Space rocks could have delivered DNA building blocks to Earth

By Alexandra Witze

Web edition: August 10, 2011

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HINTS OF LIFE

Chemists have found that this meteorite, found in Antarctica and dubbed Lonewolf Nunataks 94102, contains nucleobases, the building blocks of DNA. The nucleobases probably originated in space, the scientists say.

Credit: M. Callahan/NASA GSFC

Scientists have discovered life-related chemicals in nearly a dozen meteorites, the strongest evidence yet that space rocks contain the building blocks of DNA and could have delivered them to Earth.

Several of the chemicals are extremely rare on Earth, suggesting they really are from outer space instead of being just local contamination. “Everything points to these being indigenous to the meteorites,” says Michael Callahan, an analytical chemist at NASA’s Goddard Space Flight Center in Greenbelt, Md.

He and his colleagues published the findings online the week of August 8 in Proceedings of the National Academy of Sciences.

Researchers have spotted other extraterrestrial biological molecules in meteorites before, including amino acids, the building blocks of proteins. The new study looked instead at nucleobases, ring-shaped compounds used to store information in RNA and DNA molecules, which carry life’s genetic blueprints. Nucleobases are at least as important to life as amino acids, Callahan says, and, until now, no one had found nucleobases in meteorites that couldn’t potentially have been contaminants from Earth.

To tackle the question, Callahan’s team probed the chemistry of 12 meteorites, including nine that had been scooped up on the Antarctic ice sheet. Eleven contained at least one nucleobase, adenine, that is common on Earth. Others were more exotic. Two of the meteorites, known as Murchison and Lonewolf Nunataks 94102, turned out to host a wide variety of nucleobases, including three that are rare on Earth. These rare nucleobases are from a class called purines.

The researchers carefully analyzed the Antarctic ice, earthly soil and other control samples that could show whether terrestrial contamination was a problem. The team didn’t find the nucleobases anywhere but in the meteorites.

To figure out how the chemicals got there, the scientists mixed hydrogen cyanide, ammonia and water — common meteorite ingredients — in the laboratory. The resulting chemical reactions yielded the same kinds of nucleobases seen in the meteorites.

Callahan now wants to see whether he can find nucleobases in other space rocks.

The new study significantly bolsters the evidence that nucleobases can form in places other than Earth, says Alan Schwartz, a chemist at Radboud University Nijmegen in the Netherlands. Schwartz has looked for purines before in the Murchison meteorite, and the new study reported much lower levels of those chemicals than he did — suggesting that nucleobases, where they exist, might be spread patchily through the rock.

A second paper appearing online in Proceedings of the National Academy of Sciences the same week looks at the biological contents of meteorites from a different angle. A team led by George Cooper of NASA’s Ames Research Center in Moffett Field, Calif., discovered chemicals such as pyruvic acid and citric acid in several meteorites, including Murchison. On Earth, these chemicals are key players in the citric acid cycle, which cells need to respire and survive.

The papers both suggest that space rocks contain more of life’s ingredients than once thought.