Unformatted text preview: came cool enough to allow liquid water to rest on the surface. Life probably arose on Earth shortly after its formation, about 4 billion years ago. Life may have arisen in the deep ocean. Earth’s atmosphere has changed substantially through time. The most dramatic change began about 2 billion years ago when photosynthetic or-ganisms began producing large quantities of oxygen. Other planets and moons may have—or may have had—oceans....
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- Spring '08
- 2 billion years, 4.6 billion years, 5 billion years, 4 billion years, formation heavy materials