Oxygenic photosynthesis at first caused a mass extinction, but also allowed complex life on Earth to develop. In what ways does free oxygen allow complex life

Oxygen would be played a key role in evolution of complex organisms because this molecule is required to efficiently produce chemical energy during cellular respiration

Explanation:

Chloroplasts originated from endosymbiotic cyanobacteria 1,500 million years (mya) ago. Like chloroplasts, mitochondria originated from cyanobacteria, and also approximately at the same time. Mitochondria are organelles capable of generating energy by burning oxygen during cellular respiration (aerobic respiration). During this period (1,500 mya), complex cells became able to efficiently produce energy by using oxygen in aerobic reactions, which can be attributed to the emergence of mitochondria. Interestingly, it has been shown that during the next 500 mya, different groups of organisms composed of more than 50 different cell types evolved (including algae, fungi and early animals). Therefore, it is possible to hypothesize oxygen would be a critical molecule without which complex organisms couldn't evolve.