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Estimated at 2.48 billion years ago.

http://www.sciencedaily.com/releases/2011/10/111019181210.htm

Complete thread:

The earliest oxygen-breathing bacteria David Turell 2011-10-22, 01:38 AM
- The earliest oxygen-breathing bacteria dhw 2011-10-22, 12:24 PM
  - The earliest oxygen-breathing bacteria David Turell 2011-10-22, 10:23 PM
    - The earliest oxygen-breathing bacteria dhw 2011-10-23, 04:02 PM
      - The earliest oxygen-breathing bacteria David Turell 2011-10-23, 07:08 PM
        
        The earliest oxygen-breathing bacteria romansh 2011-10-23, 10:33 PM
        
        The earliest oxygen-breathing bacteria David Turell 2011-10-24, 12:03 AM