alt Hacker NewsI suspect that many lush worlds would not have oxygen atmospheres.
Oxygen is really a deadly poison, to many theoretical lifeforms. There's a good chance that an alien landing on Earth would be unable to breathe our air.
Of course, it could be argued that we are well on our way to not being able to breathe Earth's air[0].
Oxygen is a highly efficient electron acceptor. Anything requiring tons of energy to power a big brain will scarcely find a suitable replacement. The same properties that make it a good source of energy, also make it dangerous. Most of the metabolic stress comes from oxygen. It is a double-edged sword.