It could be a combination of both theories. Increased oxygen allowed animals take deeper vertical dives, and these deeper dives created yet more usable oxygen, making even more deeper dives possible. It could be that animals first had to reach a threshold of complexity to trigger the feedback cycle.<p>For example, there's less need to hide in the depths from predators if the predators are wimpy, and predators without enough oxygen will stay wimpy. The diving energy expenditure is not worth it under wimpy predation. Capability to dive deep enough and the predator/pray competition both had to reach a level to trigger the feedback cycle. Earlier generations couldn't get either the depth nor be potent enough predators because they were too primitive.