In an old seventies song, Joni Mitchell croons, “We are stardust…” – a pretty metaphor or a distinct possibility? For years, scientists and the general public alike assumed that life arose from substances that had been on Earth since its formation. But recently, Joni’s view has started to seem a bit less outlandish. A new field called exobiology, which considers the entire universe as the possible birthplace of life, has emerged. Researchers are exploring the possibility that amino acids, the molecular precursors to Earthly life, were actually formed in outer space and transported through the atmosphere on meteorites. Amino acids are the building blocks of proteins, which in turn serve as antibodies, catalysts, and storage and transport molecules, as well as fulfilling countless other roles. The perennially fascinating question of our primeval origins has again come into the limelight, giving rise to both reasonable investigation and wild speculation.

The quest to trace the origins of life was under way half a century ago, when University of Chicago graduate student Stanley Miller tried to simulate conditions on Earth billions of years ago, before life arose (Miller, 1996; Szaflarski et. al.). In a closed system, he replicated, to the best of contemporary knowledge, the chemical composition of the Earth’s atmosphere, using methane, hydrogen, and ammonia gases and water vapor. Underneath the gases, a primitive “ocean” of water seethed. Then, to simulate the lightning storms in the early atmosphere, Miller ran a continuous current of electricity through the mix, causing the gases to react with one another. The products of these reactions condensed into the “ocean”, where Miller collected and analyzed them. Within weeks, Miller’s experiment yielded several different amino acids. For decades, his model of amino acid generation was eagerly accepted.

Some modern-day scientists, however, have raised skeptical eyebrows. Earth’s prebiotic, or “before-life” atmosphere, they contend, may have been a much less hospitable environment for amino acid production than scientists in the fifties believed (Orgel, 1998; Bernstein et. al., 2002; Munoz Caro et. al., 2002). Amino acids form readily only when surrounded by chemical processes known as reduction reactions. Recent studies have suggested that the prebiotic atmosphere was much less reducing than Miller assumed. But if amino acids were not generated on the surface of the Earth, where did they come from?

One fledgling theory has it that the first amino acids came from the most unlikely of nurseries: outer space. In this scenario, the reactions producing amino acids were catalyzed by rays from the sun instead of by electricity (Orgel, 1998; Munoz Caro et. al,. 2002; Bernstein et. al., 2002), and the amino acids were transported to Earth on meteorites (Basiuk & Navarro-Gonzalez, 1998). This concept gained credibility when amino acids were discovered embedded in a meteorite that fell to Earth near the town of Murchison in Australia. But how were the molecules created in the first place? And how could they have survived the treacherous journey through the Earth’s atmosphere and the impact with the surface?
In an experiment similar in concept to Stanley Miller’s, C. M. Munoz Caro, of the Leiden Observatory in the Netherlands, and several colleagues tested the hypothesis that amino acids could be formed in the interstellar medium, dense clouds of dust where stars are formed. They chose a gas mixture they believed to be representative of the interstellar medium, which included water vapor, carbon monoxide, carbon dioxide, methanol, and ammonia. In a vacuum chamber, they allowed the gases to crystallize into ice on a block of ice cooled to 12 Kelvin – nearly absolute zero. They then exposed the ice to ultraviolet radiation – similar to the energy that would bombard them in outer space – which fueled reactions between the gases in the mixture (Munoz Caro et. al., 2002). After warming the ice and analyzing the residues, the researchers found several intact amino acids. They also found that a few other prebiotic molecules had been created (Munoz Caro et. al., 2002). Their results have been corroborated by the findings of another team of researchers, Max P. Bernstein and colleagues, of the SETI (Search for Extraterrestrial Intelligence) Institute.

Our second question remains unanswered, however: how did the molecules survive the journey to Earth? In search of an answer, researchers Vladimir Basiuk and Rafael Navarro-Gonzalez have studied the behavior of organic compounds under the intense temperatures sustained by objects entering Earth’s atmosphere and impacting the surface. The researchers subjected amino acids to temperature fluctuations as great as 500 degrees Celsius. About 1 to 10 percent of each sample survived, implying that a few molecules could have made the journey to Earth (Basiuk & Navarro-Gonzalez, 1998). Only these critical few would have been necessary to spark life.
Researchers are accumulating a body of solid scientific evidence for the theory that amino acids were formed in outer space and transported to Earth on meteorites. But both scientists and nonscientists have also put forth more speculative hypotheses. It is very tempting to wonder if amino acids wouldn’t make their way to other planets as well as Earth, and perhaps give rise to life there. It has even been suggested that the first amino acids to arrive on Earth could have been generated by living beings on other planets, or that those organisms could have become embedded in meteorites and traveled to Earth themselves (Szaflarski et. al.). Whether or not these hypotheses prove supportable, the experiments promise to broaden our views and spark our imagination about our origins.

References

Basiuk, V. A. & Navarro-Gonzalez, R. (1998). Pyrolytic behavior of amino acids and nucleic acid bases: Implications for their survival during extraterrestrial delivery [Electronic version]. Icarus, 134, 269-278. http://dx.doi.org/10.1006/icar.1998.5950

Bernstein, M. P., Dworkin, J. P., Sandford, S. A., Cooper, G. W., & Allamandola, L. J. (2002). Racemic amino acids from the ultraviolet photosynthesis of interstellar ice analogues [Electronic version]. Nature, 416, 401-403. http://dx.doi.org/10.1038/416401a

Munoz Caro, G. M., Meierhenrich, U. J., Schutte, W.A., Barbier, B., Arcones Segovia, A., Rosenbauer, H., Thiemann, W. H.-P., Brack, A., &

Greenberg, J. M. (2002). Amino acids from ultraviolet irradiation of interstellar ice analogues [Electronic version]. Nature, 416, 403-406. http://dx.doi.org/10.1038/416403a