A new study shows how baleen whales, some of the largest animals ever known to exist on Earth, were able to get so big — and, in evolutionary terms, it's a pretty recent development.
The short answer, per NYT: Thanks to an ice age a few million years ago, whales took advantage of environmental changes to eat. A lot.
It's like going from whales the size of minivans to longer than two school buses. Nick Pyenson, Smithsonian Insitution's National Museum of American History