RNA research sheds light on first life forms

An English chemist has found the hidden gateway to the RNA world, the chemical milieu from which the first forms of life are thought to have emerged on Earth some 3.8 billion years ago.

He has solved a problem that for 20 years has thwarted researchers trying to understand the origin of life — how the building blocks of RNA, called nucleotides, could have spontaneously assembled themselves in the conditions of the primitive Earth. The discovery, if correct, should set researchers on the right track to solving many other mysteries about the origin of life.

Other researchers believe John Sutherland, a chemist at the University of Manchester, has made a major advance in prebiotic chemistry, the study of the natural chemical reactions that preceded the first living cells.

First forms of life

Scientists have long suspected that the first forms of life carried their biological information not in DNA but in RNA, its close chemical cousin. Though DNA is better known because of its storage of genetic information, RNA performs many of the trickiest operations in living cells.

RNA seems to have delegated the chore of data storage to the chemically more stable DNA eons ago. If the first forms of life were based on RNA, then the issue is to explain how the first RNA molecules were formed.

For more than 20 years researchers have been working on this problem. The building blocks of RNA, known as nucleotides, each consist of a chemical base, a sugar molecule called ribose and a phosphate group.

Chemists quickly found plausible natural ways for each of these constituents to form from natural chemicals. But there was no natural way for them to join together.

The spontaneous appearance of such nucleotides on the primitive Earth "would have been a near miracle," two leading researchers, Gerald Joyce and Leslie Orgel, wrote in 1999.

The miracle seems now to have been explained. In article to be published today in Nature, Sutherland and his colleagues Matthew Powner and Beatrice Gerland report that they have taken the same starting chemicals used by others but caused them to react in a different order and in different combinations.

Different order

Instead of making the starting chemicals form a sugar and a base, they mixed them in a different order, in which the chemicals naturally formed a compound that is half-sugar and half-base. When another half-sugar and half-base are added, the RNA nucleotide called ribocytidine phosphate emerges.

A second nucleotide is created if ultraviolet light is shined on the mixture. Sutherland said he had not yet found natural ways to generate the other two types of nucleotides found in RNA molecules, but synthesis of the first two was thought to be harder to achieve.

Zipping together

If all four nucleotides formed naturally, they would zip together easily to form an RNA molecule with a backbone of alternating sugar and phosphate groups. The bases attached to the sugar constitute a four-letter alphabet in which biological information can be represented.

"My assumption is that we are here on this planet as a fundamental consequence of organic chemistry," Sutherland said. "So it must be chemistry that wants to work."

Sutherland's proposal has not convinced everyone. Robert Shapiro, a chemist at New York University, said the recipe "definitely does not meet my criteria for a plausible pathway to the RNA world." If Sutherland's proposal is correct it will set conditions that should help solve the many other problems in reconstructing the origin of life. Darwin, in a famous letter of 1871 to the botanist Joseph Hooker, surmised that life began " ... in some warm little pond, with all sorts of ammonia and phosphoric salts." But the warm little pond has given way in recent years to the belief that life began in some exotic environment like the fissures of a volcano or in the deep sea vents that line the ocean floor.

Sutherland's report supports Darwin. His proposed chemical reaction take place at moderate temperatures, though one goes best at 60 degrees Celsius.