I just heard the craziest thing about oxygen from long ago?

I was watching an anime (japanese cartoon) called Needless and the main character starts babbling about "Long ago, oxygen was something that killed living organisms upon contact, until some organisms evolved and absorbed their mitochondrias into their own bodies and turned oxygen into an energy source." I'm like WWWWWTTTTTTFFFFF!!???! did he just say? Is this true? What did we do before oxygen?? O.O I'm being odee mind-****** right now, excuse my language -_-