But for Pyenson, the secret to really understanding why the baleen whales got so big is to really nail down when they got so big. And more importantly, when did they get really big? The sheer size of the baleen whales can distract us from the fact that some are much bigger than others—there’s a considerable difference between a 20-foot minke and a 100-foot blue. When did the latter titans emerge?

To find out, Pyenson teamed up with Graham Slater and Jeremy Goldbogen to collect data on the size of baleen whales, both past and present. For extinct species, they measured fossils. For living ones, they turned to museum specimens, records of beached whales, and even data from aboriginal harvests. They then mapped these measurements onto a family tree that unites all of these species, showing how they’re related and when each group evolved.

The trio found that the really big baleen whales—those measuring 33 feet (10 meters) or more—only show up in the final act of the whale drama. When this group first appeared, around 35 million years ago, they were already big, but they only became Big with a capital B within the last 4.5 million years. What’s more, gigantic sizes evolved independently in at least three different lineages of baleen whales. One gave rise to the fins and humpbacks, a second led to the blues and seis, and a third produced the rights and bowheads.

The result of these repeated forays into gigantism is that “modern whales are dramatically larger than any whales in their evolutionary history,” says Pyenson. He has noticed this pattern for a long time, but his data show it plainly. Even the smallest living baleen whale—the amusingly named pygmy right whale—is 20 feet long, making it about the same size as average prehistoric whales. And it’s not just that the biggest members of the family have become bigger. Almost all the smallest species disappeared. The family as a whole ballooned in size.

Cope’s rule can’t explain this late-breaking increase. It’s also unlikely that factors like competition within the group or threats from huge predators or the advent of filter-feeding were responsible, since these were already part of whale life before they became giants. Instead, Pyenson says, “We think the ice ages had something to do with it.”

Around 3 million years ago, the planet entered a cycle where large glacial sheets would form in the northern hemisphere, expand all the way down to the northern U.S., and then retreat again. These cycles triggered a shift away from continuous warmth toward seasonal climates that varied over the year. That seasonality reshaped the oceans. By strengthening winds that blow from the south, it intensified the upwelling currents that bring nutrients up from the depths, specifically near the coasts of continents. And those coastal waters were also hit by run-offs from melting glaciers on land, which brought even more nutrients with them.