Organisms have evolved to use oxygen instead of nitrogen for several reasons:
Chemical reactivity: Oxygen is highly reactive and can readily participate in various chemical reactions, making it useful for biological processes such as energy production. Nitrogen, on the other hand, is relatively inert and less likely to engage in the necessary reactions.
Electron acceptor: Oxygen serves as an excellent electron acceptor during cellular respiration, which is the process by which organisms generate energy. Oxygen has a high electron affinity and can accept electrons from the electron transport chain, facilitating the efficient production of ATP (adenosine triphosphate), the cell's primary energy currency. Nitrogen does not possess the same electron-accepting properties as oxygen.
Atmospheric abundance: While nitrogen is indeed abundant in Earth's atmosphere, comprising about 78% of it, the form of nitrogen in the air (N2) is relatively unreactive and cannot be directly utilized by most organisms. Oxygen, in contrast, exists in a more reactive form (O2) and can readily diffuse across cell membranes, allowing organisms to access it for metabolic processes.
Evolutionary history: Oxygenic photosynthesis, the process by which oxygen is produced, evolved in early photosynthetic organisms. This event led to the oxygenation of the Earth's atmosphere over billions of years, creating an environment rich in oxygen. Organisms that were capable of utilizing oxygen had a selective advantage in this oxygen-rich atmosphere, leading to the evolution and dominance of oxygen-utilizing organisms.
It's important to note that while some organisms can metabolize nitrogen compounds, such as certain bacteria that perform denitrification, the majority of life forms have evolved to utilize oxygen due to its favorable properties and its availability in the atmosphere.