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Cell theory refers to the idea that cells are the basic unit of structure of all living things. Development of this theory during the 1800's was made possible by advances in microscopy. This theory is one of the fundamental foundations of biology. The theory says that new cells are formed from other existing cells and the cell is a fundamental unit of structure, physiology, and organization in all living organisms.

The cell was first discovered and named by Robert Hooke in 1665. He remarked that it looked strangely similar to cellula or small rooms which monks inhabited, thus deriving the name. However what Hooke actually saw was the dead cell walls of plant cells (cork) as it appeared under the microscope. The cell walls observed by Hooke gave no indication of the nucleus and other organelles found in most living cells. The first man to witness a live cell under a microscope was Anton van Leeuwenhoek, who in 1674 described the algae Spirogyra(see). Van Leeuwenhoek probably also saw bacteria .

The idea that cells were separable into individual units was proposed by Ludolph Christian Treviranus[3] and Johann Jacob Paul Moldenhawer. All of this finally led to Henri Dutrochet formulating one of the fundamental tenets of modern cell theory by declaring that "The cell is the fundamental element of organization''

Credit for developing Cell Theory is usually given to three scientists, Theodore Schwann, Matthias Jakob Schleiden, and Rudolf Virchow. In 1839 Schwann and Schleiden suggested that cells were the basic unit of life. Their theory accepted the first two tenets of modern cell theory. However the cell theory of Schleiden differed from modern cell theory in that it proposed a method of spontaneous crystallization that he called "Free Cell Formation". In 1858, Rudolf Virchow concluded that all cells come from pre-existing cells thus completing the classical cell theory.


Classical Cell Theory
All plants are made of cells (Schleiden)
All animals are made of cells (Schwann)
All cells come from pre-existing cells (Virchow)

Modern cell theory
The generally accepted parts of modern cell theory include:

The cell is the fundamental unit of structure and function in living things.
All cells come from pre-existing cells by division.
Energy flow (metabolism and biochemistry) occurs within cells.
Cells contain hereditary information which is passed from cell to cell during cell division
All cells are basically the same in chemical composition.
All known living things are made up of cells.
Some organisms are unicellular, made up of only one cell.
Others are multicellular, composed of countless number of cells.

Exceptions to the theory
Viruses are considered by some to be alive, yet they are not made up of cells.
The first cell did not originate from a preexisting cell.

http://en.wikipedia.org/wiki/Cell_theory




In the natural sciences, abiogenesis, the question of the origin of life, is the study of how life on Earth might have emerged from non-life . Scientific consensus is that abiogenesis occurred sometime between 4.4 billion years ago, when water vapor first liquefied, and 2.7 billion years ago, when the ratio of stable isotopes of carbon (12C and 13C ), iron and sulfur points to a biogenic origin of minerals and sediments and molecular biomarkers indicate photosynthesis. This topic also includes panspermia and other exogenic theories regarding possible extra-planetary or extra-terrestrial origins of life, thought to have possibly occurred sometime over the last 13.7 billion years in the evolution of the Universe since the Big Bang.

Charles Darwin made the suggestion that the original spark of life may have begun in a "warm little pond, with all sorts of ammonia and phosphoric salts, lights, heat, electricity, etc. present, so that a protein compound was chemically formed ready to undergo still more complex changes". He went on to explain that "at the present day such matter would be instantly devoured or absorbed, which would not have been the case before living creatures were formed." In other words, the presence of life itself makes the search for the origin of life dependent on the sterile conditions of the laboratory.

http://en.wikipedia.org/wiki/Origin_of_life
http://en.wikipedia.org/wiki/Panspermia


The RNA world hypothesis is a theory which proposes that a world filled with RNA (ribonucleic acid) based life predates current DNA (deoxyribonucleic acid) based life. RNA, which can store information like DNA and catalyze reactions like proteins (enzymes), may have supported cellular or pre-cellular life. Some theories as to the origin of life present RNA-based catalysis and information storage as the first step in the evolution of cellular life.

The RNA world is proposed to have evolved into the DNA and protein world of today. DNA, through its greater chemical stability, took over the role of data storage while protein, which is more flexible in catalysis through the great variety of amino acids, became the specialized catalytic molecules. The RNA world hypothesis suggests that RNA in modern cells, in particular rRNA (RNA in the ribosome which catalyzes protein production), is the evolutionary remnant of the RNA world.

http://en.wikipedia.org/wiki/RNA_world_hypothesis