To understand how life was formed, scientists try to explain how amino acids were formed. Amino acids are the raw materials from which proteins and all cellular life are composed of.

Charles Darwin was reluctant to publish his views on the origin of life, according to Scientific American. His opinions on the subject are only known from a letter that he sent to his friend and colleague Joseph Hooker.

“But if (& oh what a big if) we could conceive in some warm little pond with all sorts of ammonia & phosphoric salts,—light, heat, electricity &c present, that a protein compound was chemically formed, ready to undergo still more complex changes, at the present day such matter would be instantly devoured, or absorbed, which would not have been the case before living creatures were formed,” wrote Darwin in the letter sent in 1871.

Since then, researchers have suggested that life evolved in volcanic pools or that lightning played a key role in the creation of life. But a study published in the journal Life proposes that solar particles colliding with the Earth’s early atmosphere can form amino acids and carboxylic acids, which are the basic building blocks of proteins and organic life.

The long and convoluted path to finding the origin of life

The famous Miller-Urey experiment in 1953 was a groundbreaking advance in the search for the origin of life. Stanley Miller of the University of Chicago tried to recreate the conditions of primordial Earth in the lab. He filled a closed chamber with methane, ammonia, water and molecular hydrogen and repeatedly ignited an electrical spark to simulate lightning.

Those were the gases that were thought to be prevalent in the Earth’s early atmosphere. A week after the chamber was closed, Miller’s graduate advisor Harold urey analysed the chamber’s contents and found that 20 different amino acids had formed.

An artist’s concept of the Earth at the dawn of primordial life. (Image credit: NASA)

“That was a big revelation. From the basic components of early Earth’s atmosphere, you can synthesize these complex organic molecules, said Vladimir Airapetian, co-author of the new paper in Life, in a press statement, referring to the Miller-Urey experiment. Airapetian is a stellar astrophysicist at NASA’s Goddard Space Flight Center in Maryland.

But the 70 years since the experiment have obfuscated the inferences that can be drawn from it. Scientists now believe that ammonia (NH3) and methane (CH4) were far less abundant during Earth’s primordial phase, according to NASA. Instead, it was filled with more carbon dioxide and molecular nitrogen, which require much more energy to break down.

While these gases can still yield amino acids, they do so in reduced quantities.

In a search for alternative energy sources that could have powered the breakdown of those compounds. Some proposed that shockwaves from incoming meteors could be a source, others pointed to ultraviolet radiation. Airapetian and colleagues looked through data from NASA’s Kepler mission to look at another direction—energy particles from the Sun.

Getting the Sun in the warm pond

In 2016, Airapetian published a study in Nature Geoscience which proposed that during our planet’s first 100 million years, the Sun was about 30 per cent dimmer but had near-constant eruptions of powerful “solar superflares.” Solar superflares are intensely powerful solar explosions that we only see once every hundred years or so. According to the study, it could have happened once every three to ten days when our planet was younger.

“As soon as I published that paper, the team from the Yokohama National University from Japan contacted me,” said Airapetian.

According to NASA, Kobayashi, a professor of chemistry in the Yokohama National University, was trying to understand how galactic cosmic rays could have affected the atmosphere of Early Earth.

“Most investigators ignore galactic cosmic rays because they require specialised equipment, like particle accelerators. I was fortunate enough to have access to several of them near our facilities,” said Kobayashi, in a press statement. Kobayashi spent more than 30 years studying prebiotic chemistry and small tweaks to his experimental setup helped the scientists put new theories to the test.

Airapetian, Kobayashi, and other researchers did something similar to the Miller-Urey experiment. They combined carbon dioxide, molecular nitrogen, water and a variable amount of methane. After this, they shot the mixture with a stream of protons to simulate solar particles. They also shot such mixtures with spark discharges to replicate the Miller-Urey experiment for comparison.

The mixture shot by protons seemed to produce detectable amounts of amino acids and carboxylic acids as long as the proportion of methane in it was over 5 per cent. But the spark discharges, which were to simulate lightning, required a 15 per cent methane concentration before any amino acids were formed. “And even at 15% methane, the production rate of the amino acids by lightning is a million times less than by protons,” added

As long as the methane proportion was over 0.5%, the mixtures shot by protons (solar particles) produced detectable amounts of amino acids and carboxylic acids. But the spark discharges (lightning) required about a 15% methane concentration before any amino acids formed at all.

“And even at 15% methane, the production rate of the amino acids by lightning is a million times less than by protons,” Airapetian added.

Based on the research, it seems that solar particles are a more efficient source of energy than lightning. But according to Airapetian, things are actually much more skewed in favour of solar superflares. Lighting comes from thunderclouds formed by rising warm air. This would have been much more unlikely when the Sun was 30 per cent dimmer.