While Charles Darwin's Theory of Evolution is about how species change to adapt to their environment, it doesn't address the question of how life began originally. At one point, certainly when the planet was still hot and molten, there was no life on Earth, though we know life evolved later.

The question is, how did early Earth life forms originate?

There are several theories on how the basic building blocks of living organisms came into being. The mechanism of how nonliving matter became self-replicating living organisms and then complex life forms is not fully understood.

It has some gaps, but abiogenesis deals with interesting concepts and makes a start at an explanation.

Abiogenesis is the natural process by which living organisms arose from nonliving organic molecules. Simple elements combined to form compounds; the compounds became more structured and involved different substances. Eventually, simple organic compounds were formed and linked to produce complex molecules such as amino acids.

Amino acids are the building blocks of the proteins that form the basis of organic processes. The amino acids could have combined to form protein chains. These proteins could have become self-replicating and formed the basis for simple life forms.

Such a process could not take place on Earth today because the necessary conditions no longer exist. The creation of organic molecules presupposes the presence of a warm broth that contains the substances required for those organic molecules to appear.

Elements and simple compounds such as hydrogen, carbon, phosphates and sugars all have to be present together. An energy source such as ultraviolet rays or lightning discharges would help them bond. Conditions like this may have existed 3.5 million years ago when life on Earth is thought to have started. Abiogenesis details the mechanisms of how that might have taken place.

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Both abiogenesis and spontaneous generation propose that life can originate from non-living matter, but the details of the two are completely different. While abiogenesis is a valid theory that has not been disproven, spontaneous generation is an outdated belief that has been shown to be incorrect.

The two theories differ in three major ways. The theory of abiogenesis states that:

1. Abiogenesis happens rarely. It happened at least once about 3.5 billion years ago and probably has not occurred since then.
2. Abiogenesis gives rise to the most primitive forms of life possible. These may be as simple as replicating protein molecules.
3. Higher organisms evolve from these primitive life forms.

The theory of spontaneous generation states that:

1. Spontaneous generation happens frequently, even in modern times. For example, every time meat is left to rot, it generates flies.
2. Spontaneous generation gives rise to complex organisms such as flies, animals and even humans.
3. Higher organisms are the result of spontaneous generation, and they don't evolve from other life forms.

Scientists used to believe in spontaneous generation, but today even the general public no longer believes that flies come from rotten meat or mice come from garbage. Some scientists also question whether abiogenesis is a valid theory, but they have been unable to propose a better alternative.

How life might have originated was first proposed by Russian scientist Alexander Oparin in 1924 and independently again by British biologist J.B.S. Haldane in 1929. Both assumed that early Earth had an environment rich in ammonia, carbon dioxide, hydrogen and carbon, the building blocks of organic molecules.

Ultraviolet rays and lightning provided the energy for chemical reactions that would allow these molecules to link up.

A typical chain of reactions would proceed as follows:

1. Prebiotic atmosphere with ammonia, carbon dioxide and water vapor.
2. Lightning produces simple organic compounds that fall into solution in shallow water.
3. The compounds react further in a prebiotic broth, forming amino acids.
4. The amino acids link up with peptide bonds to form polypeptide chain proteins.
5. The proteins combine into more complex molecules that can replicate and metabolize simple substances.
6. Complex molecules and organic compounds form lipid membranes around themselves and start acting like living cells.

While the theory presented consistent and credible concepts, some of the steps proved difficult to carry out under laboratory conditions that tried to simulate those on early Earth.

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In the early 1950s, American graduate student Stanley Miller and his graduate advisor Harold Urey decided to test the Oparin-Haldane abiogenesis theory by recreating an early Earth environment. They mixed the simple compounds and elements from the theory in air and discharged sparks through the mixture.

When they analyzed the resulting chemical reaction products, they were able to detect amino acids created during the simulation. This evidence that the first part of the theory was correct supported later experiments that tried to create replicating molecules from the amino acids. These experiments were unsuccessful.

Subsequent research found that the prebiotic atmosphere of early Earth probably had more oxygen and fewer other key substances than the sample used in the Miller-Urey experiment. This led to questioning whether the conclusions were still valid.

Since then, some experiments using a corrected atmosphere composition have also found organic molecules such as amino acids, thus supporting the original conclusions.

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Even when it is established that the conditions for the generation of simple organic compounds were present on prebiotic Earth, the path to living cells has been in dispute. There are three possible ways relatively simple compounds such as amino acids could eventually become self-sustaining life:

1. Replication first: The organic molecules become more and more complex until they include DNA segments that can replicate themselves. The self-replicating molecules develop cell behavior and metabolism.
2. Metabolism first: The organic molecules develop the ability to sustain themselves by integrating and changing substances from their surroundings. They become proto-cells and develop the ability to replicate.
3. RNA world: The organic molecules become precursor RNA segments that can produce DNA molecule copies. They develop metabolism and cell-like behavior at the same time.

The steps from amino acids on were a serious problem, and none of the different theoretical paths have, as of May 2019, been successfully simulated.

There is no doubt that a simulation of the early Earth atmosphere can produce comparatively complex molecules that are the building blocks of the organic molecules found in living cells. There are, however, several problems getting from the complex molecules to actual life forms. These include:

• There is no detailed theoretical path to go from complex organic molecules to a life form.
• There are no successful experiments supporting the formation of molecules more complex than amino acids.
• There is no mechanism for RNA building blocks to develop into the purine/pyrimidine bases of full RNA.
• There is no consensus on how the replicating/metabolizing molecules become life forms.

If abiogenesis does not take place in the way the theory describes, alternative ideas have to be considered.

With progress on abiogenesis seemingly blocked, alternative theories for the origin of life have been proposed. Life might have originated in a way similar to the abiogenesis theory but in geothermal vents under the sea or within the Earth's crust, and it may have happened several times in different places. None of these theories have any more hard data support than classic abiogenesis.

In another theory that abandons abiogenesis altogether, scientists have proposed that complex organic compounds or complete life forms such as viruses may have been delivered to Earth by meteorites or comets. Early Earth (primitive Earth) was subjected to heavy bombardment during Hadean time (about 4 to 4.6 billion years ago) when life may have started.

Without more hard data, the only conclusion is that exactly how life on Earth originated is still a mystery.