Good question. Oxygen is very important to all life on Earth. We had to do a bit of looking for this one. Pierre Kerr, firstname.lastname@example.org, gave us this answer.
First of all, what is Oxygen? The Compton's Interactive Encyclopedia says:
"OXYGEN. The most abundant element on Earth is oxygen (chemical symbol O), and it is essential to all the planet's life forms. As the gas O2 it is in the lower atmosphere in the air that is breathed, and in the upper atmosphere as ozone (O3) it shields the Earth's surface from the sun's harmful ultraviolet rays. Ozone is a pale blue gas that is explosive and poisonous even in low concentrations."
"Oxygen constitutes about two thirds of the human body and nine tenths (by weight) of the water in rivers, lakes, and oceans. In combination with other chemical elements, it forms hundreds of thousands of compounds, making up by weight almost half of the rock and sand in the Earth's crust. In its free state, oxygen is a gas without color, odor, or taste. It turns into a pale blue liquid at temperatures below -297° F (-183° C) and becomes solid at about -360° F (-218° C)."
But, that's not what you asked. How did it get here. Scientists tend to agree that the early atmosphere of the Earth, 3 to 4 billion years ago, had very little oxygen but mostly hydrogen. Early life forms were mostly algae plants living in oceans. These plants produced oxygen.
Plants are still producing oxygen today. Next time you walk by a tree, say "Thanks for the oxygen."
Answer #% % filename Document5 page% % page 1 of% % numpages 1 % % date \@ "MMMM d, yyyy"March 24, 1995