The odds are improving that life exists beyond Earth. A European-U.S. team reports that a meteorite that formed billions of years ago and eventually crashed on our planet harbors two important components of RNA and DNA, the fundamental molecules of life. The findings could help explain how life got started on Earth, and they suggest that the ingredients for life have been liberally sprinkled throughout the solar system, if not the galaxy.
So far, the clues for extraterrestrial life are tantalizing but inconclusive. Radio astronomers have found organic chemicals floating in clouds of dust between the stars (ScienceNOW, 28 March). NASA's twin Mars Exploration Rovers have discovered evidence that liquid water once flowed, at least fitfully, on the Red Planet's surface (ScienceNOW, 13 December 2006). And planetary probes have uncovered organic molecules in the atmospheres of Jupiter, Saturn, and Saturn's giant moon Titan, as well as amino acids in meteorites.
The latest evidence is not a slam dunk, but it does suggest that life's most important molecule could form off our planet. The researchers dissolved and purified fragments from the Murchison meteorite, which was found in Australia in 1969. Chemical analysis showed that the meteorite contains xanthine and uracil, substances called nucleobases that are necessary for RNA and DNA to form their base pairs as part of their replication process. Furthermore, the researchers report in the 15 June issue of Earth and Planetary Science Letters, the carbon atoms in the two substances are in a heavy form that is extremely rare on Earth. As such, the findings represent the most complex molecules ever found to have originated in space.
"The two [molecules] present in the meteorite are definitely extraterrestrial," says astrobiologist and lead author Zita Martins of Imperial College London. She says it's unclear whether they were synthesized on the meteorite's parent asteroid or were formed in space and then stuck to the meteorite. "But we know that they may be formed in a variety of cosmic environments." From there, the molecules "were delivered ready-made to the early Earth," says astrobiologist and co-author Mark Sephton, also of Imperial College London. "If the early solar system was a store cupboard, it would have many of the simplest ingredients for the recipe of life."
Experts agree that the find is tantalizing. The work shows that the chemistry needed to make "the information-storage systems of life can take place in the extreme environment of space," says astrobiologist Andrew Steele of the Carnegie Institution of Washington. These molecules might not lead to life directly in space, he adds, but they might have jump-started it once they arrived on Earth. Astrobiologist Michael Mumma of NASA's Goddard Space Flight Center in Greenbelt, Maryland, adds that "the research demonstrates that bodies like Murchison likely delivered DNA precursors to planets in the solar system when they were young. ... If anything, the findings intensify interest in the possibility of life on Mars and large moons such as Titan and Europa, which are suspected of containing liquid water beneath their ice shells."