From land to sea: Whale evolution and adaptation
Palaeontologist Hans Thewissen should have been content with his new discovery: the fossil of an ancient four-legged animal. But he wasn’t.
‘I was so frustrated…I had this nice complete skeleton in three wooden crates, but I didn’t know what the stupid thing was.’
It was 1992, and Thewissen and his team had spent three arduous days digging the 49-million-year-old fossil out of the Pakistan desert. They took it back to the compound where they were staying, and wrapped it carefully in plaster and toilet paper, to prepare it for shipment back to Thewissen’s lab in the United States for future study. But Thewissen couldn’t wait that long.
He began unwrapping a few pieces, starting with the ear, and immediately noticed an unusually heavy bone. This left him more baffled than ever, because it did not match an animal group related to elephants and sea cows—his best guess for the mystery fossil’s identity.
‘So then I unwrapped the lower jaw. I expected to see these low molars, like in elephants and sea cows, but they were these high teeth. Suddenly it hit me, “Oh, this is a whale”. And then I thought, “Oh my God, I understand why that ear looked like that!” It was a whale ear.’
Thewissen was shocked because he didn’t expect to find something that had never been found before—very large hind limbs on a fossil whale. ‘That was a real game changer,’ he says.
Named Ambulocetus natans, or ‘walking whale that swims’, Thewissen’s find joined a lengthening procession of remarkable, unexpected and sometimes baffling fossil cetaceans. Teased from rock over the past two and half decades, they provide one of the best-documented and most spectacular cases of evolutionary transformation.
Around 50 million years ago, whales’ mammal ancestors began their journey towards an aquatic lifestyle, reversing a process begun 200 million years earlier when animals first emerged from the seas. But evolution never truly goes backwards: whales did not develop gills and scales, but instead a whole new suite of aquatic adaptations. Other mammals have turned to the oceans—sea manatees, dugongs, sea otters, polar bears, seals, sea lions and walruses—but no other group surpasses whales in terms of diversity and number of species.
Indohyus: Ancient link to whale origins
Before turning to whales’ ancient ancestors (called the Archaeoceti), it helps to understand a fossil that tells us about their origins and relatives. Indohyus was the size of a cat but shaped like a deer, and belonged to the order Artiodactyla, or even-toed ungulates. This large group today includes deer, hippopotamus, cows, giraffes, sheep and pigs.
Indohyus skulls have a signature whale feature already mentioned: a thickened bone (auditory bulla) in part of the middle ear, likely an adaptation for underwater hearing. This shows that whales originated from artiodactyls. Additional evidence from DNA tells us that hippopotamus (also artiodactyls) are whales’ closest living relatives today.
Indohyus had unusually thick bones in its legs, which would have made it less buoyant and better able to walk on river or lake bottoms, as hippos do. Its bone chemistry also suggests an aquatic lifestyle. But what use would these aquatic adaptations be to tiny Indohyus?
Some living artiodactyls suggest a possibility. To avoid predators, African mouse deer, Hyemoschus aquaticus, take to the water and stay submerged for up to four minutes. Fifty million years ago, the India–Pakistan border area where Indohyus was found was a subtropical region of forests and streams. Like the mouse deer, Indohyus may have turned to water for escape. Another theory holds that food may have tempted whales’ ancestors into the water.
Pakicetids, the oldest whales
The oldest known ancient whales are a family called pakicetids. Known from 50-million-year-old fossils, they had four legs, had tails, and may have looked much more like wolves than whales. Their mouths were full of sharp teeth with high cusps, worn in a way that suggests a fish diet. They share with Indohyus the signature whale ear and unusually heavy bones—adaptations suggesting a lifestyle that was at least partially aquatic. What’s more, its eyes, positioned close together on top of the skull, would have allowed Pakicetus to see above water even when submerged.
When pakicetids lived in what is now India and Pakistan, the region experienced alternating periods of heavy rain and drought. Rivers would have swelled then dried up, leaving stagnant pools and small lakes. ‘I think Pakicetus would have hung out at lakes and waited for animals to come and drink,’ says Thewissen. It may have chased and attacked its prey, but more likely ambushed them, since it was not a fast mover. Other palaeontologists suspect that Pakicetus spent less time on land, and more time in water as a semi-aquatic, foot-powered swimmer.
Ambulocetus, the Walking Whale
Nature’s grand cetacean experiment continued with Ambulocetus natans. Much bigger than Pakicetus, Thewissen’s walking whale was found in northern Pakistan, the site of a shallow sea 49 million years ago. Ambulocetus’ bone chemistry suggests it lived partly in fresh water, possibly where a river mouth met the sea.
‘This is not a place you would want to go swimming,’ says Thewissen. ‘Not only is there Ambulocetus, which is about four metres long, with lots of teeth and not very smart—but also a lot of crocs about the same size or bigger, and lots of sharks. So there are all kinds of things ready to eat each other.’
Thewissen suspects Ambulocetus natans looked and hunted like modern crocodiles, eating marine fish and maybe even ambushing animals that ventured too close to the water’s edge. With sideways-facing eyes high on its head, similar to those of hippos, it could have seen prey above water even when submerged. It has ear adaptations for underwater hearing, and a lower jaw with a large cavity that, in modern whales, is filled with fat and helps transmit sound to the ear.
Ambulocetus also had front limbs ending with flexible wrists and fingers, and its strong hind limbs had even bigger feet. In water it may have swum like an otter, or like a dog. Or it may have walked like modern hippos along the bottom. Its legs could have supported its full weight, but on land Ambulocetus was probably clumsy and slow. With Ambulocetus, ancient whales had taken one step further towards the fully aquatic lifestyle of modern whales.
Protocetids and the mother whale
Next in our procession of ancient whales are the protocetids, a diverse group of whale ancestors that spread across the globe between 49 and 40 million years ago. These foot-powered swimmers had jaws and ears modified to enhance their ability to hear in water.
Some unusually complete protocetid skeletons were discovered in Pakistan last decade by Philip Gingerich and his team, who are based at the Museum of Paleontology at the University of Michigan. Like fossils versions of the Rosetta stone, the skeletons provide a glimpse into the lives and capabilities of these ancient whales: like today’s sea lions, they probably hunted in water but came to land to mate, give birth and nurse young.
Amongst these discoveries were fossils of Maiacetus inuus or ‘mother whale’, so-named because one of them contains a foetus. ‘It’s interesting because the foetus is about to be born like a land mammal, not a whale,’ Gingerich says, ‘that is, about to be born head-first, not tail first.’