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Did Twisted Starlight Set Stage for Life?
30 July 1998 7:00 pm
In their quest to trace the origins of life on Earth, scientists keep confronting a puzzle: How did vital molecules get their distinct twists? Nearly all the amino acids in proteins are "left-handed" (L), a designation for one of two mirror-image configurations of atoms around a carbon center. Now comes fresh evidence that this uniform handedness, or homochirality, could have preceded life: In tomorrow's issue of Science, astronomers report finding high levels of a type of polarized light streaming from a region of intense star birth in the Orion Nebula. Five billion years ago, such light could have seeded our infant solar system with L amino acids, which then rode to Earth aboard comets, meteorites, and dust.
Those who favor an unearthly genesis for homochirality have for years pointed to circularly polarized light--in which the electromagnetic wave rotates steadily--as a possible trigger. Astronomers have seen high levels of such radiation near binary stars and in other exotic settings with strong magnetic fields. Now a team led by Jeremy Bailey of the Anglo-Australian Observatory near Sydney has spotted circularly polarized light in an environment much like the one that spawned our solar system. They studied the Orion Molecular Cloud, a cauldron of star formation, with an infrared camera on the 3.9-meter Anglo-Australian Telescope. They found that up to 17% of the infrared light streaming from Orion was circularly polarized. "That was a big surprise," says Bailey, who had expected levels of 1% to 2%.
The researchers calculate that this infrared light would be accompanied by circularly polarized ultraviolet (UV) light, powerful enough to destroy organic molecules such as amino acids. (Bailey's colleagues could not see UV light from Orion because of obscuring dust.). If such high-energy UV light from a nearby star cascaded through our early solar system, it could have broken the bonds in enough right-handed (D) amino acids to yield one extra L amino acid for every 10 molecules--enough of an excess for early organisms to seize upon and amplify.
The findings are "quite exciting," says organic geochemist John Cronin of Arizona State University in Tempe, who has found a surplus of L amino acids in two meteorites that hit Earth this century and thinks such space-borne amino acids might have set the pattern for ones made later on Earth. Origin-of-life experts have a different spin. "There are so many problems" with the scenario, says biogeochemist Jeffrey Bada of the Scripps Institution of Oceanography in La Jolla, California, who is skeptical that large quantities of amino acids from space would have survived the journey to Earth or hung around long enough to influence early biology. "I doubt this will settle the issue of how homochirality arose."