Answer
The sudden appearance of oxygen in the atmosphere about 2.5 billion years ago, during the Great Oxygenation Event, had significant impacts on the development of modern life-forms.
a. Oxygen-rich environment: The increase in atmospheric oxygen created an oxidizing environment, which was toxic to many anaerobic (oxygen-intolerant) organisms but provided opportunities for the evolution of aerobic (oxygen-using) organisms.
b. Origin of eukaryotes: The rise of oxygen allowed for the evolution of eukaryotic cells, which are more complex than prokaryotic cells and eventually gave rise to multicellular life forms.
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