Scientists have wrangled for decades over the origin of life on Earth, speculating from lightning bolts to warm ponds and underwater vents. But a revolutionary new idea is put forward in a new study from Stanford University: the spark of life might have come from the small lightning bursts created by breaking waves and waterfalls. These “microlightning” flashes, which are invisible to the naked eye, could have had enough energy to form the building blocks of life.
The 1952 Miller-Urey experiment first showed that organic molecules can be generated when a mixture of gases approximating Earth’s early atmosphere is subjected to electricity. It has been this same traditional experiment, however, which spawned the proposal that lightning rods may have provided the energy for transforming inorganic substances into amino acids and other essential molecules. But critics ever since have detected flaws in such a hypothesis as the infrequency of lightning events and the diffusion of organic molecules throughout the wide ocean.
Stanford chemist Richard Zare and his colleagues have now reversed this story. Their experiments demonstrate that microlightning, small electrical discharges among oppositely charged water droplets, will form the same organic molecules that appeared in the Miller-Urey experiment—without added external electricity. “Microelectric discharges between oppositely charged water microdroplets make all the organic molecules observed previously in the Miller-Urey experiment, and we propose that this is a new mechanism for the prebiotic synthesis of molecules that constitute the building blocks of life,” said Zare, Stanford’s Marguerite Blake Wilbur Professor of Natural Science.
The researchers’ experiments started with probing the electrical nature of water droplets. When water is splashed or sprayed, bigger droplets are positively charged and the smaller ones negatively charged. As these oppositely charged droplets move toward one another, small sparks jump from one to the other a process Zare describes as microlightning. While these flashes last only billionths of a meter, they pack enough punch to power chemical reactions. High-speed cameras allowed the researchers to record these brief sparks, giving visual confirmation of their existence.
To our astonishment, the researchers sprayed room-temperature water into a gas tank filled with gases thought to mimic the ancient atmosphere of Earth: nitrogen, methane, carbon dioxide, and ammonia. The results showed that organic molecules with carbon-nitrogen bonds, including hydrogen cyanide, glycine, and uracil, were formed. Glycine is an essential amino acid for protein formation, while uracil is a nucleobase in RNA, a molecule at the heart of genetic coding and cellular function.
This discovery slices the long-held assumption that lightning bolt strikes were the prime force behind prebiotic chemistry. While lightning is a hit-or-miss affair, water sprays are ubiquitous. “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,” Zare said. Such water-droplet-abundant and rock-surface-abundant settings could have been the best environments for organic molecules to settle and further react.
The results of this study have implications beyond Earth. The study indicates that microlightning may have been plentiful in water-rich environments on early Earth, possibly powering prebiotic chemistry in regions where other sources of energy, such as UV radiation or volcanism, were minimal. The study also provides new opportunities for the search for extraterrestrial life. If microlightning can initiate the development of organic molecules on Earth, then it is likely that the same happens on other planets or moons with liquid water.
This research also points to the unexpected reactivity of water droplets, which are generally thought of as harmless. Zare’s group has already shown that water droplets can spontaneously generate hydrogen peroxide and be involved in ammonia synthesis, a major component of fertilizers. “We usually think of water as so benign, but when it’s divided in the form of little droplets, water is highly reactive,” Zare said.
Although this initial test did not yield all of the molecules required for life, the researchers are hopeful about the prospects of future research. If we can run the experiment for longer, we should be able to detect more, said Yifan Meng, a postdoctoral researcher at Stanford and co-author of the study. Meng pointed out that microlightning may be able to facilitate a wider array of prebiotic synthesis, similar to subsequent versions of the Miller-Urey experiment broadened their capabilities.
The possibility that life might have emerged from infinite minute sparks instead of a single grand event provides a new window into one of the science world’s biggest mysteries. As Zare rightly pointed out, “This is a real contribution to understanding how you can go from non-life to life.” By moving the discussion away from sporadic lightning and towards ubiquitous water droplet chemistry, this study emphasizes the strength of small, continuous processes in charting the path forward for life.
The study, in Science Advances, invites scientists to re-think the origin of life and explore the as-yet untapped potential of micro lightning. “It opens up an array of possibilities that we need to explore further, using different gas and fluid compositions,” says Dr. Eva Stueeken of the University of St Andrews.
Whether life started in a hot pond, beside a crashing waterfall, or in the mist of ocean waves, the tale of its beginning is still unfolding. And with every new find, we are one step closer to understanding the complex chemistry that transformed Earth’s primordial soup into the teeming, living world we see today.
