Once upon a time, when our planet Earth was very young and very new, there was not a single scrap of life on it to be found.
Then, somewhere, somehow, some quirk of chemistry happened, and the molecular building blocks of our very first single-celled ancestors emerged: the amino acids and nucleic acids that came together in just the right way to continue a chain reaction that gave rise to life.
We're not entirely sure of the details of this emergence, which took place billions of years ago, and left no trace on the fossil record. But using what we know of the chemistry of early Earth, scientists have found a new series of chemical reactions that could have produced those biological building blocks on Earth, all those eons ago.
"We've come up with a new paradigm to explain this shift from prebiotic to biotic chemistry," said chemist Ramanarayanan Krishnamurthy of 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 could have unfolded is largely experimental. Scientists take what they know of current biological processes, and try to recreate them in laboratory settings using the chemistry of early Earth, prior to 3.7 billion years ago.
Evidence suggests that one of the molecules present was cyanide; deadly to consume, but possibly instrumental to 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 their success trying to create more complex organic molecules – amino acids, of which all proteins in living cells consist.
Today, the precursors of 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 early Earth, enzymes didn't, which has led scientists to the conclusion that amino acids had to have formed from precursors called aldehydes instead. That raises a bunch of other questions, though, such as when α-keto acids took over.
Krishnamurthy and his colleagues thought that there may be a pathway via 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 also present on early Earth, was added next, to contribute the required nitrogen. It took a bit of trial and error to figure out the final part, but, just as the researchers had found with their previous work, the key ended up being carbon dioxide.
"We were expecting it to be quite difficult to figure this out, and it turned out to be even simpler than we had imagined," Krishnamurthy said. "If you mix only the keto acid, cyanide and ammonia, it just sits there. As soon as you add carbon dioxide, even trace amounts, the reaction picks up speed."
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 by-product 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 that take place in living cells today, which means the finding would negate the need to explain why cells made the switch from aldehydes to α-keto acids. The team therefore believes that their finding 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. In turn, this 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 Nature Chemistry.