The ability of cyanobacteria to perform oxygenic photosynthesis is highly significant. Early cyanobacteria in stromatolites are thought to be largely responsible for increasing the amount of oxygen in the primaeval Earth's atmosphere through their continuing photosynthesis. They were the first known organisms to photosynthesis and produce free oxygen. After about a billion years, the effect of this photosynthesis began to make a huge change in the atmosphere. The process, called the Great Oxygenation Event, actually took a long time. Eventually, it killed off most of the organisms which could not live in oxygen, and led to the kinds of environment we know today, where most organisms use and need oxygen.