For most of earth’s history, called the Precambrian time, living things consisted of single-celled organisms. For almost 2 billion years the only living things were prokaryotes, then, later, eukaryotes appeared. Some early prokaryotes were like cyanobacteria of today (blue-green algae) and could photosynthesize. These organisms were very important in that they produced oxygen, a substance lacking in the primitive atmosphere. The increasing levels of oxygen allowed the development of aerobic respiration. It also resulted in the formation of the ozone layer that shielded ultraviolet radiation from the earth’s surface which made colonization of the land possible.