ORIGIN OF LIFE:
Did Twisty Starlight Set Stage for Life?

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. On the other hand, the sugar backbones of DNA and RNA always spiral to the right. This uniform handedness, or homochirality, could have arisen in the course of evolution, either by chance or because such shapes somehow aid DNA replication or protein synthesis. Or it may have preceded life: Some researchers argue that our infant solar system was seeded with L amino acids formed in cool interstellar clouds, which then rode to Earth aboard comets, meteorites, and dust.

That scenario receives a boost this week with a report on page 672 describing the first evidence of a possible space-borne mechanism. A team led by Jeremy Bailey of the Anglo-Australian Observatory near Sydney has spotted circularly polarized infrared light--in which the electromagnetic wave rotates steadily--streaming from a region of intense star birth in the Orion Nebula. Ultraviolet (UV) light polarized this way can selectively destroy either left- or right-handed (D) amino acids, depending on the direction of spin. If similar radiation bathed the dust around our newborn sun 5 billion years ago, says team member James Hough of the University of Hertfordshire in Hatfield, England, "it could have created the necessary precursors to life's [handedness]. This process would produce a much higher excess [of L amino acids] than anything that could occur on Earth."

The findings are "quite exciting," adds 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 doubts 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."

Those who favor an unearthly genesis for homochirality have for years pointed to circular polarization 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, Bailey's team has found it 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, presumably by scattering off fine dust grains aligned in a magnetic field. "That was a big surprise," says Bailey, who had expected levels of 1% to 2%.

Infrared light, however, does not pack the energy needed to destroy organic molecules. That would take UV light. Although Bailey's colleagues could not see UV light from Orion because of obscuring dust, they calculate that a similar percentage of UV light should also be circularly polarized. If such light from a nearby star cascaded through our early solar system, it could have broken the bonds in enough 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. Other planetary systems, depending on the direction of polarization, might see an excess of D amino acids.

Even so, Bailey and Hough acknowledge, many events must fall into place to render their scenario plausible. Those steps include making huge amounts of amino acids in space and delivering them to Earth without losing the surplus to "racemization"--the spontaneous transformation of homochiral molecules to an even-handedness that happens quickly at high temperatures and in water. "I consider each of those steps to be possible," says planetary scientist Christopher Chyba of the University of Arizona, Tucson, noting Cronin's recent discovery of L amino acid excesses ranging from 3% to 9% in the Murchison meteorite, which fell in Australia in 1969 (Science, 14 February 1997, p. 951), and in a 1949 meteorite from Kentucky. "The open question is, would such an excess be important to the origin of life?"

Bada and his colleague at the University of California, San Diego, chemist Stanley Miller, think not. "Once the amino acids get to Earth, they would racemize in very short order," Miller says. "I've always felt that homochirality arises by chance."