How did life begin on Earth? While there are several theories, an Indian-origin scientist working in the USA has proposed a new set of "origins of life", chemical reactions that can lead to the development of amino acids - the building blocks of proteins and DNA – from chemicals and gases present on ancient Earth.
Four billion years ago, Earth looked very different, devoid of life and covered by a vast ocean. Over the course of millions of years, in that primordial soup, life emerged.
There are multiple hypotheses on how life emerged from inorganic molecules. Now, a team led by Ramanarayanan Krishnamurthy at Scripps Research Institute, La Jolla, California has discovered a new set of chemical reactions that use cyanide, ammonia and carbon dioxide — all thought to be common on the early earth — to generate amino acids and nucleic acids, the building blocks of proteins and DNA.
"We came up with a new paradigm to explain such a shift from pre-biotic to biotic chemistry," said Krishnamurthy, lead author of the study.
"The experiments led to the formation of the amino acids that are produced by the modern citric acid cycle - glycine, alanine, aspartic acid and glutamic acid. In modern biology, amino acids are converted to proteins by polymerisation and also to nucleobases," Krishnamurthy, who did his graduation from Vivekananda College in Chennai and M.Sc from IIT-Bombay, told DH.
The experiment also required water that was abundant on the carth and a chemical named alpha keto acid, which may have been formed by ultra violet-light mediated chemical reactions of carbon dioxide.
Alpha keto acids are precursors from which amino acids are generated in cells in the presence of nitrogen and enzymes.
In the 1920s Russian scientist Aleksandr Oparin and English scientist J B S Haldane separately proposed that life on Earth could have arisen from non-living matter through a process of "gradual chemical evolution".
Three decades later, Stanley Miller and Harold Urey in 1953 demonstrated that it was indeed possible to create organic molecules required for life on the primordial earth spontaneously.
In their quest to get an answer to one of the most fundamental questions, Krishnamurhty, his colleagues Sunil Pulletikurti and Mahipal Yadav, and Greg Springsteen from Georgia Institute of Technology, Atlanta decided to use cyanide to set off chemical reactions that can convert prebiotic molecules and water into basic organic compounds required for life.
They got the cue from their earlier success when they used cyanide to drive other chemical reactions. The plan was to try cyanide, without enzyme, and see if it helps turn alpha-keto acids into amino acids.
As they knew nitrogen would be required in some form, they added ammonia — a form of nitrogen that would have been present on early earth. Then, through trial and error, they discovered a third key ingredient: carbon dioxide. With this mixture, the team began to see formation of amino acids.
Unlike previously proposed reactions, this one worked at room temperature and in a wide acid-base range.
"We were expecting it to be quite difficult to figure out, but it turned out to be even simpler than we had imagined," said Krishnamurthy. "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."
Because the new reaction is relatively similar to what occurs today inside cells —except for being driven by cyanide instead of a protein — it seems more likely to be the source of early life, rather than drastically different reactions, the researchers say.
The study – published in Nature Chemistry last week - also helps bring together two sides of a long-standing debate about the importance of carbon dioxide to early life, concluding that carbon dioxide was key, but only in combination with other molecules.