With apologies to Carl Sagan, we are
With apologies to Carl Sagan, we are “comet stuff.” A comet’s organics, detected by the Rosetta mission, figure in a new RNA World mechanism. [SDenisov/Getty]

“The cosmos is within us,” said Carl Sagan. “We are made of star stuff.” Memorable words, but they don’t explain how “star stuff”—pretty much all the chemical elements—could have come together in various ways to form life’s chemical building blocks—proteins and nucleic acids. No doubt Sagan was aware that the story of chemical evolution still needed to be worked out in detail, so if he were still with us today, he might have amended his words in light of new information. He might have said that we are made of “comet stuff.”

New information has indeed been found. And appropriately enough, it has come from outer space. Recent discoveries made by the European Space Agency's Rosetta mission to the comet 67/P/Churyumov-Gerasimenko (67P/C-G) have led scientists to propose an efficient mechanism for the prebiotic synthesis of a vital class compounds—the large purine bases A and G. These bases presumably arose from simple organic compounds, and, in living organisms, they help encode the genetic information stored in RNA and DNA. They also form part of the molecules adenosoine triphosphate (ATP) and guanosine triphosphate (GTP), both of which serve as energy sources for biochemical reactions and as molecular switches in the control of protein function.

Yet how could A and G have accumulated in the first place? Such questions are entertained by scientists working on the “RNA World” hypothesis, which posits that RNA was one of the first self-replicating molecules leading to the origin of life. Among these scientists are chemists at Ludwig-Maximilians-Universitaet (LMU) in Munich. They analyzed simple organic molecules that were detected on the surface of the 67P/C-G comet, and they developed a plausible mechanism for how these molecules could have served as purine base precursors.

The LMU scientists, led by Thomas Carell, presented their work May 13 in the journal Science, in an article entitled “A High-Yielding, Strictly Regioselective Prebiotic Purine Nucleoside Formation Pathway.” In this article, the LMU scientists assert that their mechanism is more efficient than one that had been proposed earlier. The earlier mechanism called for a single prebiotic pathway for purine nucleosides, but it lacked regioselectivity and would have produced low yields.

“We report that the condensation of formamidopyrimidines (FaPys) with sugars provides the natural N-9 nucleosides with extreme regioselectivity and in good yields (60%),” the authors of the Science article wrote. “The FaPys are available from formic acid and aminopyrimidines, which are in turn available from prebiotic molecules that were also detected during the Rosetta comet mission.”

The LSU authors added that this nucleoside formation pathway can be fused to sugar-forming reactions to produce pentosides, providing a plausible scenario of how purine nucleosides may have formed under prebiotic conditions.

When Rosetta's lander module Philae first made contact with the 67P/C-G comet's surface, it bounced off and dust was wafted into its mass spectrometer. The ensuing analysis enabled mission scientists to identify 16 simple organics in the sample. In addition to water and carbon monoxide, the catalog included a number of nitrogen-containing components, such as formamide and hydrogen cyanide.

“We have now looked for ways in which these very simple substances could have given rise to the complex organic building-blocks of life under conditions similar to those that are thought to have existed on the young Earth. In particular, we were interested in the synthesis of key components of RNA,” Carell explained.

“Some 70% of the products of the FaPy pathway are purines, with adenosine—an important subunit of RNA—accounting for about 20%. With the FaPy mechanism, we have thus discovered a synthetic pathway that provides central biochemical components of life in high yield and with high specificity,” Carell continued. “So the FaPy mechanism constitutes an experimentally attested scenario that can explain how the process of chemical evolution could have proceeded during the phase prior to the formation of the first cells.”

By developing the FaPy mechanism, the LMU scientists have not only added support to the RNA World hypothesis, they have also contributed to an even larger story, one that Sagan characterized in these words: “We are a way for the cosmos to know itself.”