We know that the earth is full of life. What is difficult to say is how that life began on earth billions of years ago. There are a number of theories with no proof of anything specific. One such theory is the primordial soup theory. Charles Darwin first put this theory into unforgettable words so many years ago when he said life may have begun in a "warm little pond, with all sorts of ammonia and phosphoric salts, lights, heat, electricity, etc. present, so that a protein compound was chemically formed ready to undergo still more complex changes".
Darwin's statement sums up the theory, though the idea that it could really be that simple has long since fled scientists mind. In order for life to have been formed in the soup there would have to have been chemcials and energy that would have formed amino acids. These acids would then have to be connected into proteins. These proteins would then have to be formed into nucleic acids which would then be the beginning of new life on earth.
The theory was first tested in 1953 by Stanley Miller with the assistance of Harold Urey. In their experiment they took a combination of gases they believed to have been prevalent on earth at the time that life began and bottled it up. This included ammonia, hydrogen, and water, with a significant amount of methane. They then sent sparks through it to simulate lightning. This produced a number of amino acids. After adding different types of energy to the mixture they eventually created all 20 amino acids. This was believed to be proof that life could indeed begin in the conditions of Earth at the time.
Later is was shown that this had one major flaw. Methane and ammonia were a huge part of the Miller-Urey experiment, but they weren't major components of Earth during the time that life began. It was later discovered that carbon dioxide and nitrogen were the main gases on Earth during that time. Miller repeated his experiment using these gases in 1983 only to find that the amino acid rich soup created in his first experiment did not exist in the second. Few amino acids were produced in the new gases when energy was passed through them.
This put a big hang up in the Primordial Soup Theory. After all, in order for life to begin amino acids had to be present. These amino acids would have to combine into proteins and these proteins into nucleic acids. However, recent research has brought about exciting findings.
Recent research and experiments by Jeffery Bada has brought about new information. The reason the combination of carbon dioxide and nitrogen failed to produce amino acids is because the produced a liquid that was full of nitrate and very acetic. This combination removed the possibility of amino acids. However, if you add iron and carbonate materials and repeat the process of adding energy you get a "soup" full of amino acids of all sorts. This has spurred life into the Primordial Soup Theory.
Additional research done by the University of California, San Diego may even show us the next step. All living organisms us co enzyme A to connect amino acids into proteins. While this enzyme wouldn't have existed in early Earth waters or atmosphere, it is possible that pantetheine, a chemical that if very similar to co enzyme A could very well have been produced through a chemcial reaction between pantoyl lactone, beta- alanine and cysteamine could have been. Pantoyl lactone, beta- alanine, and cysteamine are all believed to have been on Earth during its beginnings. These chemicals could make pantetheine and other chemicals at 40 degrees Celcius which is rather cool for the temperatures of the time. This finding brings about hope that there will be significant evidence to show that life could have formed on Earth through Primordial Soup.
Even with all of the evidence that has brought the theory farther then many thought possible there are still several doubts. Many scientists against the theory continue to offer doubts. For one, a protein is a combination of 100 amino acids. With 20 different amino acids to choose from getting the 100 right and in the right order is no small feat. Many believe that the possibility of this happening on its own seems unlikely. Additionally, there is little evidence to support the idea that the building blocks of life could have gotten more complex over time rather then simpler. Without additional evidence this and other origins of life theories will remain theories without concrete proof. One thing remains true, we have not seen this process be reproduced either in nature or in a laboratory.