Origin of Life: Are we really natives of earth?
How many of us ever worried if the life started on the earth or was brought from outer space? I guess many of us do. Otherwise you wouldn’t be reading this article. Until 17th century people believed in the so called theory of “spontaneous generation” proposed by Aristotle to explain the origin of life. According to this theory, life came from inanimate matter as food. It was said that maggots can be produced from rotten meat and rats from piece of soiled cloth wrapped around wheat husk in an open jar for about 21 days. Do you remember hearing such stories? But later on Louis Pasteur and Francisco Redi proved that the microbes in the air are responsible for the life produced in food.
Stanley Miller’s experiments with Urey in 1950 were the serious attempt to understand the origin of life. Their experiments involved passing electric spark through a brew of methane, ammonia, water vapor and hydrogen – a mixture thought to be mimicking of early earth atmospheric condition- to produce molecules that are important constituent of life (amino acids). From then on, there have been mainly two groups of researchers: One who argue that life originated on the earth and the other who would oppose and say, “No, no! Earth lacked suitable conditions to form or support life and therefore life came from outer space (exogenesis)” pointing to alien life. Life on earth could have started from volcano eruptions or in the deep oceans and would involve complex chemical reactions. For exogenesis to happen, small molecules in outer space must undergo chemical reactions under varying temperature and radiation giving rise to new and larger molecules. These molecules would eventually form key molecules of life as amino acids and DNA and they would have been delivered to earth by meteorites that hit earth numerous times in the past.
Each of them is involved in research intensively to solve the mystery and to prove their hypotheses. In the last quarter alone there were two breakthrough results that provide cues on the origin of life.
In January this year, researchers from the Arizona State University at Tempe reported that they have stronger evidence for exogenesis. In collaboration with the University of California at Santa Cruz, Dr. Pizzarello analyzed a meteorite piece treated under high temperature (300oC) and pressure (100 Mega Pascal). The meteorite released enormous amounts of ammonia than any other samples analyzed to date. While constant supply of ammonia (the reduced form of nitrogen) is indispensable to form nitrogen containing molecules as amino acids, DNA and RNA earth did not have sufficient amount of it and often was destroyed by photochemical reactions. This provides hints that life on earth was made possible by extraterrestrial ammonia brought to earth by meteorites. Previous studies on other meteorite samples revealed presence of amino acids as well as aromatic hydrocarbons.
Then, in February this year, a research led by Jeffrey Bada reports formation new sulfur containing amino acids necessary for life. The team rerun the Millers’ experiment with his own apparatus and produced sulfur containing molecules that were not reported by him. When they analyzed the original samples from Millers’ experiment they were able to find the molecules in smaller proportions and perhaps Miller wasn’t able to identify the compounds with his crude analysis. Many scientists believe that early earth could have had reducing atmospheric conditions near volcanoes and transiently. In fact volcanoes are the major sources of atmospheric H2S and lightning would have started the reaction of H2S with other reduced molecules as CH4 and NH3. Does this study mean life originated on the earth? Not quite. Analyses show that relative amounts of amino acids produced by Miller’s experiment is same as found in certain carbon-rich meteorites suggesting meteorites could have brought the amino acids to the earth. Before the formation of earth interplanetary lightning would have converted dust and gas surrounding the newborn sun partly to amino acids.
Although each of them has evidences, they are not sufficient to prove any one of their hypotheses. Origin of life does not stop if formation of amino acids or presence of H2S could be explained. For any one of these hypotheses to be accepted, each stage in the formation of life must be explained and proven. How did the amino acids polymerize to proteins? What is the mechanism of DNA and RNA formation? Or how did the DNAs (RNAs) self-replicate?
If you would ask me at this stage, I am a supporter of exogenesis. In the last two years I have been studying aromatic hydrocarbons films and effect of radiation that is ubiquitous in the space. Researchers from NASA have found hydrocarbons in the interstellar medium with their spectroscopy techniques and the meteorites also provide evidence of hydrocarbons. The hope is to identify possible chemistry in aromatic hydrocarbons which could lead to formation of new molecules.
So, which theory do you believe in?