One likely reason that only microbes existed for the first approximately 2.5 billion years of life on Earth is that the concentration of _______ was too low to support more complex life.

Question

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Oxygen was not present in the Earth's atmosphere until about 2.5 billion years ago, during the Great Oxygenation Event. Prior to this, only anaerobic microbes could survive in the oxygen-free environment. The concentration of oxygen was too low to support more complex life forms.

One Likely Reason That Only Microbes Existed for the First