The deep sea contains more than 90% of the water in our oceans, but only about a third of all fish species. Scientists have long thought the explanation for this was intuitive — shallow ocean waters are warm and full of resources, making them a prime location for new species to evolve and thrive. But a new study reports that throughout Earth’s ancient history, there were several periods of time when many fish actually favored the cold, dark, barren waters of the deep sea.The deep sea contains more than 90% of the water in our oceans, but only about a third of all fish species. Scientists have long thought the explanation for this was intuitive — shallow ocean waters are warm and full of resources, making them a prime location for new species to evolve and thrive. But a new study reports that throughout Earth’s ancient history, there were several periods of time when many fish actually favored the cold, dark, barren waters of the deep sea.