Piece of the puzzle: The chemical structure of RNA.
Credit: Wikimedia
PARIS: Chemists have figured out key new steps in the process by which life on Earth may have emerged from a seething soup of simple chemicals.
Genetic information in living organisms today is held in deoxyribonucleic acid (DNA), the famous "double helix" molecule of sugar, phosphate and a base.
But DNA is too sophisticated to have popped up in an instant, and one avenue of thought says its single-stranded cousin, ribonucleic acid, or RNA, came first.
RNA first?
RNA plays a key role in making proteins and, in viruses, is used to store genetic code. It is chemically similar to DNA but is simpler and tougher in structure, and thus looks like a good candidate for Earth's first information-coding nucleic acid.
But for all its allure, the "RNA first" theory has run into practical problems. Its three ingredients – the base, ribose sugar and phosphate – must have formed separately and then combined to form the molecule, according to conventional thinking.
Critics, though, say that RNA, while somewhat simpler than DNA, is still a complex molecule and could not have been assembled spontaneously. These doubters have been comforted by the failure to find any feasible chain of chemical events to explain how the three components all came together.
New theory
But a paper published today in the British journal Nature, by chemists at the University of Manchester, in England, puts forward a different explanation. The team, led by John Sutherland, venture that an RNA-like synthesis took place through a series of chemical reactions and an important intermediate substance.
Their lab model used starting materials and environmental conditions that are believed to have been around in early Earth and are also used in the standard "RNA first" scenario.
Their theory starts with a simple sugar called glycolaldehyde, which reacts with cyanmide (a compound of cyanide and ammonia) and phosphate to produce an intermediate compound called 2-aminooxazole.
Gentle warming from the Sun and cooling at night help purify the 2-aminooxazole, turning it into a plentiful precursor which contributes the sugar and base portions of the new ribonucleotide molecule.
The presence of phosphate and ultraviolet light from the Sun complete the synthesis.
Elegant explanation
In a commentary also published by Nature, U.S. molecular biologist Jack Szostak hailed the research as an elegant explanation as to why the sugar and base would not have to form separately before forming the new molecule.
"It will stand for years as one of the great advances in prebiotic chemistry [the term for the study of the chemical processes that led to life on Earth]," he enthused.
Opinions vary as to when the first organisms appeared on Earth. One estimate, based on fossilised mats of bacteria found in Australia, is that this happened around 3.8 billion years ago, around 700 million years after the planet was formed.
