Evolving into the sea

While this course is primarily about modern whales and their interactions with humans; their story actually starts a few million years ago, with the fascinating story of how air-breathing, land-dwelling animals found their way permanently into the ocean.

Whales are mammals, so have the features and systems of mammalian anatomy including breathing air with lungs and nursing their young on milk. But whales today also differ significantly from almost all other mammals – a result of their gradual move from land to sea millions of years ago.

Changing bodies

Changing habitats so dramatically required equally dramatic physical changes. This encompassed things like the development of a tail fluke (with the distinctive up and down movement that contrasts with the side to side motion of fish), the loss of hind limbs, the adaptation of its front limbs from hooves to flippers, and the development (in the absence of gills) of a blowhole on the crown, from what were once nostrils on the tip of a snout.

Later still came the development of even more unique and specialised morphology, like baleen instead of teeth, and the technical marvel that is echolocation.

Evolution

^{Replica skeletons and models of three archeocetes (ancient cetaceans) show their evolution from the furry land mammal Pakicetus (right) to the amphibious Ambulocetus natans (centre) then the fully aquatic Dorudon (left). Photograph Andrew Frolows/ANMM}

This astounding transition came shortly after the rise of modern mammal groups, around 55 million years ago – millions of years after dinosaurs and large marine reptiles had disappeared.

During this hot period in the Earth’s history, one group of hoofed mammals spent more and more time in the water, living on the abundant food there. Eventually they left the land altogether – to become whales.

The video below illustrates the evolutionary journey, though it should be noted that the common idea of evolution as a single progressive ‘chain’ does not capture the full complexity of what actually happened – it was a much wider adaptive radiation; different branches evolving in a wide range of mixed aquatic/terrestrial habitats, with many ending in evolutionary dead ends.

That said, the species discussed below demonstrate some of the major stops along the way to the whales we know today.

Once upon a time

This video, by Factor Trace, looks at this fascinating transition of ancient cetacean ancestors from the land into the deep ocean, culminating in the ancestral species of all modern whales today.

From ancient to modern

Surprisingly, modern baleen whales actually still develop teeth in utero, but resorb them before they are born and only grow in their adult baleen after they have weaned from their mother’s milk. This curious evolutionary artefact evidences their shared common ancestry with toothed whales. It has recently been hypothesised that suction feeding may have been a transitional evolutionary step between using teeth to grab prey and filter feeding using baleen.

What’s in a name?

Before we go any further, let’s clear up some confusing names that might pop us as we go on. This has to do with the distinctions between whales, dolphins, and porpoises. All of these belong to the order Cetacea. All cetaceans are considered ‘whales’.

Within Cetacea, there is a division into the two sub-orders that encompass baleen whales (Mysticeti), and toothed whales (Odontceti). Toothed whales includes dolphins, porpoises, and all other whales with teeth, like sperm whales and beaked whales.

Click to expand
^{Whale taxonomy chart (simplified), courtesy of Whale & Dolphin Conservation USA, all rights reserved.}

Clear as mud

People often use the term ‘whale’ to refer just to the larger animals within Cetacea, but there are whales that are smaller than humans (like the pygmy sperm whale), and dolphins that are larger (like the killer whale, or orca).

Compounding all this confusion is some blurry naming practices. One example you might have noted above is that the killer whale is more accurately a dolphin. This is also the case for the false killer whale, and the melon-headed whale, among others.

Dolphins and porpoises

Dolphins and porpoises are both toothed whales, but have their own subclassifications based on anatomical and behavioural differences. Dolphins usually have a beak and always have conical teeth that taper to a fine point. They’re also more social, and found in all the world’s oceans.

Porpoises on the other hand have no beak and are smaller and stouter than the dolphins. Their teeth are flat and spade-shaped, and they are only found in the Pacific Ocean.

^{Finless porpoise by cotaro70s via Flickr. CC BY ND2.0}

We’ll learn more about the fascinating lives and habits of both baleen and toothed whales in the next step.

Further resources

A Different Kind of Whale, National Geographic

When whales walked on four legs, Natural History Museum UK

The evolution of whales, Berkeley University of California

Is an orca (killer whale) a whale or a dolphin?, Whale & Dolphin Conservation USA

Seeing The Ocean With A Buzzing Nose (a look at the mysterious evolution of echolocation), Carl Zimmer