Evolution is usually depicted as a consequence of the never-ending war between living things — a “Struggle for Existence amongst all organic beings,” as Charles Darwin put it. The physical environment around those living things plays a part, but it’s secondary to the intimate relationships that organisms have with one another. Whether talking about symbioses or the ongoing competitions between species called Red Queen’s races, researchers largely take it for granted that ecological relationships are the primary force of evolution.

But in the grand scheme of life, that may be a surprisingly recent development. In the oceans, for example, ecological success was closely tied to the inanimate forces steering ocean chemistry until only about 170 million years ago.

Complex organisms had been evolving since at least the beginning of the Cambrian period. “That’s, what, 300 million years of evolution in the Paleozoic? And all that time they were still so susceptible to these very basic geochemical shifts,” said Kilian Eichenseer, a doctoral candidate at the University of Plymouth. But as he, his adviser Uwe Balthasar and their colleagues showed in a recent paper in Nature Geoscience, that all changed when certain tiny marine creatures emerged in the Jurassic — and evolution in the ocean was never the same again.

A Plankton Revolution

About 250 million years ago, at the end of the Permian period, life on Earth suffered an unprecedented blow. Something — massive volcanic activity in what are now the Siberian Traps is the likeliest culprit — devastated the global ecosystem and eliminated 90% or more of all marine species and upward of 70% of those on land. The nearly cleaned slate marked the dawn of a new era: the Mesozoic.

Life was slow to recover, and for the first few tens of millions of years, the seas were dominated by groups of animals like the tough, thick-shelled brachiopods, ammonites and mollusks, some of the scant survivors of the Permian extinction. In the middle of the Mesozoic, however, the ocean began to teem with agile animals that had much thinner shells. “If you look at the fossil record, we see animals becoming bigger, becoming more active — predators become more fearsome,” Eichenseer said.

This dramatic shift in marine life came to be known as the Mesozoic marine revolution. And it was in this time of renewal that a new kind of life emerged: the calcifying plankton — a diverse classification of tiny organisms that build shells or skeletons of calcium carbonate.