Breathe in … breathe out.
Ahh. Hooray for oxygen, the element that keeps much of life on Earth humming.
Element No. 8 on the Periodic Table of the Elements is a colorless gas that makes up 21 percent of Earth's atmosphere. Because it's all around, oxygen is easy to dismiss as dull and inert; in fact, it's the most reactive of the non-metallic elements.
Earth has been oxygenated for about 2.3 billion to 2.4 billion years, and levels began to creep up at least 2.5 billion years ago, according to a 2007 NASA-funded study. No one knows quite why this lung-friendly gas suddenly became a significant part of the atmosphere, but it's possible that geologic changes on Earth led to oxygen produced by photosynthesizing organisms sticking around, rather than being consumed in geologic reactions, according to the study researchers.
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