Do all whales have the same ancestor?

Thanks to Dr Ellen Coombs for the answer.

Will - Did you know the closest living relative of cetaceans, that’s whales and dolphins, is the hippopotamus? Well, whilst that might be true, the word closest in this context… isn’t all that close. In fact cetaceans split off from the rest of the mammal world around 55 million years ago. Since then, they have evolved into a hugely diverse array of marine mammals. Today, there are two main groups: the baleen whales, which filter krill with giant plates in their mouth; and toothed whales, which have… teeth. But did all of this variety stem from one lone organism? Dr Ellen Coombs from the Smithsonian Institution takes us back in time, and to perhaps a rather unlikely source.

Ellen - If we travel back about 50 million years to what is now Pakistan, we would see a goat sized mammal called Pakicetus. Pakicetus had fur, four legs, a long tail, and importantly, its nostrils were on the tip of snout, just like in any other terrestrial mammal. But despite appearances, Pakicetus was actually the first known whale.

Will - At some point around that time, Pakicetus realised that there was a lot of rich pickings to be had in the warm and vibrant waters of the Tethys Sea, and so began an amphibious life of catching fish, and whatever else was in those waters.

Ellen - Gradually over time, Pakicetus and later Archaeoceti became beautifully adapted to the aquatic lifestyle where they evolved high frequency underwater hearing, powerful tails. They lost their hind limbs and their nostrils moved from the tip of the snout to the top of the head to facilitate breathing at the water's surface. So in around eight to 12 million years, these ancient whale ancestors looked much more whale-like. These animals evolved from being land dwelling to fully aquatic in an evolutionarily short eight to 12 million years.

Will - But it wouldn’t be unreasonable to think that toothed and baleen whales came from two different organisms, given how different they are today. The two groups diverged an estimated 39 million years ago, but how the transition to baleen came about is less clear. Obviously Pakicetus had teeth, so toothed whales came first, and it was assumed that baleen evolved from teeth. but stronger evidence now suggests that these whales lost their teeth completely before growing baleen separately. And in the interim, sucked fish directly down their throats. Isn’t nature a marvel? But whatever the case, all of this evolution and speciation happened once whales were well established in the water, so we can be sure that all forms of whale including the largest organism ever to have lived, come from a humble beach side mammal.

Thank you very much to Bert for the question, and Dr Ellen Coombs for providing an answer. If you have a query of your own, do send it in to chris@thenakedscientists.com