Origin of Life
The sun and its planets, the solar system,  formed between 5 and 4.6 billion years ago as matter in our solar system began to coalesce because of gravity. By about 3.9 billion years ago, the Earth had an atmosphere that contained the right mix of hydrogen, oxygen, carbon, and nitrogen to allow for the formation of life. Scientists believe that the energy from heat, lightning, or radioactive elements caused the formation of complex proteins (Organic substances primarily composed of carbon, hydrogen, nitrogen, and some other minor elements which are arranged in about 20 different compounds known as amino acids. The various amino acids found in a protein are linked together by peptide bonds) and nucleic acids into strands of replicating genetic code. These molecules then organized and evolved to form the first simple forms of life. At 3.8 billion years ago, conditions became right for the fossilization of the Earth's early cellular life forms. These fossilized cells resemble present day cyanobacteria. Such cells are known as prokaryotes. Prokaryote cells are very simple, containing few specialized cellular structures and their DNA is not surrounded by a membranous envelope. The more complex cells of animals and plants, known as eukaryotes, first showed up about 2.1 billion year ago. Eukaryotes have a membrane-bound nucleus and many specialized structures located within their cell boundary. By 680 million years ago, eukaryotic cells were beginning to organize themselves into multicellular organisms. Starting at about 570 million years ago an enormous diversification of multicellular life occurred known as the Cambrian explosion. During this period all but one modern phylum of animal life made its first appearance on the Earth. 
Important Questions about Life:
The Origin of Life on Earth
Where did the raw materials for life come from?
How did monomers develop?
How did polymers develop?
How was an isolated cell formed?
How did reproduction start?

The Orign in of Life: One common misconception concerning  the origin of life  is the belief that, for billions of years, organic molecules floated around and collide in a primordial organic "soup" which after trillions and trillions of collisions, resulted in a single-celled organism crawling out onto land, Not so. The oldest crustal rocks are approximately 3.9 billion years old, although the rocks were subsequently metamorphosed, some had originally been sedimentary rocks. Fossils have been found in these earliest sedimentary rocks (although the organic nature of these structures Is disputed by some). Thus, it appears that life did not begin slowly and gradually. Several lines of evidence and careful analysis indicate that life appeared on the earth just as soon as the earth was cool enough to support it. 

Four groups of biological entities are currently recognize :

Archaea - are a group of recently discovered organisms that sometimes live in extremely hostile habitats like thermal volcanic vents, saline pools, and hot springs. Archaea are single-celled organisms that are similar in appearance to bacteria. However, they are biochemically and genetically very different from bacteria. Many books and other forms of scientific literature refer to them as archaebacteria

Bacteria - are simple single-celled organisms that generally lack chlorophyll (an exception is cyanobacteria. Bacteria have a prokaryote cell type. They also generally obtain energy for survival through the breakdown of organic matter via fermentation and respiration. Bacteria such as Rhizobium spp. and cyanobacteria play an important role in the fixing of atmospheric nitrogen. Without these bacteria ecosystems would be severely short of nitrogen for plant and animal growth. The oldest fossils of life on Earth are bacteria-like organisms.

Eukaryota - are all organisms with a eukaryote cell type. This group of life includes the kingdoms Protista, Fungi, Animalia, and Plantae.

Viruses - are fragments of DNA or RNA that depend on host cells that they infect for their reproduction. They are not cells. Viruses are thought to be parts of the genetic code that originated from either eukaryote or prokaryote cells. These code fragments contain enough genetic information for self-existence. At times, viruses are metabolically inert and technically non-living. Viruses cause a variety of diseases in eukaryote organisms. In humans they can cause smallpox, chicken pox, influenza, shingles, herpes, polio, ebola, AIDS, rabies, and some types of cancer.

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The oldest known fossils are cyanobacteria like this filamentous Palaeolyngbya from Archaean rocks of western Australia, dated 3.5 billion years old. 

©1994 by The Museum of Paleontology of The University of California at Berkeley and the Regents of the University of California

I may be small but I'm old
Bacteria like filaments?
Materials found in Martian meteorite. Compare with cyanobacteria specimen above.
Image source: NASA/JSC
Another misconception concerning the origin of life is that, to explain how life arose on earth chemists and biologists must somehow be able to mix different chemicals in a test tube and have a living cell form. Again, not so-- Creating cellular life in the laboratory is currently impossible. Instead, scientists try to postulate a reasonable set of conditions under which life could have arisen on the early earth, and for the most part, they have succeeded. 

The four biological entities described above share some unique characteristics that can allow us to distinguish them from non-living things. These include: 

(1) Organisms tend to be complex and highly organized. Chemicals found within their bodies are synthesized through metabolic processes into structures that have defined purposes. Cells and their various organelles are examples of such structures. Cells are also the basic functioning unit of life. Cells are often organized into organs to create higher levels of complexity and function. 

(2) Living things have the ability to take energy from their environment and change it from one form to another. This energy is usually used to facilitate their growth and reproduction. We call the process that allows for this facilitation metabolism. 

(3) Organisms tend to be homeostatic. In other words, they regulate their bodies and other internal structures to certain normal parameters. 

(4) Living creatures respond to stimuli. Cues in their environment cause them to react through behavior, metabolism, and physiological change. 

(5) Living things reproduce themselves by making copies of themselves. Reproduction can either be sexual or asexual. Sexual reproduction involves the fusing of haploid genetic material from two individuals. This process creates populations with much greater genetic diversity.

(6) Organisms tend to grow and develop. Growth involves the conversion of consumed materials into biomass, new individuals, and waste.

(7). Life adapts and evolves in step with external changes in the environment through mutation and natural selection. This process acts over relatively long periods of time.

 Five fundamental questions are asked concerning the origin of life, that need to be answered when discussing the origin of life . These are: (1) Where did the raw materials for life come from? (2) How did monomers develop? (3) How did polymers develop? (4) How was an isolated cell formed? and (5) How did reproduction start? 
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Where did the raw materials for life come from?: The early earth: provided all the elements and chemicals needed for life to begin. 

How did monomers develop? addresses the question of how organic materials could form from inorganic processes. Monomers are simple organic compounds, such as amino acids and simple sugars. For a long time scientists believed that there was a barrier between inorganic processes and organic processes. Surely, inorganic chemical reactions could not produce organic molecules. But this idea was proved wrong by several very famous experiments performed in the late 1940s and early 1950s -the Miller-Urey experiments, named after co-workers Stanley L. Miller and Harold Urey. Through their experiments, Miller and Urey proved that organic molecules, amino acids in particular, could be produced by inorganic processes. They placed the ingredients thought to have been present at the start of the earth in a large flask. These ingredients included water, hydrogen, methane, and ammonia. The mixture was then heated to boiling. The steam and water droplets were subjected to electrical sparks, to simulate early storm events, and then the gas was cooled and recirculated. Miller and Urey continued the experiment for just over one week, At the end of that time, the mixture in the bottom of the flask contained abundant amino acids and many other simple organic compounds. 

Since then, these types of experiments have been performed many times, with similar results, The one basic requisite for the success of the experiments is the absence of oxygen, Oxygen prevents the simple organics from forming. Abundant evidence exists to show that the atmosphere was devoid of free oxygen during the early Archean, when processes similar to this were occurring, In summary, the Miller-Urey experiments have shown that anon acids could have easily formed under postulated Precambrian conditions, 

Another source for amino acids, excluding inorganic chemical processes, is meteorites. This idea is relatively new and highly controversial. Upon careful examination of the interiors of some meteorites, researchers have found amino acids and other organic structures. Perhaps when the earth was very young, the profusion of meteorite impacts "seeded" amino acids in the developing oceans. But there is one major problem with this idea. How do we know the amino acids were originally present in the meteorites? Perhaps the meteorites became contaminated with amino acids when they struck the earth or when a geologist handled the rock. Whether this idea proves to be possible or not, amino acids could have easily formed in the developing oceans by inorganic processes, which satisfactorily answers the question regarding the origin of monomers. 
The famous ALH84001 Mars Metorite
Image source: NASA/JSC
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How did polymers develop? concerns how simple organic molecules linked into longer chains of organic molecules, a process called polymerization. Examples of polymers include various types of proteins and nucleic acids (such as RNA), Again, numerous models have been development to explain polymerization. One recent idea is that the simple organic molecules may have become concentrated on the surfaces of chemically complex minerals, such as clays, and grown into longer chains, Another idea is that water containing amino acids and other simple organic molecules may have totally evaporated near volcanic vents on the shores of ancient oceans. Experiments have shown that when water containing amino acids evaporates completely, the amino acids spontaneously form longer chains-polymers. Many other models of polymerization have also been used, Overall, the question of polymerization has also been answered. 
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How was an isolated cell formed? -concerns the development of the cell wall. Once complex organic molecules formed, how were they isolated into a cell-like structure? Only a few models for this process have been proposed, and they are not without flaws. To date, the question of the development of a cell wall has proved difficult to answer. 

How did reproduction start? remains, essentially, unanswered. 
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