When Earth was so young, there was no trace of life on it, then somewhere, somehow, a strange chemical reaction took place that gave rise to the molecular building blocks of our first single-celled ancestors: amino acids and nucleic acids that came together the right way to continue The chain reaction that led to the emergence of life on Earth.
The role of prebiotic chemistry in the emergence of life on Earth
But scientists are still not entirely sure about the details of this appearance, which occurred 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 likely produced the biological building blocks of Earth eons ago.
“We have come up with a new paradigm to explain the transition from pre-biochemistry to biochemistry,” said chemist Ramanarayanan Krishnamurthy of the Scripps Research Institute. “We suspect that the type of interactions we described likely occurred early in the Earth’s life.”
And figuring out how biochemistry emerged is largely experimental, as scientists gather what they know about current biological processes and try to recreate them in laboratory environments using early Earth chemistry, 3.7 billion years ago.
Cyanide helped create life on Earth
Evidence suggests that cyanide was one of the molecules present at that time; It is a toxic substance, but it may have helped the emergence of life on Earth. Teams around the world have explored the role of cyanide in the process. Earlier this year, Krishnamurthy and colleagues demonstrated how cyanide can readily produce basic organic molecules at room temperature and across a wide range of pH conditions. The speed of this reaction is increased by adding a little carbon dioxide.
This prompted the researchers to wonder if they could replicate their success in trying to create more complex organic molecules – such as the amino acids that make up all proteins in living cells.
The precursors of amino acids are molecules called alpha-keto acids, and they react with nitrogen and enzymes to produce amino acids. Alpha-keto acids may have been present on Earth early on, but enzymes were not, leading scientists to conclude that amino acids must have formed from precursors called aldehydes. This raises a host of other questions, such as when alpha-keto acids replaced aldehydes.
Carbon dioxide is the key to life on Earth
Krishnamurthy and colleagues suggest the existence of a pathway that enables alpha-keto acids to synthesize amino acids without the enzymes present. They started with alpha-keto acids, of course, and added cyanide, which their previous experiments had shown to be an effective catalyst for chemical reactions that produce organic molecules.
The team then added ammonia to contribute to the formation of the required nitrogen, knowing that ammonia is a compound of nitrogen and hydrogen that was also found in the early stages of the Earth’s life. It took a bit of trial and error to figure out the last part, and the researchers found, as they had found in their previous work, that the key was carbon dioxide.
“We were expecting it to be hard to figure out, but it turned out to be simpler than we thought. If you just mix keto acid, cyanide and ammonia, it will stay the same for a long time,” Krishnamurthy said. “And once you add even a trace of carbon dioxide, the reaction speeds up.”
The team’s overall findings suggest that carbon dioxide was a vital component of life on Earth – but only when combined with other ingredients. The team also discovered that the byproduct of their reactions is a molecule similar to a compound produced in living cells called orotes [orotate]It is one of the basic building blocks of nucleic acids, including DNA and RNA.
The team’s findings are similar to the reactions that occur in living cells today, meaning that the result eliminates the need to explain why cells dispense with aldehydes and switch to alpha-keto acids. Therefore, the team believes that their discovery represents a scenario for the emergence of prebiotic chemistry molecules and supports the aldehyde hypothesis.
The next step is to further experiment with this chemical mixture to see what other prebiotic molecules might appear. In turn, this will help demonstrate the plausibility and implausibility of the various scenarios that describe the humble beginnings of all life on Earth.
Nature Chemistry