Cetaceans are a group of animals containing whales, porpoises, and dolphins, including some of our planet’s most iconic aquatic mammals. However, despite living their lives in water, these creatures must breathe air to survive. This seems like a pretty inconvenient way of going about things – especially considering the fact that their ancient ancestors actually lived on land. So why did cetaceans make this evolutionary U-turn and decide to take a permanent dip?

Meet the cetacean family tree

The earliest cetaceans are known from fossils found in Pakistan and India. The cetacean species Pakicetus lived about 50 million years ago and was a wolf-sized, four-legged land animal that lived near riverbanks and lakes. Wear on their teeth indicates that pakicetids hunted fish.

Then, 49 million years ago came the arrival of Ambulocetus. These creatures had more flipper-like limbs and were 3.4 to 3.7 meters (11 to 12 feet) long.

Next came Remingtonocetidae, with fossils dated 48-41 million years ago. They had a powerful tail, short legs, and a long snout.

Protocetids lived in the middle Eocene, around 49-37 million years ago. One genus called Rodhocetus lived around 47 million years ago. Their nostrils were further back on their snouts and they used their tails for propulsion while swimming.

Basilosaurids emerged around 41 million years ago and were the first to be completely aquatic. Their nostrils were pushed back, resembling a blowhole, and their tiny hind limbs would have been pretty useless on land. The genus Basilosaurus translates as “king lizard” – they were originally thought to be dinosaurs! Another genus, Dorudon, gave birth to their young underwater.

Modern cetaceans emerged around 34-30 million years ago and are split into two categories, mysticetes (baleen whales) and odontocetes (toothed whales).

Why switch from the land to the water?

So why did cetaceans spurn a land-based lifestyle in the first place? As Dr Curt Stager told North Country Public Radio in 2016: “It actually makes perfect sense really to an evolutionary biologist.”

“The really kind of the underlying principle here would be that whenever there’s an opportunity, based on your body design for mutations, if there’s an opportunity to make a living, something’s probably gonna move in there and do it,” Stager explained. “And if there was a niche in the water, something that got adaptations that let it survive there, it will survive there.”

One 2018 paper explored the question of why land animals began to live in the sea, finding 69 cases of this happening. The study suggests that this phenomenon is related to an abundance of food in the watery depths.   

How cetaceans' bodies changed for life in the water

First of all: cetaceans are mammals, and mammals nurse their young on milk, so how on earth does this work underwater? Well, their nipples are hidden under folds of skin called mammary slits, inverted to make them more hydrodynamic. Although footage of breastfeeding is rare, sperm whale calves have been observed making their way into the mammary slit using their lower jaw to get at some thick, fatty nourishment.

As alluded to before, cetacean nostrils gradually migrated from the snout to a location on top of the skull, becoming blowholes. Cetaceans also gradually lost the need for hind limbs over time, so did away with them, and their front limbs became the flippers that now propel them through the water.

Toothed whales have evolved echolocation, which helps with navigation and finding prey in waters where visibility is low. They generate sounds in their heads, which bounce off things and are thus reflected back to the whale. This lets them piece together a picture of what’s going on around them.

Baleen whales get their name from the comb-like filter-feeding apparatus inside their mouths that they have instead of teeth. This allows them to feed on teeny weeny prey such as krill and plankton, gulping down gigantic mouthfuls of water and then pushing it out again to trap their dinner in their mouth. Baleen is made of keratin, much like human hair and fingernails.

How cetaceans’ genes changed during their aquatic metamorphosis was explored in a paper published in 2019. Researchers screened for genes that code for proteins that were lost somewhere along the evolutionary timeline, before toothed whales and baleen whales became distinct but after cetaceans split from Hippopotamidae – fun fact, whales and hippos are each other’s closest relatives alive today!

The team found 85 inactivated genes, some of which could be related to aquatic adaptations. One of these was SLC4A9, which is involved in the secretion of saliva – food doesn’t really need much else to lubricate and wash it down when you’re gulping it down with a nice big mouthful of seawater. Blood coagulation-associated factors F12 and KLKB1 were also lost. This could be because diving induces blood vessels to narrow, and this happening frequently could increase the risk of blood clots.

A timeline of cetacean evolution

“Within eight million years, the ancestors of whales go from being fully terrestrial, such as the four-legged, furry Pakicetus which lived around the edge of the Tethys Sea, to fully aquatic,” explained cetacean researcher Dr Ellen Coombs in a 2022 statement. “This is super quick in evolutionary terms.”

Coombs is lead author of a paper on what cetacean skulls tell us about the story of their evolution. “We've gathered the most expansive cranial data set for whales which exists on the planet. Our data came from both specimens of living species and fossilized whales,” Coombs said.

This data spanned around 53 million years, cetaceans’ whole evolutionary history. This included 88 living species, representing about 95 percent of cetaceans alive today, and 113 fossil species.

“Because the cranium captures many of the most extreme shifts in feeding, respiration, and sensory structures, it is ideal for understanding these rapid and radical changes, but no previous study has reconstructed the evolution of the cetacean cranium through the full breadth of their extinct and living diversity,” added Coombs.

Examining these cetacean noggins revealed that cetaceans evolved in three bursts: one 47.8-42 million years ago in the late Eocene, another around 39 million years ago in the mid to late Oligocene, and finally a lesser one around 18-10 million years ago in the mid Miocene.

The first burst involved ancient cetacean ancestors called archaeocetes first dipping their toes into an aquatic lifestyle alongside walking about on land, doing things such as catching freshwater prey, and adapting accordingly. Their skulls and facial features evolved quickly, which the paper authors attribute either to high productivity prompting rapid change or having little competition.

The second wave involved the origin of neocetes, which is a group that diverged into two sub-groups to suit different niches: mysticetes and odontocetes. The researchers suggest that this fast diversification could have been due to constraints that needed to be overcome via evolution. The skull features of mysticetes were observed to evolve slower than odontocetes. The third wave mostly involved the specialization and diversification of odontocetes.

This incredible example of evolution has resulted in the emergence of many fascinating and beautiful creatures – including the critically endangered vaquita, the world’s rarest marine mammal. This group of animals also includes a contender for the heaviest animal ever, the extinct Perucetus colossus; and the biggest animal on our planet, the blue whale!