Scientists theorize possible scenarios on how life could have first originated on Earth.
Back in 1953, Jim Kasting
said, scientists thought they had the origin of life figured out. Chemists Stanley Miller
and Harold Urey
at the University of Chicago had simulated that crucial instant
around 3.9 billion years ago when a batch of simple inorganic molecules, zapped by a bolt of lightning (or maybe just the sun's warmth during a break in the clouds), fell together to form the prototypes for the complex organic compounds that life is made from.
Now that was a moment. Remember it on Star Trek? The muddy puddle of ooze on the edge of Nowheresville? The awful humidity? The onset of bubbling? Before, everything was dead as Play-doh. After came a chain of eye-popping events that just keeps unfolding, across the eons, into alligators and astronauts, puppies and banana figs, mosquitos and lichens and particles of ebola virus . . .
In their lab, Miller and Urey shot flashes of lightning, in the form of cascades of sparks, through a flask containing an "ocean" of liquid water and an "atmosphere" of strongly reduced (that is, hydrogen-rich) gases - methane, ammonia, hydrogen sulfide, and water vapor. After a couple of days, they tested what was left. "They had formed all sorts of compounds," Kasting said, "including large quantities of amino acids," the molecules that join to form proteins. This simple experiment seemed to corroborate a vision Darwin (and not Gene Roddenberry) had described a hundred years earlier, of life emerging "in some warm little pond, with all sorts of ammonia and phosphoric salts, light, heat, electricity, etc., present."
But the Miller-Urey experiment, important as it was, had a flaw. Urey had based his primitive-Earth atmosphere on astronomical data just then coming in, the first spectra from the giant planets in our Solar System: Jupiter, Saturn, Uranus, and Neptune. These characteristic bands of color showed that the giants were swathed in atmospheres rich in methane and ammonia, thought to be left over from the planets' formation.
At the time, people thought all of the planets had once shared a "primordial" atmosphere, the result of their common birth. Because of their stronger gravity, the giants were believed to have retained this early atmosphere, while the atmospheres of Earth and the other, smaller planets had lost some of their lighter gases, hydrogen among them, to space. Thus, Urey reasoned, an early Earth atmosphere, before its hydrogen had escaped and the life-driven process of photosynthesis had boosted its oxygen, would have been a lot like a present-day giant's.
Shortly after the Miller-Urey experiment was published, however, geologists came up with new findings on Earth's volcanic emissions - and threw the old reasoning for a loop. "What comes out of volcanoes is not methane and ammonia," Kasting said, "but about 80 percent water vapor, 15 to 20 percent carbon dioxide, and traces of carbon monoxide and molecular hydrogen." James C. G. Walker, one of Kasting's graduate advisers at the University of Michigan during the 1970s, took these emissions data and balanced them against the rate at which hydrogen would be expected to escape from a planet with Earth's gravity. ("He did all this stuff on the back of an envelope," Kasting said.) What Walker came up with was a much different picture of Earth's early atmosphere: an oxygen-rich mix of carbon dioxide, nitrogen, and water vapor.
The catch is that oxygen, although an absolute necessity for multicellular, advanced life, is poison to pre-biotic synthesis. Do a Miller-Urey experiment in an oxygen-rich atmosphere, Kasting said, and "you don't form things like amino acids. There are too many oxygen atoms in there." So, over the years, "enthusiasm for the warm little pond theory has waned."
Two competing theories have emerged instead. The discovery of microbes and other small organisms living in and around hydrothermal vents - underwater hot springs boiling from the ocean floor - has led to the idea that life may have started at the bottom of the sea. Sharp differences in temperature and oxygen concentration at the boundaries around these vents make good catalysts for chemical reactions, Kasting said. "The problem with this theory is that the complex organic compounds likely to form life cannot remain stable for long at such high temperatures." Amino acids, instead of joining up, would tend to break down.
The other scenario has life first coalescing in the frigid climes of outer space - specifically, within the cold dark hearts of interstellar dust clouds. "Long, complex organic molecules can be made when ionizing radiation leads to ion-molecule reactions," Kasting explained. "The intense cold prevents them from breaking down." In this so-called "seeding from space" model, these complex molecules are brought to Earth by incoming meteorites and comets. The weak link here is that most of a meteor is vaporized on impact with our atmosphere. "The survival potential for organisms is low. They get pyrolized: Burned to a crisp."
Kasting, for his part, is not ready to give up on the warm little pond. Using computer models of light-triggered atmospheric processes, he is working to reconcile Darwin's vision with the constraints imposed by a relatively oxygen-rich atmosphere.
"My idea," Kasting said, "is that this atmosphere did contain some methane: just enough to allow for the formation of hydrogen-cyanide molecules, one of the key starting materials for making both amino and nucleic acids. Ten to 100 parts per million would be enough."
Present-day life, he explained, requires three types of molecules: DNA, to store the genetic information that allows cells to replicate; RNA, which transfers that genetic information from the nucleus to the rest of the cell; and the proteins that catalyze these reactions. "It's a very complicated system." Yet in 1989, molecular biologists Thomas Cech of the University of Colorado and Sidney Altman of Yale shared a Nobel prize for showing that under some circumstances RNA can replicate on its own. Not only that, but it can store genetic information.
RNA, in other words, can do it all. "Early life is now believed to have passed through a stage in which only RNA was present," Kasting said: the so-called "RNA world." All you need for life, besides those crucial amino acids, are the ingredients for RNA: ribose, a sugar; phosphate, a salt; and the four bases - adenine, cytosine, guanine, and uracil (the last replaces the thymine in DNA). The question is, can you get these molecules in an atmosphere where significant oxygen is present? The answer, Kasting said, is yes - assuming there's a little bit of methane around.
Ribose, Kasting explained, "is simply five molecules of formaldehyde strung together," and formaldehyde is easy to make where there is carbon dioxide and light. Phosphate occurs routinely with the weathering of rocks. And all four bases, A, C, G, and U, can be synthesized from hydrogen cyanide, for which you need that sprinkling of methane.
"So the key to making Darwin's little pond," Kasting said, "is to figure out if there was a good source for methane in the early atmosphere." That source, he suggests, is under the sea, in the volcanic activity that fires up those super-hot hydrothermal vents. Currently, the carbon released from the vents run about 99 percent carbon dioxide, he said, and about one percent methane, a slightly different mix than what comes from volcanoes on land. "And there are good geochemical reasons to believe that the Earth's mantle 3.9 billion years ago was much more strongly reduced than it is today, which means the methane component of these emissions would have been that much higher." Plenty high enough to allow for the formation of organic molecules.
That's not to say this is the way life sparked into being, Kasting quickly added. But it's a plausible scenario. And if it did happen that way here, what's to stop the same process from repeating itself, around the universe, wherever conditions happen to be the same?
Produced by the Office of Research Publications at Penn State with funding from the Penn State Astrobiology Research Center; the Pennsylvania Space Grant Consortium; the Education Office of the NASA Astrobiology Institute; Pfizer Inc.; and the Eberly College of Science.
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