A form of "quasi-life" that is thought by most biological scientists to be the precursor to life on Earth. A protocell is similar to the modern cell in many ways; they are both primarily composed of proteins, they both divide, and they both have a cell membrane. However, protocells do not have any organelles, and their metabolism processes are still unknown. Because of this, protocells are not classified as cells and so are described as "quasi-life".
Protocells were thought to have formed about 4 billion years ago, when the Earth had a reducing atmosphere composed of such gases as ammonia, methane, water vapor, and hydrogen. It is thought that an electric spark from lightning or ultraviolet rays from the sun "fused" these chemicals into amino acids, which then formed into protocells.
This process was re-created in the laboratory with positive results. Stanley Miller ran an electric discharge through a beaker filled with the gases from the primitive reducing atmosphere (described above). Amino acids and other biological compounds started to form at the bottom of the flask. A few years later, Sidney Fox heated a solution of amino acids, and artificial protocells were formed.