As it turns out, we really do need oxygen to survive.

It might seem like an obvious thing, but oxygen really does make our world go round. About 4.6 to 2.5 billion years ago, there wasn't free oxygen in our atmosphere like there is today. In fact, methane, which is a very potent greenhouse gas, was the main control on our atmosphere. Only extremophiles could survive and exist in such conditions. That was until, the GREAT OXDIATION EVENT occurred!!! Thanks to the cyanobacteria that was able to survive in such extreme conditions, they were able to photosynthesize and release oxygen into our oceans and atmosphere. Through chemistry, the methane that was dominating interacted with the oxygen and created carbon dioxide. Carbon dioxide is still a greenhouse gas but it is much less potent than methane. So much so that the Earth actually cooled and was frozen over for nearly 30 million years. 

Large amounts of carbon dioxide poses to be an issue in our society today but those little microbes way back then were probably so excited to be interacting with a new atmosphere that wasn't so hard to stay alive in. Science is really cool and without little microbes doing their thing, we wouldn't be here today.