How Life Became Left-Handed

Lefties are just 11 percent of the world population. But take a close look at anyone's amino acids, which are protein's and thus life's tiny building blocks, and left-handers rule.

Left-handed amino acids are mirror images of their right-handed buddies, and though similar, the mirrored molecules are chemically different. For some reason, nearly every single organism exclusively relies on lefties.

Why does life discriminate on a molecular level? Neutron starlight might have zapped amino acids riding on comets and asteroids into a bias, and a little water might have concentrated them after they crashed into Earth, a team of scientists now say.

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If so, our planet's first inhabitants would have little choice other than to rely on the lefty acids.

"People have been exploring this question for years, and I think we've put some pieces together that make it not so puzzling," said Ronald Breslow, a chemist and biologist at Columbia University in New York.

Selective destruction

In nature, chemical reactions that make amino acids form equal amounts of lefties and righties, which is why the extreme bias for the left-handed molecules on Earth is such a mystery.

Breslow said scientists have discovered ancient amino acids that hitchhiked onto Earth on meteorites in the past, but no one could adequately explain why there was five to 10 percent more of the left-handed types in the chunks of rock.

"The molecules are loaded with heavy isotopes, so we knew they came from space," Breslow said. "What is so mysterious is that there isn't a 50-50 mix of right- and left-handed versions."

But Breslow explained that neutron stars — super-dense, fast-spinning remnants of supernovas — could be the culprit as they emit circularly polarized light, sort of like a laser. High-energy beams coming out of one pole are polarized in a right-handed way, while the other pole shoots out left-handed rays.

While right-handed light breaks apart some of both versions of amino acids, it preferentially zaps more of the righties because their molecular structure better absorbs the light energy. (The process is comparable to how dark objects heat up quicker in the sun than light objects.)

If amino acid-peppered asteroids or comets soaked up more right- than left-handed neutron starlight before they smacked into Earth, Breslow said it could explain why the space rocks show a bias for the lefty molecules.

From concentrate

Add a little desert heat and occasional water to meteorite debris, Breslow said, and the story gets more interesting. As the water dries up, the amino acids crystallize and leave behind a concentrated residue of lefties.

"Amino acids like to stack up one right-handed molecule to one left-handed," Breslow said. "It's as if you had a big pile of gloves with a few more left gloves. If you pair them up, pretty soon you've just got the lefty gloves."

Nobody knows how amino acids and other chemistry mixed or was zapped to become life. But the lefty acidic concentrate would have been the feeding stock for Earth's first organisms, the thinking goes, as most of the right-handed versions would be locked up in crystallized form. Breslow said our proteins might have been made of all right-handed amino acids if we were in another neck of the cosmic neighborhood.

"The take-home lesson is that the universe doesn't have any great preference for left-handed amino acids," Breslow said. "Our solar system just happens to be getting more light here that preferentially destroys right-handed stuff. Who knows, there could be life out there that is a mirror image to us on a molecular level."

The research was funded by the National Science Foundation. Breslow presented it April 6 at the American Chemical Society national meeting in New Orleans.