Around 4 billion years ago, oceans began to condense on earth while volcanic eruptions released nitrogen, carbon dioxide, methane, ammonia, and hydrogen into the primordial atmosphere. However, organisms with the characteristics of life were not initially present on earth. Scientists have used experimentation to determine how organisms evolved that could grow, reproduce, and maintain an internal environment.
In the 1920s, the scientists Oparin and Haldane proposed the idea that simple biological compounds could have formed on the early earth. More than 30 years later, Stanley Miller and Harold Urey at the University of Chicago tested this hypothesis by simulating the conditions of the early earth's atmosphere and oceans in a laboratory apparatus. Using electricity as an energy source, the Miller-Urey experiment generated amino acids and other organic molecules, showing that the environment of early earth was conducive to the formation of biological molecules. More recent experiments have yielded comparable results and suggest that amino acids may have formed near areas of volcanic activity or hydrothermal vents in the ocean.
Amino acids and small organic molecules may then have self-assembled to form more complex macromolecules. For instance, dripping amino acids or nucleotides into hot sand can result in the formation of the corresponding polymers, proteins, and nucleic acids, respectively. A class of macromolecules called lipids may have then formed vesicles providing a separate, internal environment. This ability to separate the inside from the outside is one of the key characteristics of life. Another characteristic of life is the possession of genetic material; RNA was likely the first heritable genetic information. Specialized vesicles, called protocells, probably contained RNA that could replicate itself. These simple protocells could also grow and evolve, setting the stage for the formation of cellular life on earth.
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