Earth started off with no oxygen in the atmosphere. Then it was emitted initially by plankton in oceans, later by plants on land.

Mammals came along much later and ? their size appears to be related to more efficient aerobic respiration.

But, it came at a price as they and by extension, we are literally burning ourselves up as oxygen is incompatible with the organic matter under uncontrolled situations.

Hence we age and ultimately die. We are a mere error in the play of natural selection. 😅

Full article at https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2720141/