Life originated on earth nearly ................. years ago.

Explanation

**Step-by-Step Solution:**

1. **Understanding Earth's Origin**: The Earth formed approximately 4.6 billion years ago. This is the starting point for understanding when life began.

2. **Conditions on Early Earth**: At the time of Earth's formation, the atmosphere was not conducive to life as we know it today. It consisted mainly of water vapor, methane, carbon dioxide, and ammonia, released by the molten mass of the Earth.

3. **Formation of Water and Ozone Layer**: Over time, the UV rays from the sun caused chemical reactions that led to the formation of hydrogen and oxygen. As water vapor condensed, it contributed to the formation of oceans. The oxygen eventually combined with methane and ammonia, leading to the formation of the ozone layer.

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