Oxygen changes everything
The earliest living things were bacteria, which grew by using chemicals in the sea. When the chemicals started to run out, it became harder for life to survive. Some living thing, called cyanobacteria, found a new way of growing, called photosynthesis. Photosynthesis works by using the energy in sunlight, and it produces oxygen as a waste product. As this new gas began to build up, the Earth's atmosphere changed, creating the kind of air we breathe today.
Some prehistoric bacteria from the start of life have descendants that we can study. This bacteria cannot tolerate oxygen, because it was not part of Earth's ancient atmosphere. They live in the black mud at the bottom of marshes, or in other places without oxygen. They make food by chemical reactions, and do not rely on sunlight like other living things.