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 • Basic Overview
Light and Early Life
Single-celled organisms transformed the Earth. Some prokaryotes developed a way to turn the sun’s energy into the energy they needed to live. They use sunlight to drive a chemical reaction called photosynthesis, which turns carbon dioxide and water into sugars for food.
Photosynthesis creates a waste product: oxygen. Over two billion years, photosynthetic bacteria releasing waste oxygen into the oceans and atmosphere altered our planet dramatically in two ways:
It rusted the iron in oceans.
Free oxygen—oxygen molecules not bonded with other chemical molecules—causes iron to oxidize, or rust. Massive amounts of waste oxygen released by photosynthetic bacteria rusted iron particles in the oceans.
 It created our oxygen rich world.
As oxygen from photosynthesis saturated the oceans, it escaped into the atmosphere—creating the air we breathe today.
Some bacteria could not live in the presence of free oxygen, and went extinct. But over time, oxygen paved the way for new life. By forming the ozone layer, which blocks the sun’s harmful radiation, oxygen created a protective environment in which new life forms evolved that flourished in the presence of oxygen.
Continue to Early Life Evolves. >>
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