New set of chemical reactions may finally explain how life started on Earth

New set of chemical reactions may finally explain how life started on Earth

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

Then, somewhere, somehow, some quirk of chemistry happened, and the molecular building blocks of our very first single-celled ancestors appeared: the amino acids and nucleic acids that came together in just the right way to continue a chain reaction which 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 the chemistry of the early Earth, scientists have found a new series of chemical reactions that could have produced the biological building blocks on Earth all those eons ago.

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

Reconstructing how biotic chemistry might have unfolded is largely experimental. Scientists take what they know about current biological processes and try to recreate them in a laboratory environment using the chemistry of early Earth, before 3.7 billion years ago.

Evidence suggests that one of the molecules present was cyanide; deadly to consume, but possibly instrumental in the emergence of life on Earth. Cyanide’s role 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 across a wide range of pH conditions. With some carbon dioxide thrown in, this reaction really picks up speed.

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

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

Krishnamurthy and his colleagues believed that there may 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 that it is an effective driver of chemical reactions that produce organic molecules.

Ammonia, a compound of nitrogen and hydrogen that was also present on early Earth, was then added to contribute the necessary nitrogen. It took some trial and error to figure out that last part, but just as the researchers had found 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 the keto acid, cyanide and ammonia, it just stays. As soon as you add carbon dioxide, even trace amounts, the reaction speeds up.”

Together, the team’s overall results suggest that carbon dioxide was an important 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. This is one of the building blocks of nucleic acids, including DNA and RNA.

And the team’s results are very similar to reactions taking place in living cells today, meaning the finding would obviate the need to explain why cells switched from aldehydes to α-keto acids. The team therefore believes that their findings represent 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. In turn, this will help establish the plausibility and improbability of the various scenarios describing the humble beginnings of all life on Earth.

The research is published in Natural chemistry.

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