Steven Haddock/Monterey Bay Aquarium Research Institute

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 University of Washington study led by Elizabeth Miller 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.

“It’s easy to look at shallow habitats like coral reefs, which are very diverse and exciting, and assume that they’ve always been that way,” said Miller, who completed the study as a postdoctoral researcher in the UW School of Aquatic and Fishery Sciences and is now a postdoctoral fellow at the University of Oklahoma. “These results really challenge that assumption, and help us understand how fish species have adapted to major changes to the climate.”