Finding signs of life on planets in other solar systems could be unexpectedly tough, because they may not have the oxygen-rich atmosphere usually taken as the signature of life.
Earth was oxygen-poor when photosynthesising bacteria made their debut at least 2.7 billion years ago. Even then oxygen concentrations crept up only slowly until 2.4 billion years ago, when they jumped by a factor of 1000. Now a model developed by Colin Goldblatt and colleagues at the University of East Anglia in Norwich, UK, has shown that the jump occurred after oxygen reached a critical level that allowed ozone to form,…
