In order to attempt to understand how life started billions of years ago, we need to first look at what scientists believe to be the conditions of our pre-biotic earth. It all started about five billion years ago. Our solar system was filled with hot gases and dust, which swirled and revolved around a white hot core. When the core approached one million degrees Fahrenheit, our sun was born. The gaseous dust clouds gradually condensed and formed asteroids. It is estimated that over 100 trillion planetesimals, or large asteroids, existed when our solar system was formed. (Picture) As these huge pieces of matter were revolving around the sun, many of them collided with one another. While some of these collisions destroyed the planetesimals, others caused them to combine. As their mass increased, gravity pulled in more particles and debris, and the planetesimals grew larger. This process, called accretion, is how the earth and the other planets were formed.
Over hundreds of millions of years, the earth continued to change due to the bombardment of asteroids.
These asteroids released such an enormous amount of energy upon impact that they began to melt the earth's crust. Radioactivity from the earth's core also changed the surface of the earth. Radioactive decay caused heat and gases to build up in the earth's core. This caused huge volcanoes to spew forth molten rock, or lava, as well as various gases which had been trapped under the surface of the earth. As this lava covered the earth, evidence of the early craters began to be erased.
As the planet earth continued to be hit by meteorites, cosmic H2O was released from the meteorites and from the crust of the earth. This gaseous H2O rose into the atmosphere, combined with CO2 and other gases, and formed incredibly dense clouds above the earth. These clouds formed a reflective shield above the earth, keeping solar heat from penetrating to the surface. As the frequency of meteorite impacts declined, the surface of the earth began to cool. When this happened, the immense clouds which had emerged began to pour rain over the entire planet, cooling the molten rock, and creating lakes and oceans. Water and wind from the atmosphere slowly began to erase the enormous craters which covered the earth.
For a long time it was thought that the early Earth had a reducing atmosphere. A reducing atmosphere contains reductants, or molecules saturated with hydrogen atoms, which are able to reduce other molecules. Many scientists believed that the atmosphere consisted of CH4, NH3, and H2. This is the mixture of gases Miller and Urey used in 1953 to mimic the conditions of the early earth. Their experiment showed that abiotic molecules could be used to create important biotic compounds thought to be necessary for the origin of life.
However, most of the scientific community now believes that the early Earth's atmosphere was not reducing. Instead, scientists beleive the atmosphere was full of oxidants, such as CO2 and N2. An oxidizing atmosphere is essentially neutral, and does not permit organic chemistry to occur.
There is much known about the environment and composition of the early earth. However, there is even more which is uncertain and not known. Because of this, scientists are studying and searching for the conditions which they believe were present when life began. If we know these conditions then perhaps we can discover the building blocks from which life came.
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