Micro-sparks between water droplets may have started life on Earth
How did life begin on Earth? Nobody knows with a high level of certainty. Hypotheses have been around for a long time. Basically, what is needed is the right chemicals to mix, a way to mix them, the right atmospheric conditions, and energy to boost their reactions. And time, lots of time. The most common elements needed are carbon, hydrogen, oxygen, nitrogen, sulfur, and phosphorus. This article will talk about only the energy sources researchers have speculated could help make the building blocks.
In the 1920s, Soviet biochemist Alexander Oparin and British scientist J.B.S. Haldane came up with concepts to explain how life might have started from the raw materials on the planet. Both thought that it all started in the oceans and used lightning and UV light from the sun as energy sources. Oparin described organic molecules formed in the ocean into droplets (coacervates) where chemical reactions could take place more efficiently. Haldane described the overall situation as a "hot dilute soup" (later changed by various sources into "primordial soup") and focused on the possible chemical reactions in general.
At the time, scientists had a poor understanding of the chemistry involved in geology. Most felt that since the atmosphere today has a lot of oxygen, it must have always had it. But Oparin and Haldane disregarded that idea. Some pointed to the fact that oxygen is necessary for fire, not just life. Microbiology was not as advanced as today, so they also thought that anaerobic bacteria (bacteria that cannot live in the presence of oxygen) were just defective life forms.
Geochemistry results in the 1930s-1950s showed that extremely old mineral deposits like iron did not contain oxygen. That suggested the atmosphere lacked oxygen, and that's when origin of life hypotheses began using that idea as background for further studies.
Oparin and Haldane proposed only hypothetical notions; they conducted no experiments. Twenty years later in 1952, PhD student Stanley Miller and his mentor & Nobel laureate Harold Urey put together a lab experiment to test the Oparin-Haldane concept. Using information they thought to be accurate at the time about the ancient Earth's atmosphere, they did the following. Methane, ammonia, and hydrogen were sealed in a 5-liter glass flask to simulate the atmosphere at the time. They ran electrical sparks through it to simulate lightning that would have occurred. This was connected to a 500-milliliter flask half-filled with water that was heated to boiling to simulate the oceans (minus salt content). Water vapor flowed into the atmosphere flask. Water vapor that cooled and condensed flowed into a trap where samples could be taken. If any chemicals in the atmosphere mixed with the water vapor, they could be analyzed for changes. In just one day, trap water was pink; after a week, it was dark red and cloudy.
Five amino acids were found. At the same time (1952-1953), other investigators were conducting similar experiments. One found no change, and another found a sticky resinous mixture too complex to analyze. Miller repeated his work in 1957 with a different atmosphere, based on new assumptions (like what gases might have come out of volcanoes).
- 1952 atmosphere: methane, ammonia, hydrogen, water
- 1957 atmosphere: different combinations of methane, ammonia, hydrogen (or nitrogen), water, hydrogen sulfide, carbon monoxide
He found 22-23 amino acids in his mixture, along with hydrogen cyanide (HCN) and chemicals called aldehydes and other organic materials. HCN is a very reactive molecule important in forming other organic (carbon-containing) compounds.
After Miller's death in 2007, Professor Jeffrey Bada inherited Miller's equipment and found samples from the 1952 work. Analysis showed the old mixture now had over 20 amino acids.
Since Miller's ongoing work from the 1950s and beyond, Bada and others conducted additional experiments with different conditions, usually using electrical sparks as the energy source, but some also used just heat. Ultraviolet (UV) radiation from the sun likely had a stronger effect on an early Earth than it does now because there was no ozone layer back then to block it. It is thought to be a strong influence on creating HCN, for example.
- Joan/John Oró (1961) showed that HCN could form building blocks of nucleic acids leading to DNA & RNA. He and colleagues also in 1971, showed the high abundance of amino acids and various hydrocarbons in the Murchison meteorite in 1971.
- Carl Sagan and Bishun N. Khare (1979) examined the tar-like residue from experiments such as Miller's (and their own where they used UV radiation). They named it tholins, and it is a mixture of many polymers that can be broken down into many types of organic molecules (sugars, amino acids, nucleic acid building blocks).
- In 2012, Sarah Horst and her colleagues treated gases found in Titan's atmosphere with UV radiation and generated tholins. They also found that these could make amino acids and nucleic acid precursors.
The heat (60°C-405°C) from these alkaline vents comes from underground magma mixing with seawater that seeps through pores. It comes out mixed with many minerals and gases, and temperature gradients form between the vent and 2°C seawater. The heat plays a minor role in providing energy for the chemical reactions. It is the chemistry itself that is so reactive that changes happen on their own. The smoky plumes eventually spread sideways for thousands of kilometers before settling. Inside the vent chimneys, pockets can form with isolate chemical reactions and provide more concentrated conditions.
Finally, aside from lightning, UV radiation, heat, and direct chemical reactions underwater, another source of energy has been suggested to produce organic molecules. In shallow pools, a combination of UV radiation, mild underground heat, and a wet/dry cycle has been shown to produce organic molecules. Montmorillonite clay and mica are the two most common examples of surfaces where these pools might form. Both are composed of aluminum silicates but are different enough to give researchers separate ideas on forming primordial compounds. The clay swells and shrinks a littlein wet/dry cycles (such as those found in caves, where montmorillonite is commonly found), but mica does it more; any trapped organic molecules might be mixed better with that mechanical action. Clay's sodium and calcium are better for polymerizing reactions than the potassium in mica, but mica holds the compounds more tightly. Each has been shown to form the following molecules.
- Clay and mica: RNA precursors
- Clay: peptides and nucleobases
- Mica: protocells and flavin mononucleotide (a molecule that helps chemical reactions take place)
Recently, Richard Zare and his team at Stanford University have come up with another means to inject energy into the process of making organic molecules. They feel that lightning strikes may not have been frequent enough in an early Earth, and that even when they did occur, the changes in atmospheric components caused by these infrequent electrical discharges would simply be diluted in the oceans and washed away.
Cloud lightning is formed when a mass of negative and positive charges build up in the top and bottom of storm clouds. These charges are formed when ice crystals (0.01-0.5 mm diameter) bump into graupel (2-5 mm diameter) and transfer an electron. Graupel is a soft hail-like ice crystal that falls through supercooled water and gets coated in a frosty layer. It is usually negative and heavier, and air currents send the positive lighter ice crystals higher into the cloud.
Instead of volcanic or regular cloud lightning, Zare has recently presented data in April 2025 on "microlightning" as an energy source to form organic molecules. Zare read a 2024 report from a former colleague Anubhav Kumar from the Indian Institute of Science Education and Research. Kumar and his researchers noticed that microdroplets of water emitted from steamers, humidifier, and spray bottles changed atmospheric nitrogen (N2) the way lightning did. They determined it was due to a weak electric discharge (corona discharge) at the air–water interface.
Zare and associates conducted 2 experiments. In the first, they built a chamber and filled it with methane, nitrogen, carbon dioxide, and ammonia, then used a nebulizer to inject water in microdroplets. Liquid samples were removed, but gaseous samples were analyzed directly in the chamber with a device called a mass spectrometer.
The second setup involved suspending a drop of water with sound waves, then using the same waves to split smaller drops from the main one. A photon detector measured light coming off the different sizes of drops when they collided.
- Results from the first experiment showed the production of glycine (an amino acid), uracil (a building block of nucleic acids), urea (useful in making more amino acids), and organic molecules with cyanide components (cyanoacetylene, cyanoacetaldehyde, and cyanoacetic acid).
- In the second experiment (conducted in the dark with high-speed cameras), they saw flashes of light when large and small microdroplets of water touched each other.
