A classic experiment that has sat on the shelf for more than a half-century is yielding new clues about how life may have arisen on Earth, according to a team of scientists that has gone back and analyzed the data with modern techniques.
In 1952, Stanley Miller of the University of Chicago in Illinois and his colleagues conducted one of the most famous experiments in all of science. They repeatedly sent electric sparks through flasks filled with the gases thought to resemble Earth's early atmosphere, including water vapor, hydrogen, methane, and ammonia. After 1 week of near-continuous zapping, the simulated lightning had converted a substantial portion of the gases into organic compounds, including several of the amino acids needed to produce proteins, indicating that this might be how life began on our planet.
In the next few years, Miller and his colleagues repeated the experiment with the same lab equipment and procedures but with different sets of gases. For some reason, the results of the experiments were shelved but not analyzed, surfacing again only after Miller died and colleagues began poring through his archives. In 2008, researchers reported the results of one of those experiments, in which the half-century-old residues yielded 22 amino acids, 10 of which hadn't been detected in the original 1952 experiment.
Now, researchers have analyzed the results of another of Miller's studies, one conducted in 1958. In that research, the team sent sparks through a mixture of methane, ammonia, carbon dioxide, and hydrogen sulfide. These gasses may have been similar to the noxious blend spewing from early volcanoes, and thus they may have been more representative of the environment in and around volcanic plumes than the gasses used in the 1952 experiment. The resulting dried sludge has been stored in glass vials inside cardboard boxes and kept at room temperature for more than 50 years.
Organic chemist Henderson "Jim" Cleaves of the Carnegie Institution for Science in Washington, D.C., and his colleagues have now analyzed that sludge using modern techniques and instruments that are more than 1 billion times more sensitive than the methods used by Miller in the 1950s.
Their results, reported online today in the Proceedings of the National Academy of Sciences, suggest that the 1958 experiment produced 23 amino acids, including six that contained sulfur. The residue samples included nearly equal proportions of left-handed and right-handed versions of several amino acids, a sign that the organic chemicals had been generated during the experiment and not by microorganisms that had somehow made their way into the sealed glass vials. Living cells use and produce only left-handed versions of amino acids, Cleaves says.
Not only did the newly analyzed experiment yield more amino acids than did the one conducted in 1952, two of the sulfur-containing amino acids produced in Miller's 1958 experiment—cysteine and methionine, which weren't found in the results of the 1952 experiment—play particularly important roles in biological processes, says Nicholas Hud, a biochemist at the Georgia Institute of Technology in Atlanta, who was not affiliated with the team. Moreover, he notes, "it's difficult to believe that life as we know it wasn't incorporating sulfur-containing compounds early on."
In addition to the sulfur-containing amino acids, the 53-year-old residues contained threonine, leucine, and isoleucine, amino acids important for certain biological processes that weren't detected in any other electric-discharge experiments that Miller conducted.
The presence of hydrogen sulfide in the 1958 experiment seems to have played a key role in producing the wealth of prebiotic chemicals Cleaves's group found, says Hud. And although researchers still argue about the precise composition of Earth's early atmosphere, most agree that volcanic eruptions would have contributed hydrogen sulfide, he adds.
The origin of life is a hotly debated topic, and the source of the prebiotic chemicals in Earth's so-called primordial soup is one of its enduring mysteries. Some researchers have argued that life could have begun around deep-sea hydrothermal vents, where a warm, chemically active, and mineral-rich broth spews from the ocean floor. But the new analyses—like many of Miller's previously reported results—hint that many of those substances could have formed in the lightning-riddled, steam-filled plumes of volcanoes.
However, the new analyses also suggest that the chemicals could have fallen to Earth in meteorites, Cleaves says, because the relative amounts of amino acids produced by Miller's 1958 experiment and those found in certain carbon-rich meteorites are intriguingly similar.