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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.