How did life begin? To many thoughtful people that’s a highly charged question which inspires vexing debate. To others, the answer is perfectly simple. Life began in the primordial soup.
In 1953, at the University of Chicago, Stanley L. Miller, then a graduate student, with the help of his professor, Harold Urey, did an experiment which any middle school science teacher can replicate. He sealed up a selection of chemical compounds in flasks, heated some sterile water to add its vapor to the mixture, and then ran electricity through the combination. The result? It could be called life.
Dr. Miller was trying to recreate what he believed were conditions on primordial earth, earth before life began. One of his assumptions was that there was no free oxygen on early earth. Plants free oxygen from carbon dioxide through photosynthesis, and there were no plants. So he used methane, ammonia, and hydrogen sealed inside two glass flasks connected one above the other with tubes. The lower flask held water. He heated the contents of the lower flask, and then zapped the atmosphere thus produced with electrodes in the upper flask. (This was intended to replicate lightening storms above the oceans of early earth.) Then he cooled the system, so that the vapor condensed and trickled back into the lower flask. He repeated this cycle, over and over, for days.
At the end of a week, at least 10 to15% of the carbon he had begun with was now in the form of organic compounds. Two percent of the carbon was now in amino acids, which, of course, are building blocks of carbon-based life. Many other organic compounds essential to life were formed, although DNA and RNA were not. Later experimenters were able to produce the components of DNA in somewhat similar fashion, by adding other compounds to their mixture.
Did this experiment, first performed 55 years ago, prove that life arose on earth, by itself, out of the primordial soup? It doesn’t exactly. There are some scientific objections to the primordial soup theory.
Miller and Urey shot sparks through the mixture over and over. Scientists currently believe that lightening would not have been constant in the atmosphere of ancient earth. Would it have been frequent enough to add enough energy to the soup to let it form new bonds? Maybe not, although earth’s lightening storms would have continued for millennia. Even if organic compounds did form, it is possible that other molecules would have also formed that would have tended to break down the life-like ones. Nitrates in the earth’s atmosphere, if there were some, could have broken down the compounds as they formed.
Another objection to the primordial soup explanation is a competing scientific theory. Comets or meteorites may have seeded the infant earth with life. Astronomers have noticed the signatures of organic compounds in space. Such compounds have appeared in meteorites, and been observed in comets. So life on earth may have begun “a long time ago in a galaxy far, far away.”
Another competing theory says earth life could have begun at hydrothermal vents. Dr. Miller himself experimented with a model intended to replicate volcanic vents in 1953. There is life at vents in the ocean floor, although it’s unlike the oxygen dependant, sunlight driven life of the surface. Chemosynthetic bacteria make food out of the sulfur compounds that flow from the vents, and form the base of an ecosystem essentially indifferent to the sun.
Science can’t yet explain how life on earth began. The answer may lie in some refinement of the primordial soup theory, or in some new way of looking at the problem. It is definitely one of the great questions.
http://www.issol.org/miller/miller1953.pd
http://www.chem.duke.edu/~jds/cruise_chem/Exobiology/miller.html
http://www.arn.org/docs2/news/ucsdoriginoflife062003.htm
http://www.space.com/scienceastronomy/051018_science_tuesday.html
http://www.resa.net/nasa/ocean_hydrothermal.htm
