The understanding of the origin of life was largely speculative until the 1920s, when Oparin and Haldane, working independently, proposed a theoretical model for "chemical evolution."
The Oparin-Haldane model suggested that under the strongly reducing conditions (no oxygen believed to be present 4.0 billion years ago), inorganic molecules would, with an energy source, spontaneously form organic molecules (simple sugars and amino acids).

Miller
Urey
In 1953, Stanley Miller, along with his graduate advisor Harold Urey, tested this hypothesis by constructing an apparatus that simulated the Oparin-
Haldane "early earth."
When a gas mixture based on predictions of the early atmosphere was heated and given an electrical charge, organic compounds were formed
 water (H
2
O
 methane
(CH
4
)
 ammonia
(NH
3
) and
 hydrogen
(H
2
)
 but no oxygen

Panspermia Theory
This theory suggests that life seeds came from outer space and planets exchanged life. Panspermia literally means seeds everywhere.
Panspermia suggests that life could have existed on another planet and moved to Earth. Statistics have showed 7.5% of rocks from Mars reach Earth. The rocks would travel between less than 100 years to
16,000 years and more to get to earth.

This meteorite blasted off of the surface of the planet Mars since its creation some 180 million years ago maybe by the impact of an asteroid on the surface and traveled through space to the earth, where it landed on Antarctica.
This meteorite contains traces of Martian atmospheric gases and shows signs of having reacted with Mars’ groundwater.

Evidence for Panspermia
1. Bacteria can survive harsh environment of space a. Ultraviolet radiation b. Protons bombardments c. Cold
2. Evidence that meteorites contain life a. Amino acids b. Bacteria c. Carbon d. Protected inside rocks
3. Bacteria can live for a long time in sleeping state until awakened
4. Mars safer than Earth (less bombardments and less gravity)
5. Mars not as hot as Earth in early development
6. Mars had oxygen back when earth did not

Gaia theory is a class of scientific models of the biosphere in which life fosters and maintains suitable conditions for itself by affecting
Earth's environment. The first such theory was created by the
English atmospheric scientist,
James Lovelock , in 1969 . He hypothesized that the living matter of the planet functioned like a single organism and named this self-regulating living system after the Greek goddess , Gaia .

1.
Nearly all major and minor components of Earth's atmosphere are out of chemical equilibrium by 10 to 30 orders of magnitude.
2.
Solar luminosity has increased from 25% to 70% during the last 4.5 billion years (Earth's age) . By this estimate, Earth's surface temperature should have increased significantly. However, geologic evidence indicates that surface temperatures on Earth have remained remarkably stable for at least 3 billion years.

is a controversial set of arguments which assert that empirical evidence supports the conclusion that life on Earth was deliberately designed by one or more intelligent agents.
ID advocates argue that the standard scientific model of evolution by natural selection is insufficient to explain the origin, complexity, and diversity of life. More specifically, ID adherents believe that there exist instances of irreducible complexity , which in their view are improbable to evolve and therefore it is highly probable to have been created by an intelligent designer or designers.

Heterotroph Hypothesis
1. Alexandar Oparin - 1936 - Stated that the earliest living organisms on earth were simple heterotrophs (organisms that cannot make their own food)
The strength of the heterotroph hypothesis is that it gives the first forms of life a source of "food" that doesn't itself come from living things: the primordial soup. This is how the rest is supposed to have happened...

The ability to self-organize.
This requires some already-formed building blocks, from the soup, and a source of energy that would serve to help drive them into increasingly complex forms.
Experiments in the early 1950s began to confirm that such processes could at least begin. Those experiments used lightning, confined to a
"primordial soup bottle," to stimulate production of complex materials. Since that time, all of the various forms of energy available on the early earth have been tested, with varying results. The current
"leading contender" for life-organizer are Hydrothermal vents , openings between Earth's surface plates at the bottom of the oceans.
There, water mixes with hot magma and releases a hot soup of materials even today. They have an energy source - heat - a source of materials - once soup, now magma - and, perhaps most importantly, are a stable, long-lasting ecosystem and a place to "work the bugs out" of the earliest living systems.

The ability to reproduce.
As these early self-organizing molecules grew, only those which could make and spread copies of themselves had any real future. Life on today's Earth uses DNA code to store all of the information it needs to make the proteins it actually runs on, but DNA has little activity beyond that, and proteins generally can't duplicate themselves. There are theories that try to address those problems, but the leading current theory is that the first really complex systems were of RNA, a hypothesis usually called the RNA World hypothesis .
RNA has DNA's coding abilities and some protein-like activity, and it isn't difficult to see the evolution of a DNA-coded protein system growing quickly from an RNA ancestor.

The ability to evolve.
Once you have a planetwide ocean full of selforganizing molecules able to reproduce themselves, you have a competitive ecosystem where selection can take place. This stage of molecular evolution would have favored those who could work most efficiently, or best accumulate building blocks, or reproduce the fastest, or work with other molecules in a cooperative fashion, perhaps linking RNA or DNA codes for particularly good proteins together to work as a unified system. And these unified systems might work even better with some confinement and protection...

refers to the merging of two separate organisms to form a single new organism. The idea originated with Konstantin
Mereschkowsky in his 1926 book Symbiogenesis and the Origin of
Species , who proposed that chloroplasts originated from cyanobacteria captured by a protozoan . Today both chloroplasts and mitochondria are believed to have such an origin; see endosymbiotic hypothesis .