A) The Earth’s early atmosphere had no oxygen and was made mostly of nitrogen, with some carbon dioxide, water vapour and hydrogen compounds such as methane and ammonia. Within million of years life evolved in the oceans. 3.2 billion years ago photosynthesis had evolved with the first oxygen producing cyanobacteria, similar to today’s blue-green algae. At first, the oxygen given off simply reacted with the rocks, but gradually it built up in the oceans and started to bubble out into the atmosphere. To start with, it was a toxic gas harmful to life, but soon life evolved to make use of the oxygen through aerobic respiration. About half-way through earth’s history, there was oxygen in the atmosphere. Without oxygen, there was no ozone in the
