4.6 billion years is a very long time. If you stacked 4.6 billion pieces of paper on top of each other, the enormous pile would be 460 kilometers (286 miles) high.

About 4.6 billion years ago our Earth looked very different than it does today. Instead of the beautiful blues, greens, and whites of today, it would have looked red and orange. The surface of our planet was covered in oceans of hot lava. Instead of breathable oxygen, the atmosphere was made of a mix of deadly poisons.

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For the first 500 million years, the Earth would remain as a giant lifeless ball of fiery lava. For another 300 million years after that, it would be too hot for liquid water to form. Finally, after 800 million years, an unimaginably long period of time, our home planet cooled enough for liquid water to begin to form. Everywhere rains began to fall, filling the lower portions of the Earth with water, forming lakes, oceans, and rivers.

Still, there was no life. No fishes to fill these oceans, no plants to cover the rocky surface, no animals to graze the wilderness. Just a barren wet rock, orbiting the Sun. The Earth's atmosphere now contained a mix of very poisonous gases, including carbon monoxide, carbon dioxide, nitrogen, hydrogen sulfide, methane, and cyanide. Breathing this air would cause death to most life forms as we know them.