11.14: Conditions on Early Earth

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.

How did cells first emerge on planet Earth? Based on experimental observations, scientists think that specific conditions on early Earth led to the formation of cells in several steps. A series of volcanic eruptions occurred on Earth around four billion years ago, contributing ammonia, methane, hydrogen, and other gases to an atmosphere full of water vapor. As the Earth cooled, the water vapor condensed into oceans.

In the 1920s, scientists suggested that UV radiation or lighting could've caused the formation of small organic molecules in these oceans. In 1952, Stanley Miller and Harold Urey tested this idea in the laboratory. The Miller-Urey experiment simulated Earth's early atmosphere and oceans. In the presence of electricity, they observed the formation of amino acids, the building blocks of proteins. Since then, scientists have suggested that these building blocks may have formed near hydrothermal vents in the ocean, near volcanic activity, or as the result of meteorites hitting Earth.

How did the complex molecules form that we know today? Recent studies have shown that nucleotides can spontaneously link together to form nucleic acids. This indicates that the earliest biomolecules self-assembled from smaller building blocks. Another class of macromolecules, called lipids, can self-organize and form vesicles, which separate the inside of the vesicle from the outer environment. The constant environment within the vesicles may have facilitated the formation of protocells, a critical step in the evolution of life.

These protocells likely contained RNA as genetic material. The RNA self-replicated and was passed on to subsequent generations. The extreme conditions on early Earth enabled the formation of these RNA-containing protocells from which DNA-containing cells likely evolved.