Many life forms on earth are anaerobic, powered by chemical reactions that do not involve O2. There are even species for which O2 is toxic. It most likely inclu

Many life forms on earth are anaerobic, powered by chemical reactions that do not involve O2. There are even species for which O2 is toxic. It most likely includes the very first lifeforms. There are many definitions of life, some of them broad enough to include things like compute programs, there is a debate about viruses, but I don't know of any definition that requires breathing O2.