Gases in both Earth's atmosphere and oceans have changed over its history. Scientists noted oxygen spikes in both the oceans and the atmosphere that coincide with changes in species on Earth. They report that the first oxygen spike resulted in a change from the microscopic life forms of 635 million years ago to large algae around 551 million years ago. A second oxygen spike occurring around 550 and 542 million years ago, according to the fossil records, resulted in an increase in