When corals met algae 210,000,000 years ago.

• Position: Triassic Period

The mutually beneficial relationship between algae and modern corals--which provides algae with shelter, gives coral reefs their colors, and supplies both organisms with nutrients--began more than 210,000,000 years ago, according to a study by an international team of scientists, including researchers from Princeton (N.J.) University.

That this symbiotic relationship arose during a time of massive worldwide coral-reef expansion suggests that the interconnection of algae and coral is crucial for the health of coral reefs, which provide habitat for roughly one-fourth of all marine life. Reefs are threatened by a trend in ocean warming that has caused corals to expel algae and turn white, a process called coral bleaching.

Published in Science Advances, the study found strong evidence of this coral-algae relationship in fossilized coral skeletons dating back to the late Triassic period, a time when the first dinosaurs appeared and Earth's continents were a single land mass known as Pangea. Although symbiosis is recognized to be important for the success of today's reefs, it was less clear that that was the case with ancient corals.

"It is important to know how far back in time symbiosis evolved because it gives insight into how important symbiosis is to the health of coral reefs," says Daniel Sigman, professor of geological and geophysical sciences and a member of the Princeton Environmental Institute. "It appears that the origin of symbiosis corresponds to the rise of coral reefs in general."

In addition to confirming that...