1. | 20 billion years back | 2. | 10 billion years back |
3. | 4.5 billion years back | 4. | 3.0 billion years back |
I: | The first form of life could have come from pre-existing non-living organic molecules (e.g. RNA, protein, etc.) |
II: | The formation of life was preceded by chemical evolution, i.e., the formation of diverse organic molecules from inorganic constituents. |