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Go back to: Background | Next Section: Diversity of Life
A working definition of a living organism is as follows - a living organism is a form of self-organized matter that grows and reproduces. Terrestrial life is capable of further functions that enables growth, including taking in nutrition, respiration, movement, excretion and stimulus. The simplest form of life is the cell, which is capable of all the above-mentioned functions. All organism consists of cell(s); some are unicellular, while others are multicellular. Despite these differences, all cells are based upon organic compounds. All living organisms are composed of organic molecules. These molecules vary from one organism to another. However, they all share a common element- carbon. In recent years, chemists have been able to produce many carbon-based compounds, some that are not related or found in living organisms. However, these compounds are also called organic compounds. The four common organic molecules essential for life are:
Scientists have placed the age of the Earth at about 4.6 billion years. 4.6 billion years ago, the atmosphere of the Earth contained mainly nitrogen, carbon dioxide, water vapour and hydrogen with traces of ammonia, carbon monoxide and methane. There have been opposing opinions about the existence of oxygen as an atmospheric gas. At about the same time, the surface of the Earth was a chaotic and hostile environment, abundant with energetic and dynamic processes. Ultraviolet radiation and other cosmic rays bombarded the Earth's surface, which at present is shielded by the protective layer of ozone, 03. Extreme temperature ranges and such chaotic process made the surface unbearable and hostile to life. However, it is highly possible that these energetic processes themselves provided the energy for the complex chemical reactions that could have resulted in the origin of life. Out of this chaotic environment has arisen several possible theories about the origin of life. 1) Prebiotic Evolution- Organic organisms may have evolved from inorganic matter in interaction with the non-living environment, in a series of stages. 2) Extraterrestrial Origins- Life originated on an extraterrestrial environment, possibly an asteroid or another planet. This was then transported to Earth by comets, or meteors. Previous Section: Background | Next Section: Diversity of life
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