By Sid Perkins
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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It’s no surprise, then, that during geologic periods when atmospheric oxygen concentrations have been high, biological innovation has blazed brightly. At such times, insects grew to gargantuan proportions, reptiles took to the air, and the forerunners of mammals developed a warm-blooded metabolism. When oxygen concentrations fell precipitously, biodiversity was smothered. Some of the planet’s mass extinctions occurred during or after geologically sudden drops in atmospheric oxygen.