Mitochondria didn't really become advantageous until free oxygen began to increase... so maybe the chloroplast and oxygenic photosynthesis were the more importa

Mitochondria didn't really become advantageous until free oxygen began to increase... so maybe the chloroplast and oxygenic photosynthesis were the more important developments in terms of the evolution of complex life? Perhaps mitochondria were a key partner to chloroplasts, but I wonder which came first? Or maybe just the free-living chloroplasts were capable of raising oxygen levels, once they'd figured out how to prevent themselves from being oxidized by their own waste products.