The appearance of the first cells marked the origin of life on Earth. However, before cells could form, the organic molecules must have united with one another to form more complex molecules called polymers. Examples of polymers are polysaccharides and proteins.
In the 1950s, Sidney Fox placed amino acids in primitive Earth conditions and showed that amino acids would unite to form polymers called proteinoids. The proteinoids were apparently able to act as enzymes and catalyze organic reactions.
More recent evidence indicates that RNA molecules have the ability to direct the synthesis of new RNA molecules as well as DNA molecules. Because DNA provides the genetic code for protein synthesis, it is conceivable that DNA may have formed in the primitive Earth environment as a consequence of RNA activity. Then DNA activity could have led to protein synthesis (see Chapter 10).
For a cell to come into being, some sort of enclosing membrane is required to hold together the organic materials of the cytoplasm. A generation ago, scientists believed that membranous droplets formed spontaneously. These membranous droplets, called protocells, were presumed to be the first cells. Modern scientists believe, however, that protocells do not carry any genetic information and lack the internal organization of cells. Thus the protocell perspective is not widely accepted. Several groups of scientists are currently investigating the synthesis of polypeptides and short nucleic acids on the surface of clay. The origin of the first cells remains a mystery.