The theory put forth to explain the origins of the Universe, our solar system, and our planet is called the Big Bang Theory. The Big Bang Theory IS NOT EVOLUTION! (The theory of evolution deals with living organisms, once they have come into existance.) The Big Bang theory says that all matter in the Universe was, at one time, concentrated in a giant mass (a black hole?) that blew apart about 10 - 20 bya (billion years ago) and is still expanding. About 5 bya, some of the matter condensed until forces were so strong that thermonuclear reactions began, and this was the origin of our sun. A disk-shaped cloud of matter orbiting the sun subsequently condensed into the planets. Thus, about 4.6 bya, the planets coalesced, and it is thought that Earth began as cold world. Later, due to whatever factors, the planet heated up enough to melt and sort into layers by density (core, mantle, crust). It is thought that the very first atmosphere may have been hydrogen gas, but since that is so light weight and very chemically reactive, most of it would have floated off into space or reacted with other substances, thus would have been rapidly dissipated.
Alexander Ivanovich Oparin (publ. 1936), a Russian scientist, in The
Origins of Life, described hypothetical conditions which he felt would
have been necessary for life to first come into existence on early Earth.
This, thus, is referred to as the Oparin Hypothesis. He theorized
that the first atmosphere was made largely of water vapor (H2O),
carbon dioxide (CO2), carbon monoxide (CO), nitrogen (N2),
methane (CH4, and ammonia (NH3. As the surface of
Earth cooled again, torrential rains of this mixture formed the first
seas, the “primordial soup.” Some think this may be what conditions
are like, even now, on Venus. Lightening, ultraviolet (UV) radiation,
volcanic action all were more intense than they are now.
Several possible steps/stages were suggested to get from there to living
organisms. The first step is thought to have been the abiotic
synthesis (syn = with, together; thesis = an arranging) of
organic monomers, in other words, putting inorganic chemicals like
methane, carbon dioxide, and ammonia together to form simple organic
chemicals like amino acids, simple sugars, fatty acids, and nucleic acids.
This portion of the hypothesis was later tested by an experiment done by
Stanley Miller as a grad student under Harold Urey in 1953. He used a
sterile, enclosed system consisting of a flask over a heat source, a spark
chamber, and various other tubing (see illustration). He added sterile H2O,
H2, CH4, and NH3 to the sealed system.
Heat was applied under the flask to simulate volcanic action, and this was
enough to turn a significant portion of the water into steam. A spark
chamber periodically discharged electricity into the gasses to simulate
lightening. In the return tube, the mixture was cooled to condense the
water back into liquid, along with any organic compounds that might have
formed from the mixture. Water and all the gasses Miller included are all
“clear,” thus his experiment started out with transparent water and
transparent gasses. However, after only one week, Miller had a brown,
murky soup. Subsequent chemical analysis showed the presence of a number
of amino acids and other organic compounds. Other researchers have since
tried similar experiments with slight variations in the initial mix of
chemicals added, and by now, all 20 amino acids, and a number of sugars,
lipids, and nucleotides have been obtained in this manner. From this
experiment, scientists generalize that if this can happen in a lab, it
could have happened in a similar way on early Earth. Note that ALL
that was made here was simple organic chemicals!
The next step in going from non-living to living is thought to have been the abiotic synthesis of organic polymers, possibly using hot sand or finely divided clay as a catalyst (cata = down, downward; lysis = loosen, break apart), a substance which helps a chemical reaction to go without being consumed in that reaction, which caused dehydration synthesis to occur, thereby joining the smaller molecules into larger macromolecules such as proteins, carbohydrates, RNA or lipids.
Thirdly, it is thought that non-living aggregates of these polymers formed. These may have exhibited some properties characteristic of living organisms, but were NOT ALIVE, and did not have all the properties of living organisms. In a research laboratory, scientists have seen mixtures of proteins, lipids, and carbohydrates form globules. If the proteins involved happen to be enzymes, these globules can even carry on “metabolic” activity, although they have no means to replicate themselves. Simultaneous to this, the genetic code would have to have arisen. Several widely-accepted theories as to how this may have happened include the possibly involvement of damp, zinc-containing clay as a catalyst to help the nucleotides polymerize first into RNA, and later into DNA.
It is thought, then, that about 4.1 to 3.5 bya, the first prokaryotes, organisms without a true nucleus (like bacteria) came into existance. It is difficult to pinpoint a date for this because bacteria don’t have skeletons to leave behind. The first “fossils” (remains of colonies/secretions) of prokaryotes seem to be this age. These would have been very simple cells without many of the organelles present in modern cells, especially modern eukaryotes.
Note that while some of these steps have been demonstrated in a lab, Nobaoy Has Ever Made a Living Cell in a lab. While people have demonstrated bits and pieces of this process, the whole process has never been done in a lab. Rather, this is a theory of how things might have happened.
Once the first cells, the first living organisms, the first prokaryotes came into existance, then the Theory of Evolution takes over to provide an explanation for how (not why) these primitive cells diversified into the five kingdoms of life which we recognize today.
Information provided by: http://biology.clc.uc.edu