How Life Began – Formation of the Earth
Processes That Created the Solar System and Primordial Earth
Oct 31, 2008
The Earth is thought to have formed approximately 4.6 billion years ago. One billion years later it was heaving with life, largely microorganisms resembling the modern photosynthesizing cyanobacteria (often called blue-green algae). How did these life forms arise?
This is the first of a series of articles discussing current ideas on the origins of life. Here, the formation of the early Earth is described, providing conditions in which life could begin.
The Early Solar System
In order to understand how life might have arisen billions of years ago, the conditions on the prebiotic Earth must be appreciated. Originally, the solar system was comprised of hot gases and dust that were furiously mixing and revolving around a white-hot core. This core eventually reached a million degrees Fahrenheit enabling Earth's Sun to be born.
The gas and dust clouds slowly condensed and formed asteroids. Possibly greater than 100 trillion large asteroids (also called planetesimals) existed when our solar system was first created. Collisions often destroyed planetesimals but sometimes the impacts caused them to combine, increasing their mass. Gravity assisted this growth by attracting further particles and debris in a process called accretion.
How the Earth Formed
With the passing of hundreds of millions of years, the early Earth was created by accretion. It continued to transform due to the asteroid barrage releasing enormous amounts of energy, causing the Earth’s crust to melt. This was compounded by radioactive decay in the Earth’s core that generated incredible heat and caused massive volcanoes to discharge lava and gases, changing irretrievably the surface of the planet and its atmosphere.
Meteorites continued to bombard the planet, supplementing the gaseous water now being released from the Earth’s crust. In the atmosphere, this combined with other gases such as carbon dioxide to produce dense clouds, shielding the Earth from the Sun’s radiation. Eventually, helped by a reducing number of meteorite impacts, the surface began to cool and clouds released rain that formed warm lakes and seas.
Conditions on the Early Earth
The Earth’s crust began to stabilize some 3.9 billion years ago, as evidenced by the oldest known rocks. But conditions on the early planet were substantially different from the present day. The temperature was much hotter and water was present, but in the atmosphere there was little free oxygen.
Opinions have changed concerning the composition of the early atmosphere. Early ideas suggested that methane, ammonia and hydrogen were abundant and that the atmosphere was overall a reducing type, in which organic molecules were more stable. In 1953, Miller and Urey used a reducing mixture of gases to mimic the conditions of the early Earth and their experiment demonstrated that abiotic molecules could be used to create fundamental biochemicals that were thought to be necessary for the origin of life.
Overall, there is no strong geological evidence confirming the composition of the Earth’s early atmosphere. Some scientists believe that the Earth’s atmosphere contained an abundance of carbon dioxide and nitrogen, was oxidizing, and did not readily support organic chemical reactions. So, if a reducing atmosphere, as suggested by Miller and Urey, was not available on the early Earth, then how else might the first biological molecules have arisen?
In the next article, How Life Began – Origin of Biochemicals, some theories are presented that might explain the source of both biochemicals and the first biological cells on Earth – termed abiogenesis.
Related reading:
Arny, T.T. (2000). Explorations: An Introduction to Astronomy. Boston: McGraw-Hill Company.
Miller, S. (1953). A production of amino acids under possible primitive earth conditions. Science 117:528-529.
Miller, S. and H. Urey. (1959). Organic compound synthesis on the primitive earth. Science 130:245-251.
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