Right-handed people may predominate here on Earth, but all of us are built from amino acids that are chemically "left handed." Now two NASA scientists studying meteorites older than our planet have found a majority of left-handed amino acids, suggesting that our solar system has always had a preference for southpaws.
Since the days of Louis Pasteur, scientists have known that amino acids, the building blocks of proteins, come in two varieties. There are left-handed and right-handed versions, with a hydrogen atom representing the "thumb" in both cases. For unknown reasons, organisms on Earth contain only left-handed amino acids. The question is why?
For the past 4 years, astrobiologists Daniel Glavin and Jason Dworkin of NASA's Goddard Space Flight Center in Greenbelt, Maryland, have been searching for the answer by carefully studying the molecular deposits inside six meteorites found in Antarctica and Australia. The space rocks are more than 4.5 billion years old, making them older than Earth. Today in the Proceedings of the National Academy of Sciences, the researchers report that they expected to find a 50:50 ratio of right- to left-handed amino acids in the rocks, but instead they found the ratio of amino acids tilted toward left-handedness in all six specimens. In one of the rocks, the imbalance was 18%, the largest ever reported for a meteorite. "I have to admit I didn't believe it at first," Glavin says.
So how did the left-handed molecules gain such an early advantage? Glavin says the process might have started when the amino acids made contact with melting ice inside the meteorites' parent asteroids--water tends to help left-handed amino acids multiply and dominate. But that's only part of the answer, he says. Polarized ultraviolet radiation in space might also have helped shift the balance toward southpaw molecules.
The relative abundance of left-handed molecules may explain why organisms on Earth preferred them, Glavin says. Whatever the reason, he notes, life as we know it could just as easily have been given a nudge toward the right-handed side in a different environment.
That possibility raises an intriguing question, Dworkin says. If life elsewhere is based on right-handed molecules, humans might have some difficulty interacting with it. When right-handed and left-handed amino acids come together in an organic environment in the lab, says Dworkin, they don't interact in any biologically useful way. "It all turns out to be scrambled eggs."
The study doesn't close the case on whether the solar system truly prefers left-handed amino acids, says biochemist Sandra Pizzarello of Arizona State University in Tempe. For one thing, she says, most meteorites quickly become contaminated when they land here. "Researchers have seen bacteria happily growing inside them," notes Pizzarello, who co-authored the first paper on the preponderance of lefty amino acids in meteorites in 1997 in Science. She says that if these bugs prefer to eat right-handed amino acids, they could skew the ratio. As to whether a skewed left-handed amino acid ratio in meteorites was responsible for the bias seen on Earth, cosmogeochemist Jeffrey Bada of the University of California, San Diego, has his doubts. "It's a lot to ask from a natural geochemical process, which, basically, we know nothing about," he says.