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.
