I don't get why Oxygen is being used as an indicator for life? Why can't alien inhale some other Stuff? Why Oxygen == life? Oxygen is needed by us earth residents only.,(reply>), Oxygen combines with carbon in a way that gives off massive amounts of energy. Organisms will naturally use the most effective fuel, just as on earth. In prehistory animals used other fuels, but they died out as oxygen was superior.,(reply>), As she explained in the interview, because it's a reactive element, a large amount of oxygen molecules are unlikely to be found unless there is a large system that is continuously replenishing it.