Most of life's time on earth was single celled (life started ~4.1 Billion years ago, multi-cellular ~1.7 Billion years ago). If the world wasn't filled with O2

Most of life's time on earth was single celled (life started ~4.1 Billion years ago, multi-cellular ~1.7 Billion years ago). If the world wasn't filled with O2 or sexual reproduction didn't start life on earth could still be single celled.