The blue whale is the largest vertebrate to ever have lived – believed to be bigger, even, than the largest dinosaurs.
But how did they get that way?
Whales and their ancestors weren’t always ocean-dwelling creatures. In fact, their closest living relatives aren’t even marine species. Believe it or not, it’s the hippopotamus!
But whales didn’t actually evolve from hippos. 50 million years ago, whales were actually land animals called Pakicetus. These 4 foot (1.2 meter) long animals are unique, in that its body looks distinctly terrestrial, but evidence suggests they actually ate fish and their heads more closely resemble that of modern whales.
Check out this side-by-side comparison of a Pakicetus skull (right) and the skull of a beluga whale (left).
One of the most interesting features of this progression from land to ocean animal lies in the positioning of their nostrils.
Think about that for a minute. Most terrestrial animals’ noses are located on the front of their faces. But for species that spend a substantial amount of time in the water, it’s actually better to have your nose on the top of your head. This means you can come to surface to take a breath without lifting your entire head out of the water.
To further support this idea of a migration from land to sea, take a closer look at a whale skeleton.
Everything looks to be where it should be. But what’s that little bone in the abdomen that’s not attached to anything else?
Those bones are actually the whale’s pelvis! At one point, they supported legs! While modern whales have no need for a pelvis to support their legs and body weight, their terrestrial ancestors did!
But that doesn’t explain their massive size. Why are whales so big?
Many possible answers have been proposed; water is better able to support their massive bodies or they had to be big to deter predators.
A recent study estimated the size of modern whales and their fossil ancestors. It turns out that whales were pretty big throughout much of their evolutionary history. But they only grew to their currently massive size about 4.5 million years ago. And that happens to be approximately the time of the start of the first ice age. The scientists believe that the expansion of coastal glaciers caused coastal blooms of plankton – one of the whale’s favorite foods.
For more information about whale evolution, check out this Smithsonian article.