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

Once upon a time, when the planet Earth was very young and very new, there was no part of life on it.

Then, somewhere, somehow, something strange happened in chemistry, and the molecular building blocks of our first unicellular ancestors emerged: amino acids and nucleic acids that came together in just the right way to continue the chain reaction that gave rise to life.

We are not entirely sure of the details of this apparition, which occurred billions of years ago, and left no trace in the fossil record. But using what we know about Earth’s early chemistry, 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 model to explain this shift from prebiotic to biochemistry,” said chemist Ramanarayanan Krishnamurthy of the Scripps Research Institute. “We think the type of reactions we’ve described are most likely what would have happened on early Earth.”

Reconstructing how biochemistry might unfold is largely experimental. Scientists are taking what they know about current biological processes, and trying to recreate them in laboratory environments using the chemistry of the early Earth, 3.7 billion years ago.

Evidence suggests that one of the molecules found was cyanide; Fatal for consumption, but perhaps beneficial 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 colleagues demonstrated how cyanide can readily produce essential organic molecules at room temperature and across a wide range of pH conditions. With some CO2 thrown in, this reaction really gets faster.

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

Today, amino acid precursors are molecules called alpha-keto acids, which react with nitrogen and enzymes to produce amino acids. Although alpha-keto acids may have been present on Earth early on, the enzymes did not, leading scientists to conclude that the amino acids must have been formed from precursors called aldehydes instead. This raises a host of other questions, such as when alpha keto acids take over.

Krishnamurthy and his colleagues thought there might be a pathway by which alpha-keto acids could synthesize amino acids without the presence of enzymes. They started with α-keto acids, of course, and added cyanide, as their previous experiments had shown to be an effective driver of chemical reactions that produce organic molecules.

Ammonia, a compound of nitrogen and hydrogen also found in early earth, was then added to contribute the needed nitrogen. It took a little trial and error to figure out the last part, but, just as the researchers found in their previous work, the key ended up being carbon dioxide.

“We were expecting it to be very difficult to figure out, and it turned out to be simpler than we had imagined,” Krishnamurthy said. “If you just mix keto acid, cyanide, and ammonia, it’s still there. Once you add carbon dioxide, even trace amounts, the reaction speeds up.”

Taken together, the team’s overall results indicate that carbon dioxide was a vital component of the emergence of life on Earth – but only when combined with other components. The team also discovered that the 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.

The team’s results are very similar to the reactions that occur in living cells today, meaning that the result would eliminate the need to explain why cells switch from aldehydes to alpha-keto acids. So the team believes that their discovery represents a more likely scenario for the emergence of prebiotic molecules than the aldehyde hypothesis.

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

The search was published in nature chemistry.

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