1. Multi-cellular life began because the surface of the earth had water on it and the atmosphere began to fill with oxygen. Plus the heavy bombardment ended so life wouldn't be killed by the explosions. With this environment life evolved from single-cellular to multi-cellular.
2. The Archaean Eon because Earth, the oceans, and the moon were formed. Also the heavy bombardment ended to, so the most important events happened during the first eon.
3. Humans are not significant at all compared to the entire history because the Earth is 4.5 billion years old and humans first were around like 20,000 to 40,000 years ago. So the Earth has been human free for around 4,499,980,000 years, so they haven't been around long enough to be significant.
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