Blue whales are the biggest creatures to have ever lived on Earth. Measuring up to a hundred feet long and 200 tons in weight, they have a heart the size of a small car. Using song, these giants of the deep communicate with each other in a language researchers still don’t fully understand.

But beyond the mysteries of whale behavior is another mystery—how did they get so big. Surprisingly, whales weren’t always gargantuan water-dwelling mammals. In fact, their ancestor Pakicetus, was the size of dog and walked on land. Pakicetus was a shore-dwelling creature with webbed feet that lived around 49 million years ago. Scientists were able to link Pakicetus to the evolutionary lineage of whales because of its distinct, dense ear bones. Animals like Pakicetus gave rise to the modern-day giants of the deep. It is one of the most remarkable stories of physical transformation in the annals of evolution.