggered the formation of glycine, hydrogen cyanide, alanine, and other building blocks of life. But there have been important objections to the Miller-Urey hypothesis, Zare acknowledges.

Size matters

The first objection was that lightning strikes were intermittent and unpredictable. Compounds made by lightning strikes in the atmosphere would have become widely dispersed, which matters a lot, because making simple building blocks is not enough. These building blocks need to remain in proximity to combine into larger molecules like proteins. To really jump-start life, the lightning proposed by Miller and Urey would need to dump material into the same puddle over and over again, which seems rather unlikely.

The first hints at an alternative to lightning appeared in 2024, when a team of Indian scientists led by Shidbas Banerjee observed that electrically neutral, micron-size water droplets from sonic sprays, humidifiers, spray bottles, et cetera could spontaneously ionize surrounding gas molecules, just as lightning strikes do. Banerjee attributed this phenomenon to electrical discharges in a corona surrounding individual water droplets, but Zare wanted to investigate this phenomenon a bit further.

It turned out Miller and Urey might have been right that life started with something very much like lightning. It’s just the lightning was likely much, much smaller than they thought.

Levitating droplets

Zare and his team, just like Banerjee and his colleagues, started with electrically neutral droplets of water. They suspended these droplets mid-air using acoustic levitation and recorded the droplets’ behavior with a high-speed camera. When a droplet was squashed by moving the acoustic hardware closer together, smaller droplets started to split off its surface and levitate around it. At this point, both small and large droplets became electrically charged. “The electrons jumped from the large droplets onto the smaller droplets,” Zare explains. So the large droplets ended up with a positive charge, while smaller droplets gained negative charges.

To confirm this electric charge buildup mechanism, the team placed negatively and positively charged electrodes on opposite sides of the levitating droplets. The camera registered small droplets gravitating toward the positively charged surface. But it also registered something else. “We could see charge jumping between negatively and positively charged droplets,” Zare says. “This really was a surprise. It’s news! We’ve all seen lightning, but no one has ever seen micro-lightning in droplets.”