The continuation of life on Earth relies on the Earth System maintaining planetary conditions in an equilibrium suitable for its continuation. But the Earth System did not just spring into existence – it developed slowly as complex life evolved and modern-style ecosystems appeared. In this chapter I review the history of early life, beginning with the oldest Snowball Earth event (about 700 million years ago) through to the Cambrian Explosion (about 480 million years ago). A field trip down Brachina Creek in the Flinders Ranges of South Australia that start in the early Cambrian and ends at the boundary of the Cryogenian and Ediacaran Periods sets the scene for the discussion. Intimately associated with the evolution of complex organisms and the appearance of the Earth System is the story of oxygen. This history of this important gas and its role in the evolution of life is briefly reviewed.