The Origin of Life

Step 1: A Biotic Synthesis

A first step in the formation of a living cell is to begin to assemble the complex molecules from which that cell is constructed. Our starting materials are very simple, for we are restricted to those molecules which are naturally occurring in the oceans and atmosphere. These are therefore:

Clearly, the precise mix of molecules depends upon the environment, and it is important to remember that in the early Earth the composition of the atmosphere was very different from that at present. To a lesser extent the oceans also had a different chemical composition. An illustration of the nature of the pre-biotic Earth is given at http://cmex-www.arc.nasa.gov/VikingCD/Puzzle/Prebiot.htm.

A first step in a-biotic synthesis is the formation of molecules known as amino acids from simpler molecules. An amino acid has a structure illustrated below:

Amino acid structure diagram.

Amino acids vary from one another in the occupancy of the 'R' position. The simplest amino acid is glycine, and has a hydrogen atom in this position.

An early experiment, conducted to test whether or not more complex molecules can be made from simpler molecules was conducted by Miller and Urey in the 1950s. Scroll down the following web pages to find details of their experiment: http://www.worc.ac.uk/departs/envman/Staff/Mike/env103web/L2life.html#orig and more recent results http://exobio.ucsd.edu/miller.htm.

An electrical discharge, to simulate lightening, was passed through a mixture of 'atmospheric gases', in a glass chamber in order to investigate which molecules might be generated by this process. In these early experiments the Earth's atmosphere was thought to be a mixture of the gases steam-hydrogen-ammonia-methane. The results were very positive and amino acids and purines (a component of DNA) were formed.

More recent research, in part based on our knowledge of the atmospheres of the other terrestrial planets, suggests that the Earth's early atmosphere was carbon-dioxide rich. Unfortunately, the Miller-Urey experiment does not produce amino acids from a carbon-dioxide atmosphere. This means that the quest is still on for a process to produce amino acids from simpler molecules.