Earth came into existence about 4.6 billion years ago, and about 3.8 billion years ago, the chemical composition of Earth’s surface began to change. Scientists estimate that at about 3.5 billion years ago, the first cells were in existence.
Scientists believe that the first cells lived within the organic environment of Earth and used organic foods for their energy. The type of chemistry in those first cells was somewhat similar to fermentation, which uses organic molecules such as glucose. The energy yield, although minimal, is enough to sustain simple living things (see Chapter 6). However, organic material would soon have been used up if this were the sole source of nutrition, so a new process had to develop.
The evolution of a pigment system that could capture energy from sunlight and store it in chemical bonds was an essential evolutionary breakthrough. The organisms that possess these pigments are commonly referred to as cyanobacteria (at one time, they were called blue-green algae). These single-celled organisms produce carbohydrates by the process of photosynthesis. In doing so, they produce oxygen as a waste product (see Chapter 5). For about 1 billion years, photosynthesis provided oxygen to the atmosphere, which gradually changed until it became oxygen-rich, as it is today.
Another group of organisms that was present at the same time as the cyanobacteria was a group of bacteria called archae. Archae differ from other species of bacteria (known as kingdom eubacteria and representative of domain Bacteria) in that archae have a different ribosomal structure, different cell membrane composition, and different cell wall composition. The archae have been traced to a period of about 3 billion years ago. They are able to multiply at the very high temperatures that were present on Earth then, and their nutritional requirements reflect the composition of primitive Earth.