Left-handed people may be in the minority, but left-handed amino acids rule the Earth. Researchers have long known that the building blocks of proteins can be constructed in either "left-handed" or "right-handed" versions that are mirror images of each other, but that almost every living organism on Earth uses left-handed amino acids. Now, a new study gives weight to a theory of how that preference came to pass. NASA researchers examined meteorites that predate the Earth's formation, and say that those early rocks also have a preponderance of left-handed molecules.
"Meteorites would have seeded the Earth with some of the prebiotic compounds like amino acids that are needed to get life started, and also biased the origin of life to the left-handed amino acid form," says [study coauthor] Daniel Glavin [New Scientist].
Researchers note that if
you make amino acids from scratch in a lab using their chemical components, you inevitably get half of the right-handed version and half of the left handed version. So it might be expected that if nature makes amino acids in space using similar chemistry, you'd also get a fifty-fifty mixture [CBC].
Yet that's not what Glavin and his colleagues found when they studied the molecular deposits in six meteorites that are more than 4.5 billion years old. 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 [ScienceNOW Daily News].
The new study, published in the Proceedings of the National Academy of Sciences, also suggests various methods by which lefty molecules may have gained the upper hand.
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 [ScienceNOW Daily News].
Not everyone is convinced by the new findings. Astrobiologist Sandra Pizzarello argues that
most meteorites quickly become contaminated when they land here. "Researchers have seen bacteria happily growing inside them," notes Pizzarello.... She says that if these bugs prefer to eat right-handed amino acids, they could skew the ratio [ScienceNOW Daily News].
Still, if the results bear up under future scrutiny, it suggests that we humans would have something in common with any extraterrestrial life that might exist in our solar system. Studyd coauthor Jason Dworkin says that
"if the bias toward left-handed amino acids began in space, it likely extends across the solar system, so any life we may find on Mars, for example, will also be left-handed" [Astrobiology Magazine].
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