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5 BCE
Spontanenous Generation
According to this theory, living organisms arose from the combination of inanimate matter and a unique energy known as vital force. This hypothesis was widespread in ancient times until very recently. Alleged examples included the seasonal generation of mice and other animals from the mud of the Nile, the emergence of fleas from inanimate matter such as dust, or the appearance of maggots in dead flies. -
Panspermia Theory
The panspermia theory propose that microbes capable of surviving in space may become trapped in debris ejected into space after collisions between planets and small solar system bodies that harbour life. This debris, containing the lifeforms, can then be transported by meteors between bodies in a solar system, or even across solar systems within a galaxy. This theory contrasts with the spontaneous generation theory, which posits that life originated from inanimate objects. -
Molecular Panspermia
According to this theory, many of the small organic molecules necessary for life originated in space and were able to support the development of life on Earth due to favourable chemical and energetic conditions. This text describes a subset of the panspermia theory, which proposes that life on Earth originated from distant planets; the panspermia theory as a whole proposes that life came to Earth from other planets. -
Prebiotic synthesis and chemical evolution
The Oparin-Haldane hypothesis, proposed by Alexander Oparin and J.B.S. Haldane, suggests that the first molecules that formed the earliest cells self-organised from a primordial soup. This contradicts the molecular panspermia theory of the origin of biomolecules from space, and instead suggests that the first living organisms emerged from inorganic matter through various physico-chemical processes facilitated by the conditions on the primitive Earth. -
Current Hypothesis of Prebiotic synthesis and Chemical evolution
Stanley Miller and Harold Urey carried out a chemical experiment to demonstrate the Oparin-Haldane hypothesis. The experiment used methane, ammonia, hydrogen and water vapour to produce simple organic monomers, including amino acids (described in the previous hypothesis). However, these gases were not present in the primitive atmosphere. It is believed that the prebiotic synthesis and the chemical evolution could have taken in deep-sea hydrothermal vents, which expelled reducing compounds.