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Wednesday, June 18, 2008

Were Meteorites the Origin of Life on Earth?

A new study finds that a pair of chemical building blocks similar to those in genetic material was present in a meteorite before it fell to Earth in the 1960s. Researchers say the finding makes it slightly more plausible that meteorite bombardments may have seeded ancient Earth with life's raw materials, potentially paving the way for life itself.

Part of the scientific mystery of how life emerged is the origin of chemical building blocks: Were they created by chemical reactions on Earth or did they, perhaps, hitch rides on meteorites that may have germinated our and other planets in our solar system with the same molecules? By studying meteorite fragments, such as those that fell in 1969 near the town of Murchison, in the southeastern Australian state of Victoria, researchers have learned that they contain carbon-based compounds that most likely formed in space, including sugars and amino acids.

They were not sure, however, about a class of compounds called nucleobases, which when fused with sugar molecules are the building blocks of nucleic acids such as DNA (the stuff of genes) and its close cousin RNA (produced when genes switch on). Researchers have speculated that life may have arisen from RNA molecules that acquired the ability to copy themselves. But they have had a hard time generating nucleobases in experiments designed to mimic chemical conditions on the early Earth, says Zita Martins, a chemist and astrobiologist at Imperial College London.

To check the origin of two nucleobases—uracil, found in RNA, and xanthine, a common cellular constituent—Martins and her colleagues analyzed the ratio of carbon isotopes in the two compounds. Most carbon on Earth consists of carbon 12, named for the number of protons and neutrons in its atomic nucleus, rather than its slightly heavier cousin, carbon 13.

Martins and her colleagues compared carbon isotopes in the Murchison nucleobases to soil samples from Murchison as well as to a common mineral. As expected if the nucleobases were forged in deep space, they were richer in carbon 13—by 44.5 percent for uracil and 37.7 percent for xanthine—compared with the other samples, the group reports in Earth and Planetary Science Letters.

"It really clarifies at least that the building blocks of genetic material, the nucleobases, were available" in the early Earth, Martins says. "We are not saying that only meteorites contributed to the building blocks of life," she adds, "but it's a very great contribution."

The concentration of nucleobases in the Murchison meteorite is relatively low. Martins and her co-workers needed 0.5 ounce (15 grams) of space rock to extract their sample, compared with milligram-size samples for other chemicals, she says. But researchers believe that space rocks and dust once rained onto Earth in billions of tons per year.

Other researchers say the finding appears to be solid, although some are skeptical of its significance. Robert Shapiro, a professor emeritus and senior research scientist in chemistry at New York University, says that because of their low concentration, extraterrestrial nucleobases were unlikely to have played much of a role in kick-starting life. "They're a subunit of a subunit of DNA," he says. "My opinion is that their amounts were utterly unimportant and insignificant." He says he would be more impressed if whole nucleosides—bases plus sugars—were found in meteorites in concentrations similar to those of amino acids.

And researchers may yet discover ways that Earthly chemistry—perhaps around hydrothermal vents—could have generated nucleobases and other compounds.

Conel Alexander, a geochemist at the Carnegie Institution of Washington who specializes in meteorites, says that without more data, claims about the amounts and sources of molecules on early Earth should be taken with a grain of salt. "It really comes down to quantitative arguments about how much was made on Earth [and] how much was brought in from space," he says. "Any honest person would keep an open mind about the whole issue."
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