The abundant formation of banded iron rock approximately 2.4 billion years ago indicates that the atmosphere was receiving large amounts of free oxygen. This observation is evidence of a change that is a direct result of what other change? 1. The increased burning of fossil fuels by humans over the past 250 years.
2. The production of greenhouse gases through volcanic eruptions more than 3 billion years ago.
3. The evolution of eukaryotes, the first nucleated cells, 1.5 billion years ago.
4. The appearance of photosynthesizing organisms approximately 2.7 billion years ago