Scientific Theories on the Origin of Life on Earth

The Miller-Urey Experiment and Chemical Evolution

Years later, the ideas of these two investigators inspired Stanley Miller to conduct an experiment that simulated the primitive conditions of the Earth in a glass bottle. He deposited several simple compounds in the bottle and irradiated the mixture with ultraviolet light and X-rays, which were supposed to exist on the surface of early Earth due to the lack of oxygen in the atmosphere. The result of this experiment was surprising, since after a while, he obtained complex organic molecules, such as some amino acids and nitrogenous bases that are essential for living organisms. Thus, Miller showed that it was perfectly possible to obtain complex organic molecules from simple chemical compounds with relative ease, which represented a sort of confirmation of the ideas of Oparin and Haldane.

Submarine Hydrothermal Vents

• According to current data, the early atmosphere would be less reducing than what Miller assumed, which meant it was harder for organic compounds to form.
• The primordial soup in the early ocean would be more dilute than what is needed for the formation of complex organic molecules from other simpler ones.

To overcome all this, the view is accepted that alternative environments more conducive to life originated in submarine hydrothermal vents, also known as black smokers or chimneys. These are places that emanate volcanic gases and can now be found on the ocean floor. They thrive with primitive organisms known as thermophilic bacteria that are capable of withstanding high temperatures. This option has the advantage of being a viable stage for the emergence of life.

The Panspermia Hypothesis

Panspermia holds that the first organisms originated outside the Earth and traveled here on an asteroid or comet. These ideas have their origin in some of the considerations of the Greek philosopher Anaxagoras. Although they were often considered pure fantasy—as any meteor on contact with the Earth's atmosphere becomes incandescent, thus eliminating any transported organic content—we also know that rocks are poor conductors of heat, and the fusion would only adversely affect the outermost layer. There were two hypotheses that emerged:

• In 1969, a meteorite fell in Australia containing numerous organic compounds, including several amino acids.
• In 1996, they found traces of fossil microorganisms reminiscent of terrestrial bacteria in another meteorite of Martian origin.

Panspermia is now considered a possible alternative. An asteroid hitting a planet and the impact splatter can end up on any other planet.