In the early 1950s, a grad student at the University of Chicago zapped simple molecules with artificial lightning to turn simulated seawater into a broth containing amino acids, key building blocks for organic life.
Ever since, "primordial soup" has served as a handy tag to describe the incubator that many biologists believe gave rise to life on Earth 3.5 billion years ago.
This week, two teams of researchers say they have uncovered another potential source for these amino acids: They can be found not only in liquid water but in the ice that forms in outer space. Call it the "primordial Frosty."
In lab experiments designed to mimic the contents and conditions in frigid clouds of interstellar gas and dust, independent teams in the United States and Europe found that amino acids form naturally from interstellar ices. Several of the acids play key roles in proteins, and others have been found in meteorites.
The results not only could help identify one of the sources for Earth's complement of amino acids. They also suggest that these acids which combine to form the biological workhorses known as proteins are ubiquitous. The basic building blocks for life are likely to be present wherever a solar system forms.
Because solar systems form from interstellar clouds, the experiments suggest that "any time you form a solar system, you can bet there'll be amino acids there from the start, before the planets even form," says Max Bernstein, a researcher at the SETI Institute's Center for the Study of Life in the Universe in Mountain View, Calif., who led one of the two teams.
The results, reported in today's edition of the journal Nature, also are giving a boost to researchers interested in testing ideas about how some of the prebiotic chemicals in Earth's early inventory arrived from extraterrestrial sources.
Those ideas gained credibility after researchers began analyzing a meteorite that landed in Australia in 1969. Known as the Murchison meteorite, the carbon-rich space rock contained more than 70 amino acids. Since then, geochemists have found amino acids in similar meteorites.
Yet to explain the presence of amino acids on these objects, many scientists held that the acids formed in liquid present on the parent body. "Amino acids are known to occur in liquids," Dr. Bernstein acknowledges. "We asked: How far can we go with ice?"
Far enough, apparently, to please at least one researcher who has been trying to see if amino acids borne on comets collections of primordial dust, rubble, and ice could survive the heat and pressure of an impact with Earth.
"I'm thrilled about this work," says Jennifer Blank, a geochemist at the Lawrence Livermore National Laboratory in Livermore, Calif., who has been using a four-ton gas-drive gun at the lab to blast canisters holding amino acids. The collisions between bullet and canister are designed to generate temperatures and pressures comparable to those of a comet collision.
She says her results indicate that amino acids can survive an impact. But there's a slight hitch. To date, amino acids have not been directly detected in comets, although researchers have seen indirect evidence that suggests they may be present.
Thus, merely showing that chemical reactions in interstellar clouds can form amino acids "is a big first step" toward explaining where they can originate and how comets might acquire amino acids for delivery to Earth, she says.
Working at the NASA-Ames Research Center at Moffett Field, Calif., Bernstein's team vaporize water, methanol, ammonia, and hydrogen cyanide chemicals astronomers have identified in interstellar clouds and injected them onto a piece of highly purified nickel. The nickel sat inside a vacuum chamber chilled to 15 degrees above absolute zero. The team then bathed the sample in faint ultraviolet light to mimic the exposure ice and dust might get from starlight if they hovered at the edge of a molecular cloud for 500 years or sat shrouded deeper inside the cloud for 10,000 years.
After analyzing the results, they found they had produce three amino acids serine, glycine, and alanine all common to proteins.
A European team led by Uwe Meierheinrich of Bremen University used a slightly different recipe and came up with 16 amino acids.
Over the past few years, astrobiologists have discovered a range of intriguing compounds vital to biology in these experiments and in meteorites. For example, some of Bernstein's experiments have shown that photochemistry in interstellar clouds should be able to make quinones, compounds that can absorb radiation and govern such processes as photosynthesis. Bacteria use quinones to protect themselves from ultraviolet radiation.
Meanwhile, another team from NASA-Ames, the SETI Institute, and the University of California at Santa Cruz analyzed the residue from their interstellar-cloud experiment and found that when added to water, the residue spontaneously formed small spheres somewhat like the walls of modern cells. In effect, the researchers noted, if such mixtures found their way to Earth, they could have provided freeze-dried housing to shield the early chemical machinery that eventually evolved into simple life forms.