A new set of chemical reactions could finally explain how life began on Earth

Once upon a time, when our planet Earth was very young and very new, there was not a single piece of life on it.

Then somewhere, somehow, a quirk of chemistry happened, and the molecular building blocks of our very first single-celled ancestors emerged: the amino acids and nucleic acids that brought together in the right way to continue a chain reaction that gave rise to life.

We are not entirely sure of the details of this emergence, which took place billions of years ago, and left no trace in the fossil record. But using what we know about early Earth chemistry, scientists have discovered a new set of chemical reactions that could have produced these biological building blocks on Earth eons ago.

“We have found a new paradigm to explain this shift from prebiotic chemistry to biotic chemistry,” said chemist Ramanarayanan Krishnamurthy of the Scripps Research Institute. “We think the kind of reactions we’ve described are likely what could have happened on early Earth.”

Reconstructing how biotic chemistry might have unfolded is largely experimental. Scientists are taking what they know about current biological processes and trying to recreate them in the lab using chemistry from early Earth 3.7 billion years ago.

Evidence suggests that one of the molecules present was cyanide; deadly to consume, but perhaps decisive for the emergence of life on Earth. The role of cyanide in the process has been explored by a number of teams around the world; earlier this year, Krishnamurthy and his colleagues showed how cyanide can easily produce basic organic molecules at room temperature and a wide range of pH conditions. With a little carbon dioxide, this reaction really speeds up.

This led the researchers to wonder if they could replicate their success by trying to create more complex organic molecules – the amino acids, which all proteins in living cells are made of.

Today, the precursors of amino acids are molecules called α-keto acids, which react with nitrogen and enzymes to produce amino acids. Although α-keto acids probably existed on early Earth, enzymes did not, leading scientists to conclude that amino acids must instead have formed from precursors called aldehydes. . This raises a bunch of other questions though, like when α-keto acids took over.

Krishnamurthy and his colleagues thought there might be a pathway by which α-keto acids can form amino acids without the presence of enzymes. They started with α-keto acids, of course, and added cyanide, since their previous experiments showed it to be an efficient engine of chemical reactions that produce organic molecules.

Ammonia, a compound of nitrogen and hydrogen also found on early Earth, was then added to provide the necessary nitrogen. It took a bit of trial and error to figure out the final part, but just as the researchers had discovered with their previous work, the key ended up being carbon dioxide.

“We expected it to be quite difficult to figure this out, and it turned out to be even easier than we imagined,” Krishnamurthy said. “If you just mix keto acid, cyanide, and ammonia, it stays there. As soon as you add carbon dioxide, even in trace amounts, the reaction speeds up.”

Combined, the team’s overall results suggest that carbon dioxide was a vital ingredient for the emergence of life on Earth – but only when combined with other ingredients. The team also discovered that a byproduct of their reactions is a molecule similar to a compound produced in living cells called orotate. It is one of the building blocks of nucleic acids, including DNA and RNA.

And the team’s findings are very similar to reactions that occur in living cells today, meaning the discovery would negate the need to explain why cells switched from aldehydes to α-keto acids. The team therefore believe that their discovery represents a more likely scenario for the emergence of prebiotic molecules than the aldehyde hypothesis.

The next step is to conduct further experiments with their chemical soup to see what other prebiotic molecules might emerge. This, in turn, will help establish the plausibility and implausibility of the various scenarios describing the humble beginnings of all life on Earth.

The research has been published in natural chemistry.

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