When Miller mapped these flip-flopping speciation rates onto a timeline of Earth's history, she was able to identify three major events that likely played a role. The first was the breakup of Pangea, which created new coastlines and new oceans, meaning there were more opportunities for fishes to move from shallow to deep water. Next was the Cretaceous Hot Greenhouse period, and during this time, many continents were flooded due to sea-level rise, creating a large number of new, shallow areas across the earth. The third event was yet another major climatic change about 15 million years ago, known as the middle Miocene climatic transition. which caused major changes in ocean circulation and cooled the planet -- all the way down to the deep sea. “Around this time we see deep-sea speciation rates really speed up,” Miller said.