In 1953 quite a bit of excitement was generated when Science magazine published a brief research paper entitled "A Production of Amino Acids Under Possible Earth Conditions" by Stanley L. Miller. In the paper, Miller described how he zapped a flask filled with methane, ammonia, hydrogen and water vapor with an electrical charge. The gas mixture simulated Earth's atmosphere thought to exist about 3 billion years ago when life first sprang into existence. The vapor condensed and drained back into a water reservoir. In a week, the water was filled with organic compounds, including amino acids, essential to sustain life.
The press releases and media reports that followed made it sound as though creation of life from scratch was a cakewalk. The implication was that the universe must be teeming with life. The news broke just after a wave of UFO sightings in the United States in 1952, including a spectacular sighting over Washington D.C. These prompted the Central Intelligence Agency and the United States Air Force each to begin formal investigations into the phenomenon (the Robertson Panel and Project Blue Book). Miller's experiment seemed to fit neatly into this mystery and led to an inescapable conclusion. We are not alone!
The excitement aroused by Stanley Miller's initial experiment cooled in the succeeding decades. No progress was made to determine how the experiment's raw organic molecules might have taken the next step and become animate, reproductive life. Even worse, the prevailing scientific models for early Earth's atmosphere changed. Hydrogen, it was now theorized, would have escaped Earth's atmosphere before life appeared, and without hydrogen Miller's experiment failed to produce the required amino acids. Scientists studying the origins of life turned to other, more exotic theories. Miller's experiment began to seem irrelevant, a mere curiosity, even though it established the breakthrough revelation that no barrier separated inorganic from organic matter. Organic matter such as amino acids can spontaneously be created from inorganic material with help from a natural energy source such as lightning.
It takes more than amino acids, though, to make life. It takes an extreme amount of organization. How extreme? The answer is mind boggling.
Amino acids are assembled into protein molecules in living cells. DNA does the assembling in living cells, but DNA doesn't function outside of living cells. To create life where none existed before the amino acids would have had to self-assemble. A typical protein molecule might consist of 200 amino acids arranged in a set sequence. There are about 20 different amino acids to choose from. The resulting combinations multiply out to 10 to the 260th power, which, according to Bill Bryson in his popular book "A Short History of Nearly Everything", is larger than the number of atoms in the universe. And that's just one protein. Bryson goes on to point out that there are hundreds of thousands of different proteins used to make a functioning human organism.
Maybe we're alone in the universe after all!
There are, however, reasons to think the odds against life are not as tremendous as the previous discussion suggests. Bryson mentions some reasons and cites "The Blind Watchmaker" by Richard Dawkins as a book that tackles this dilemma. Many complex and organized structures exist in nature: waves, crystals, snowflakes, Saturn's rings. It would only take a process akin to evolution, where conditions favored some groupings of amino acids more than others, for the odds against spontaneous life creation to be brought back to the realm of possibility. If certain groupings lasted longer they would tend to be more common. If the result of their combining with another common group enabled them to last even longer, those super-groups would dominate. This process may have led to the first protein molecules.
The first life forms, also, may have been much more primitive than those today. Single-celled organisms don't leave behind fossils, so we have no idea how they might have evolved. Sophisticated forms could have out-competed and driven to extinction the more basic forms. Smaller molecules may have proved sufficient to get assemblages of chemicals to the stage where they could split, grow and split again. Without proof, or laboratory experiments that verify that the principle is sound, such musings are pure speculation, but the odds of hitting on the exact sequence of events that led to life form #1 are still very slim.
Evidence indicates that life on Earth began almost as soon as Earth had cooled sufficiently and gained some respite from the constant bombardment of the material that formed the planets - about a billion years after it first formed. This fact strongly suggests that the odds against life's creation were not too astronomical. This in turn suggests that odds are good that life exists on other planets.
Mars became the planet in our own solar system most likely to contain life after Venus - the planet thought to be most like Earth - was discovered to contain a hellish environment with a thick, poisonous atmosphere that retained so much of the sun's warmth that any organism that survived the poisons and the crushing atmospheric pressure would be baked in an instant. Mars, by contrast, seems benign to life. It even has polar ice caps, just like Earth. What it doesn't have, though, is an abundance of water and oxygen.
The Viking probes and rovers that should have detected life on Mars came up empty. Only a couple of tests involving gases gave encouragement to the most optimistic scientists. If life exists on Mars, it is well hidden and isolated, unlike Earth where it is profuse and transformative. The search, now, is for signs that life may once have existed on Mars.
Two other planets are frequently mentioned in discussions about life elsewhere in our solar system: Jupiter and Saturn. Neither of these gaseous, giant planets seem particularly hospitable to life, but moons of each planet do seem like a good candidates.
Jupiter's Europa and Saturn's Enceladus are both icy moons, and on both a heat source - tidal, geothermal, radioactivity or a combination - generate reservoirs of liquid water beneath the icy surface, possibly similar to Earth's hydrothermal vents. Earth's hydrothermal vents were only discovered in 1977 and caused biologists to reformulate what energy is necessary to support life. Previously sunlight was thought to be essential, but the hydrothermal vent communities thrived in total darkness.
A 1997 flyby of Europa by the Galileo spacecraft prompted another breakthrough experiment from Stanley Miller. In 1973 he had froze vials of ammonia and cyanide to minus 108 degrees Fahrenheit, the approximate temperature of Europa. Scientists believed this was too cold for any reaction to occur. The results, however, confounded predictions. Described in a February, 2008, article in Discover magazine entitled "Did Life Begin In Ice?", the experiment produced organic molecules: nucleobases, which are the components of amino acids. Once again, new possibilities for the genesis of life had been discovered.
Miller's 1953 experiment was somewhat resuscitated in 1969 when fragments from the Murchison meteorite which crashed into Australia were recovered and found to contain amino acids, including varieties not commonly found on Earth. This led some scientists to theorize that meteorites brought life's necessary building blocks to Earth. Other scientists began to think that life got its start deep in the ocean's hydrothermal vent environments. But it also validated Miller's first experiment by strongly indicating that amino acids can form spontaneously outside life's biosphere. Now Miller opened the possibility that life began in little, ice-encased pockets of liquid water that can remain in ice even when the ice's temperature reaches minus 60 degrees Fahrenheit, trapping and concentrating any impurities, including organic impurities. By expanding the number of environments in which it's possible for life to start, it increases the odds that life exists on other planets.
Of course, no one can know for certain if alien life exists until it is found. The odds are difficult to calculate. It's a case where the infinitesimal meets the unfathomable: the infinitesimal chances of inert molecules arranging themselves into a life-sustaining organism, and the unfathomable number of opportunities that the inert molecules have been given to do just that. The evidence from research and space exploration, though, has been increasingly in unearthly life's favor.