Generatio spontanea

Abiogenesis, or informally the origin of life, is the natural process by which life arises from non-living matter, such as simple organic compounds. The transition from non-living to living entities was not a single event but a gradual process of increasing complexity.

Abiogenesis is studied through a combination of paleontology, chemistry, and extrapolation from the characteristics of modern organisms, and aims to determine how pre-life chemical reactions gave rise to life. The study of abiogenesis can be geophysical, chemical, or biological, with more recent approaches attempting a synthesis of all three, as life arose under conditions that are strikingly different from those on Earth today. Life functions through the specialized chemistry of carbon and water and is largely based upon four key families of chemicals: lipids (fatty cell walls), carbohydrates (sugars, cellulose), amino acids (protein metabolism), and nucleic acids (self-replicating DNA and RNA). Any successful theory of abiogenesis must explain the origins and interactions of these classes of molecules. Many approaches to abiogenesis investigate how self-replicating molecules, or their components, came into existence. It is generally thought that current life on Earth is descended from an RNA world, although RNA-based life may not have been the first life to have existed.

The classic Miller–Urey experiment and similar research demonstrated that most amino acids, the chemical constituents of the proteins used in all living organisms, can be synthesized from inorganic compounds under conditions intended to replicate those of the early Earth. Various external sources of energy that may have triggered these reactions have been proposed, including lightning and radiation. Other approaches ("metabolism-first" hypotheses) focus on understanding how catalysis in chemical systems on the early Earth might have provided the precursor molecules necessary for self-replication. Complex organic molecules have been found in the Solar System and in interstellar space, and these molecules may have provided starting material for the development of life on Earth.

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