Photosynthesis arrived. It released oxygen into the water (all life was aquatic then) and as a result all free iron radicals oxidized and settled out to the ocean floor. Today we call those former ocean floors iron mines.
Once oxygen became freely available and filled the air, forms that dwell on land had a way to get the oxygen their complex metabolisms needed. In fact, it’s easy to see that all complex life, anything from a microscopic worm on up, owes its development to the use of oxygen as part of its metabolism.
Bless that disruptive early microbe!