'Microlightning' between water droplets could have sparked life on Earth. Here's how

The building blocks of life on Earth may have been fueled by tiny sparks hopping between water droplets.

Four billion years ago, Earth was a lifeless world, but a dynamic one. Crashing waves, rushing streams, and roaring waterfalls churned up sprays of water into an atmosphere rich with carbon dioxide, nitrogen, methane and ammonia. Recent experiments suggest that those sprays of water may have helped jump-start chemical reactions that produced the building blocks of life.

According to Stanford University chemist Richard Zare and his colleagues, small electrical charges built up in water droplets and unleashed tiny bursts of electricity may have been enough to power those reactions. It's a new twist on an old, and hotly debated, theory about the origins of life on Earth.

Jump-starting the chemistry of Life

Researchers are still trying to puzzle out exactly how life made the jump from chemistry to biology (and, for that matter, where to draw the line between the two) between 4 and 3.5 billion years ago. A key piece of that puzzle is figuring out where the complex chemicals that make up living cells came from. That includes things like the lipids in our cell membranes, the nucleotides that encode our genes, the amino acids that build most of the working bits of our cells, and other molecules built around bonds between carbon and nitrogen atoms.

Most of the available information suggests that early Earth didn't have a huge supply of these complex molecules, if any at all, but it did have the raw ingredients for them: various combinations of carbon, hydrogen, nitrogen, oxygen and phosphorus just waiting to be combined in the right ways.

But building new molecules requires energy. Zare and his colleagues say that the energy required could have come from "microlightning," incredibly small flickers of electricity passing between droplets in moving sprays of water on early Earth.

In recent experiments, the team of chemists noticed that when water droplets move around — like in windblown sea spray, for instance — those droplets tend to develop different electrical charges. It's similar to the process that creates lightning in clouds, but on a miniature scale. Electrical charges build up, and eventually electrons leap from negatively-charged droplets to positively-charged ones in a tiny zap of lightning.

"We usually think of water as so benign, but when it's divided in the form of little droplets, water is highly reactive," said Zare in a statement. His lab studies chemical reactions in and between tiny water droplets. This microlightning, as Zare and his colleagues call it, is too small and too fast to see with the unaided eye, but they managed to record it with a high-speed camera.

Microlightning in a bottle

Zare and his colleagues sprayed water vapor into a chamber filled with a mix of gases meant to mimic early Earth's atmosphere, around 2 billion years ago: a noxious blend of ammonia, methane, hydrogen, and nitrogen. Tiny flashes of microlightning in the water vapor kicked off a series of chemical reactions that produced some very complex molecules: the amino acid glycine, the nucleotide base uracil, and others.

The results of this recent experiment are strikingly similar (pun intended) to a 1952 experiment by University of Chicago chemists Stanley Miller and Harold Urey, who zapped a bottle of water vapor, methane, ammonia, and hydrogen gas with an electrical spark and got amino acids. Miller and Urey proposed that life got its jump-start from lightning striking Earth's primordial ocean around 4 billion years ago.

But their hypothesis has been a lightning rod for criticism, mostly because lightning doesn't happen often enough to trigger enough chemical reactions over something as big and spread-out as the ocean.

If the jump-start came from microlightning in water sprays, rather than full-sized lightning strikes over water, Zare and his colleagues say that could solve the problem and give life to a new version of the old hypothesis.

"On early Earth, there were water sprays all over the place — into crevices or against rocks, and they can accumulate and create this chemical reaction," said Zare in a recent press release. "I think this overcomes many of the problems people have with the Miller-Urey hypothesis."

Zare and his colleagues published their work in the journal Science Advances.