Early earth may have had two key RNA bases.

At some time more than 3 billion years ago, nucleic acids formed in the primordial soup that simmered on primitive Earth. These large molecules, notably RNA and DNA, evolved into the genetic basis of life.

But that prompts the question, What sequence of reactions triggered the evolution of nucleic acids?

The emergence of RNA, believed to be life's chief precursor, presupposes that its basic building blocks--adenine, guanine, cytosine, and uracil--formed first. Although scientists have shown that the purine bases, adenine and guanine, form readily under simulated early Earth conditions, they have also noted that the pyrimidine bases, cytosine and uracil, do not.

Now, Michael P. Robertson and Stanley L. Miller, both biochemists at the University of California, San Diego, have found a way around this obstacle. They describe in the June 29 Nature an "efficient prebiotic route" for the synthesis of cytosine and uracil.

"What we're envisioning," says Robertson, "is a tidal pool filled with water and urea. As the water evaporates, the urea becomes highly concentrated. The concentrated urea would then have reacted with cyanoacetaldehyde [also present in the primordial soup] to form cytosine. The cytosine would then have gone on to form uracil."

The key to solving the problem, says Miller, lies in concentrating the urea. At low concentrations, little cytosine forms. But seawater pools slowly evaporating under a hot noonday sun can become saturated enough to spawn the requisite chemical reactions. Replicating these conditions in the laboratory, the researchers generated healthy amounts of cytosine.

The cytosine then reacted to produce uracil.

In 1953, Miller first showed that amino acids could form under primordial conditions. Recently, he and Robertson demonstrated that primordial RNA may have had greater enzymatic activity than scientists had previously assumed (SN: 5/6/95, p.279).

The latest reactions "provide a plausible route to the pyrimidine bases required in a [primitive] RNA world," the two state. "There no longer seem to be good reasons to believe that the bases used in the first [genetic] macromolecules were radically different from those in DNA and RNA."

As scientists try to piece together the path of life's chemical history, Gerald F. Joyce, a molecular biologist at the Scripps Research Institute in La Jolla, Calif., calls this latest report "another brick in the road."

"In trying to understand how life originated, people are wondering how, in a primitive environment, without enzymes, the building blocks of biological molecules could have formed. The pyrimidines have presented a nagging problem. Until now, no one has shown an efficient, plausible chemical pathway by which these molecules could have been made.

"The latest chapter in this story is the Robertson-Miller synthesis, which is darn efficient," Joyce says. "That is news."