Variations in atmospheric oxygen have affected evolution in big ways
As any camper who's blown on a flickering ember can tell you, a campfire needs a steady source of air to stay alight. Without enough oxygen, the chemical reactions that release the energy stored in firewood falter and fail. When oxygen is plentiful, the release of energy proceeds apace. The same principles apply to metabolic processes: All animals require oxygen to extract energy from their food and to fuel their activity.
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