Early Earth atmospheres (>3 billion years ago) did not contain the 21% oxygen content that we have today. When life on Earth originated (~3.5 billion years ago) the first prokaryotic cells were anaerobic chemoautotrophs, most likely occupying deep ocean and subterranean habitats. With no oxygen in the atmosphere and no ozone layer, incident UVC and UVB levels would have been extreme, forcing life to remain in dim and unlighted habitats. It was the evolution of photosynthesis that added free oxygen to the atmosphere and resulted in the formation of the ozone layer. The subsequent removal of UVC and reduction of UVB from incident sunlight very likely contributed to life moving from the oceans into illuminated terrestrial environments.
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