Systems emerge in times of abundance, and are whittled in times of scarcity.

The great oxygenation was a time of near catyclismsic scarcity for most complex organisms, as resources scale to food/energy requirements imply the most complex organisms were the most dependent on the environment, and were most impacted by changes.

Inversely, oxygenation was our most crucial abundancy pre cursor, as it provides a large substrate chemically for life to exhibit