Life is broadly defined as the condition of an organism that exhibits growth through metabolism, reproduction, and adaptation to its environment. These characteristics separate living organisms from inanimate objects. The cell is the basic unit of life, and an organism must be composed of at least one cell to qualify as a living thing. The cells of every living organism contain deoxyribonucleic acid (DNA), a nucleic acid that holds the genetic instructions for the development and functioning of a life form. It is this substance that allows a living thing to grow and reproduce. DNA was first discovered in 1869 by Swiss physician Friedrich Miescher who noticed a microscopic substance in the pus on discarded bandages. The substance was later identified as DNA. A virus straddles the definition of life and, according to scientific definition, is not technically alive. It is composed simply of DNA and ribonucleic acid (RNA) strands surrounded by a protein shell and requires a host cell to metabolize and reproduce. When not within a host cell, a virus lies dormant and is nothing more than a static organic particle. Scientists estimate the earth is about 4.5 billions years old and first began to support life forms approximately 3.7 billion years ago. While scientists universally believe that naturally occurring amino acids were mediated by nucleic acids to create the first life forms on Earth, the question of how nucleic acids originated from inanimate matter is still being studied today. There is no definitive answer yet as to the origin of life on Earth. Throughout history, two dominant theories of the origin of life on Earth have prevailed. Abiogenesis, a theory that is now widely discredited, holds that life was and is spontaneously generated from decaying organic matter. The theory of biogenesis, on the other hand, maintains that new life is produced from existing life forms.
Posted on: Thu, 09 Oct 2014 08:20:19 +0000
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