How habitable is the Earth?

Charlie Stross: "There are elaborate and fascinating competing theories to explain why it took so long for oxygenation to take off: even the Moon gets blamed (the huge 50 metre tides that churned the anoxic early oceans and would have sucked premature photoautotrophs to their doom deep below: oxygenation had to wait for the moon to migrate out far enough for the tides to die down, permitting photosynthetic organisms to thrive near the surface)."