Scientists at Scripps Research have discovered a set of chemical reactions that they believe produced the building blocks of life out of materials that might have been commonplace on the earth a few billion years ago. A nutrient-rich mixture, which is often referred to as the “primordial soup”, gave rise to the most primitive lifeforms.
At school, children learn that these chemical reactions involved interactions between molecules due to the energy obtained from events such as lightning strikes or hydrothermal vents, forming basic organic compounds — essentially amino acids — which then linked up to form peptides and proteins and, eventually, living cells.
The exact chemical reactions are more complex. Scientists are not sure of the exact process that created life on Earth, but they kept trying to concoct the elusive primordial soup, on the basis of what they thought was plentiful at the time. They exposed the mixture to different conditions to see what happened and how easily life’s precursors might arise.
The research work published in the journal Nature Chemistry says that the Scripps scientists tweaked their own primordial soup recipe and discovered a new set of chemical reactions using relatively simple ingredients that were most probably common on the planet 3.7 billion years ago. They used cyanide, ammonia, carbon dioxide and alpha-keto acids. The soup soon began producing amino acids.
Every ingredient had a distinct role to play. Alpha-keto acids are the precursors that living cells now use to make amino acids. Ammonia is a source of nitrogen, which the conversion process needs. Cyanide — a substitute for enzymes in animal bodies — converts while carbon dioxide speeds the process up.
“We were expecting it to be quite difficult to figure this out, and it turned out to be even simpler than we had imagined,” said Ramanarayanan Krishnamurthy, the lead author of the study. “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.”
According to the team, this basic process is how amino acids are formed in living cells, where, instead of cyanide, enzymes that were non-existent when life shaped up on this planet did their job in the primordial soup. The group of scientists say the simplicity and similarity to current biological processes suggest that this is a more probable source of early life than other hypotheses, which require radically different chemical reactions.
When the scientists analysed the chemical soup, they saw it produce orotate as a byproduct. Orotate forms a step before nucleotides, which make up DNA and RNA, which suggests that a whole range of ingredients of life could have been produced this way.
“What we want to do next is continue probing what kind of chemistry can emerge from this mixture,” said Krishnamurthy. “Can amino acids start forming small proteins? Could one of those proteins come back and begin to act as an enzyme to make more of these amino acids?”