The picture you see is one of the oldest known ancestors of a whale: Pakicetus, an amphibious mammal that walked on land and hunted in freshwater streams 50 million years ago. With four legs, a long snout and a fully furred body, the first whale did not look like a whale at all. However, a closer look at the Pakicetus fossils reveals a few hints of the ocean giants that would make up their descendants.
The first Pakicetus fossil was a skull found in the Kuldana formation of Khotat in Pakistan, described in 1981. Additional fossils reveal it had a fascinating combination of features for living in both land and water, including leg and ankle bones (that have been lost in modern whales) and an ear partially adapted to hearing underwater. The eye-orbits were placed close together on top of the skull, much like the eyes of hippopotamus, helping to look up from the water.
The discovery of Pakicetus and the early whales led to a ground-breaking conclusion: whales originated from land-dwelling mammals that transitioned into the ocean over millions of years. As an evolutionary biologist, this is one of my favourite things to tell people. To look at a humpback whale, and imagine that its great-great-great-grandmother was a walking wolf is why I find evolution breathtaking.
Knowing that whales once walked on land also helps us understand many curious things about their skeleton. For example, did you know that underneath a whale’s flipper there is a full set of bones making up a hand that looks just like ours? Go ahead and google “whale flipper bones” – I promise it will be worth it!
Most whale species have floating pelvic bones: the same kind that make up our hips, but reduced to a single, tiny bar-shaped bone completely disconnected from the rest of the skeleton. Scientists used to think that these bones were simply vestigial – a small reminder that, in a very distant past, whales had legs. However, despite having lost their locomotory function, the floating pelvic bones serve as an anchoring point to genital muscles that control penis movement and erection. In fact, they seem to play a vital role in the reproductive success of males. A 2014 study showed that whale species with larger testes and penises also had larger pelvic bones relative to body size. Individuals with longer penises can have a higher chance of fertilizing a female when there’s a lot of male-male competition, but that requires larger pelvic bones for support (or as Fergie would say, all that junk requires a sizable trunk).
And here’s one fact that I personally didn’t know until I started studying whales for a job: toothed whales hear sounds through their jaws! As a part of their transition from land to water, whales lost their external ears (which Pakicetus did have, by the way), and developed a new way of capturing sounds through their lower jawbones. But this “new way” was not all that new: the middle ear of land mammals, including ancestral whales like Pakicetus, derives from the jaw bones, possibly creating a shortcut to the evolution of underwater jaw-hearing.
Throughout the next posts, I invite you to join me on a journey to uncover how whales colonized the ocean, becoming the wonderful, awe-inspiring creatures we know today. We will dive into all the facts that make whales an evolutionary marvel: how they sense the world and communicate underwater, what makes them smart, social and more like us than we think, and what their DNA can tell about how they conquered waters all over the world. Sea you soon!
Image source: WikiMedia Commons
Copyright information: All illustrations on this site are copyrighted to Nobu Tamura. The low resolution versions of the images are licensed under Creative Commons Attribution-ShareAlike (CC BY-SA) license meaning that you are free to use them as long as you properly credit the author (© N. Tamura). High resolution versions are available upon request.
