Conditions on Early Earth

On Earth, around four billion years ago, a series of volcanic eruptions released gases into a water-vapor–rich atmosphere. This atmosphere was likely dominated by methane and ammonia, with smaller amounts of hydrogen and other gases.

As Earth cooled, water vapor condensed and formed oceans.

In the 1920s, scientists suggested that UV radiation or lightning could have helped small organic molecules form.

In 1952, Stanley Miller and Harold Urey tested this idea by simulating early Earth’s atmosphere and oceans in a laboratory system. Their system was based on the model of early Earth available at the time.

When electricity was applied to the system, organic molecules such as amino acids formed in a simulated early Earth environment. Scientists think these early organic molecules may also have formed near hydrothermal vents, in volcanic regions, or at meteorite impact sites.

These amino acids could later serve as the building blocks of proteins.

Next, scientists found that nucleotides can join to form short chains similar to RNA and DNA. This showed that early biomolecules may have formed from smaller building blocks.

Another class of macromolecules, called lipids, may have formed through chemical reactions on early Earth. These lipids could self-organize to form vesicles. These vesicles created an internal space that was separate from the outside environment.

This internal space may have helped concentrate molecules and support chemical reactions. This step was important for the formation of protocells.

These protocells likely contained RNA as genetic material. Scientists think that early RNA molecules may have been able to copy themselves with the help of catalytic RNA activity and pass information to later generations.

Extreme conditions on early Earth helped form these RNA-containing protocells. From these early systems, DNA-containing cells likely evolved.