Searching for Life in the Stardust
We all know that, somehow, life arrived on earth. But how and why it got started -- and where -- remains one of the great mysteries of science. Theories range from incubation in primordial ooze or lightning zapping the ammonia-rich atmosphere of the young planet to reactions nurtured in the chemically rich water flowing from undersea vents and even living microbes riding earthward in meteorites.
Now a team of scientists headed by Louis J. Allamandola of the Astrochemistry Laboratory at NASA's Ames Research Center has cooked up a new recipe: Take some frozen stardust from interstellar space and just add water.
The concoction that the researchers made in a simulated deep-space environment isn't freeze-dried soup or Jell-O, nor will it get up and walk -- or even crawl, for that matter. But it demonstrates that the complex reactions essential to life can occur in the darkest and most inhospitable regions of the cosmos.
The result of the experiment was the spontaneous formation of hollow spheres made up of complex membranes -- akin to the cell walls that shelter the chemistry of life from the outside environment. "All life today is cellular, and cells are defined by membranes," says Dave Deamer, a professor of chemistry at the University of California at Santa Cruz, who earlier observed a similar phenomenon with organic chemicals obtained from a meteorite. "When life began, at some point, it became compartmented into cells."
The results are particularly surprising because interstellar space is a very inhospitable place. The churning clouds of space dust are the debris of exploded stars. When they coalesce, these motes are the stuff from which future stars and solar systems will be born. They are the building blocks of planets, asteroids, meteorites, and comets. At their average temperature -- 10 Kelvins (about -440F or -260C) -- air freezes solid. And in the high vacuum of space, they are bathed in destructive ultraviolet radiation.
So, many critics gave the idea a cold shoulder in the mid-1970s, when Allamandola conjectured that a vibrant biochemistry could take place in such hostile circumstances. Few were ready to accept the idea that a fragile carbon-based chemical could survive under such conditions.
By analyzing the light that passes through interstellar clouds, scientists figured out that they consist of tiny sand-like grains covered by a thin layer of ice. The ice also contains some simple organic molecules such as ammonia, carbon monoxide, carbon dioxide, and the simplest alcohol, methanol.
But could ultraviolet light cause chemical reactions to occur on those icy surfaces? Allamandola turned to the laboratory -- becoming one of the founders of a discipline called astrochemistry. His group freezes water and other chemicals into a thin, solid ice at temperatures near absolute zero and in an extreme vacuum. Then they bathe them in ultraviolet light, similar to that coming from neighboring stars, to see what more-complex chemicals might form.
Two years ago, laboratory data collected by Allamandola and his colleagues confirmed that curious spectral signals observed by astronomers studying interstellar clouds matched those of a class of rather gritty organic molecules, dubbed PAHs, for polycyclic aromatic hydrocarbons. On earth, these common, highly carcinogenic -- and very durable -- components of soot are found in diesel exhaust, burned pots and pans, charred hamburgers, and cigarette smoke.
When exposed to ultraviolet radiation, these substances produced many of "the molecules found in cosmetics and medicine that will be familiar to many. For example, they are found in aloe, henna, and St. John's Wort," says Max Bernstein, an organic chemist who is working at NASA Ames through a cooperative agreement with the SETI Institute, which is dedicated to finding intelligent life elsewhere in the universe.
In their current experiment, which they described in the Jan. 30 issue of the Proceedings of the National Academy of Sciences, the Ames investigators made a variety of ices. Then they exposed them to ultraviolet radiation in simulated space conditions for up to five weeks.
CRACKING THE SHELL.
The result is just a few milligrams of material, but these smudges contained an astonishing number of complex chemicals. "Instead of finding a handful of molecules only slightly more complicated than the starting compounds, hundreds of new compounds are produced in every mixed ice we have studied," says Ames space scientist Scott Sandford.
The real surprise was what happened when some of the chemicals were mixed with liquid water. They spontaneously formed microscopic hollow droplets, or vesicles, with sizes, shapes, and structures similar to those of certain living cells. "Once in liquid water, vesicles form very quickly -- within minutes," says Allamandola.
An empty shell is a long way from a living cell. But the group's tiny bubbles share another property essential to the membranes of living cells. Some of them glow, indicating that the membrane is converting some ultraviolet radiation into visible light. "They can absorb light energy from the outside, and this can be used to power processes within," says Allamandola.
Moreover, Allamandola is not certain that the vesicles are completely empty. "There is certainly something inside," he insists. And in further experiments, the researchers plan to add hydrocarbons to the equation. These, he speculates, may generate other molecules essential to life, such as nucleic acids. "We are approaching the point at which simple chemistry becomes biology," he says.
Even now, however, the group has presented compelling evidence for a new model of the origin of life. Icy dust pervades the universe. Every year, more than 100 tons of extraterrestrial material sifts down on earth -- much of it organic material similar to that the astrochemists have produced in the laboratory. When the solar system was younger, far greater quantities would have arrived. "We now know that many of the hundreds of new compounds we make in these interstellar ice-simulation experiments have properties relevant to the origin of life," says Jason Dworkin, a biochemist on the NASA team.
Allamandola's ideas are still controversial. Indeed, even he concedes that other theories could also stand up. But he believes his experimental results have changed the rules of the game. "We've proven that the stuff that is widespread throughout space is complex and capable of behaviors not imaginable only a few years ago," he says.
The notion is an attractive one. Instead of life arising by chance in isolated corners of the universe, the fiery birth and death of stars could have seeded genesis on a myriad of watery planets and moons by providing ready-made packages of the precursors to life. We may truly be the stuff of stardust -- and we may just have some company in the cosmos.
By Alan Hall in New York
Edited by Douglas Harbrecht