Oxygen on Early Earth

Astrobiology has a good article on the substantial impact that the rise of oxygen producing organisms have had on our planet. There have been two major increases in the atmospheric oxygen levels in our distant past - both occurring between 635 and 551 million years ago.

"Today, we take oxygen for granted. But the atmosphere had almost no oxygen until 2.5 billion years ago, and it was not until about 600 million years ago when the atmospheric oxygen level rose to a fraction of modern levels. For a long time, geologists and evolutionary biologists have speculated that the rise of the breathing gas and subsequent oxygenation of the deep oceans are intimately tied to the evolution of modern biological systems."

The first change sparked a slight change in microscopic life forms, with the second having a larger impact on the evolution and diversification of early life forms, including a significant spike in complex algaes. Following these developments, the number of species nearly doubled, and shortly thereafter (in evolutionary terms, anyway) the first animals to develop external skeletal structures (shells) evolved.