"Once animals began burrowing farther down into the seafloor, though, all biogeochemical hell broke loose. Burrowing animals that punched through biomats and went down more than a centimeter abruptly (and quite rudely) introduced oxygenated water from above to sediments that had rarely encountered this element. The storming of this subsurface Bastille meant that anaerobic bacteria, which had ruled the sedimentary underworld for nearly four billion years, screamed, shriveled up, and died. Well, not completely, as these bacteria are still around today, with many of them living happily ever after in our gastrointestinal tracts. But still, anaerobic bacteria became far less common in shallow-marine sediments, restricted to smaller patches of those environments. The resources of the underworld were too rich and tempting to stay out of reach forever, and its colonization was on."
From: the Evolution Underground by Anthony J. Martin