Early Earth
Whilst it is important to understand the behavior of cells, it is also important to understand where they came from. This starts with understanding the conditions of early Earth that led to their development. It is hypothesized that prebiotic Earth was home to a reducing atmosphere. This means that no oxygen was present, as plants did not exist, and the atmosphere was rich in inorganic molecules such as H2, H2S, and NH3 with CO2 and CH4 as the carbon sources.
The physical conditions predicted on prebiotic earth consisted of:
- Oceans of water due to frequent asteroid bombardment.
- Ionising radiation from space made worse by the absence of O3 (ozone) in the atmosphere.
- Intense heat.
- Frequent electrical storms.
- Volcanic and thermal vent activity.
Prebiotic earth thus provided the conditions needed to produce simple organic molecules such as methane (CH4) and carbon dioxide (CO2), which became building blocks of more complex molecules such as amino acids, nucleotides, sugars, and lipids. These polymers may have spontaneously been produced in these conditions that ordinarily would not occur.
Miller & Urey
Miller & Urey aimed to investigate this theory that Early’s early atmosphere had conditions that favored chemical reactions forming complex organic compounds from inorganic reactants. They simulated Earth’s early atmosphere to generate complex compounds. The experiment was set up as follows:
- A gas mixture of methane, ammonia, hydrogen, and water vapor was made in a chamber.
- A water basin was heated to create steam and passed into the chamber.
- An electric arc inside the chamber was used to mimic lightning, triggering a chemical reaction between the gases.
- The apparatus was cooled down to allow the water to condense and pool back in the basin. In this collected water, new amino acids were found.
In total, Miller and Urey were able to synthesize more than the 20 naturally found amino acids. Whilst the experimental conditions cannot perfectly replicate Earth's early conditions, they were indicative that this theory of molecular generation was correct.