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This discovery explained why certain muscles used during exercise received more oxygen than lesser-used muscles.10 They were producing more carbon dioxide, which attracted more oxygen. It was supply on demand, at a molecular level. Carbon dioxide also had a profound dilating effect on blood vessels, opening these pathways so they could carry more o

The beginnings of the oxygenated atmosphere go back to what has become known as the Great Oxidation Event, which began about 2.5 billion years ago.[7] During that period, oxygen released by oceanic cyanobacteria began to accumulate in the atmosphere, but it took a long time before the gases reached their modern concentrations.

Remember those cells that evolved to eat oxygen 2.5 billion years ago and kicked off an explosion of life?