To think about the past requires an obvious and dizzying shift in perspective, because ocean life stretches behind us for more time than our human brains can instinctively grasp. We must briefly let go of our customary horizons of hours and days, years and decades, centuries and, at a stretch, millennia. Think instead like palaeontologists, who have learned to find ancient moments trapped in stone, then to gather them up and piece together stories that take millions of years to be told. While we try not to get overwhelmed by the scale and intricacy of it all, we can pick out details that tell a wider story of the changing ocean. From there we can begin to sense the rhythm and pace of ocean life.

Trilobites look oddly familiar, as if a pill bug scuttled under a rock and then emerged on the other side much larger, more ornate, and more than a half billion years older. Were I a more skilled and patient fossil hunter, I might find trilobites lodged in rocks not far from my graptolite-embossed slate. There are fossil trilobites on continents across the world. These animals existed in the earliest of three great chapters of complex life on earth—the Palaeozoic, meaning “ancient life,” which was followed by the Mesozoic (“middle life”) and then the Cenozoic (“new life”). Trilobites in the Palaeozoic weren’t the first large animals to evolve, but they were undoubtedly trailblazers. They worried Charles Darwin because they seemed to confound his theory of gradual evolution via natural selection. Trilobites emerged far too quickly, too completely, and too long ago to fit his theory, as perfect creatures pressed into stone.

Trilobites evolved shortly after a three-billion-year prelude in which the only living things were single-celled microbes colonising the ocean, followed in the fullness of time by enigmatic wisps of simple, mostly jelly-based creatures that palaeontologists are still trying to make sense of. Then the Palaeozoic era opened with a dramatic twist in the history of life on earth. This era is divided into six periods; the first was the Cambrian, when evolution suddenly accelerated and ran at full tilt, churning out a mob of animals, including many that looked wildly different to anything alive today, from nozzle-nosed predators to luxuriantly spiky worms. The trigger for this flurry of life, known as the Cambrian explosion, is still a matter of debate. It may have had something to do with the fact that, for a long time leading up to it, the whole planet was frozen. As snowball Earth thawed, likely due to volcanoes spewing planet-heating carbon dioxide, the climate became more favourable for life to flourish. Rocks on land, as yet devoid of living things, began to erode and release nutrients into the ocean that organisms used to grow and build their skeletons, including enormous numbers of trilobites.

Darwin needn’t have agonised over trilobites. It was partly a matter of timing. He was quite right when he surmised that ancient seas must surely have been swarming with life, though in his day nobody had yet found any evidence for it. When Darwin was writing and thinking about evolution, most of the world’s oldest animal fossils remained unfound underground, including the extraordinary variety of Cambrian life in the Burgess Shale in Canada’s Rocky Mountains, which wasn’t uncovered until after his death.

A recent study of trilobite fossils has shed light on the timing of the Cambrian explosion and strengthened the idea that evolution can run at different speeds and has sometimes been breathtakingly fast (geologically speaking). A team based at the Natural History Museum in London used a large new collection of Cambrian trilobites to track how their appearance changed over time. The fossils went through an early, short burst of frantic innovation, showing that the Cambrian explosion may have truly gone off with a bang, lasting a brief twenty million years. Once the explosion died down, the rate of evolution among the trilobites levelled off and ticked steadily along.

By the Ordovician, the Palaeozoic period after the Cambrian, the ocean was brimming with trilobites. They ranged from flea-size swimmers to shovel-shaped diggers two and a half feet long, although most species were neatly pocket-size, measuring between one and three inches. Their basic anatomy was a head, thorax, and tail, with a ridged shell divided lengthwise into three sections, hence the name trilobite, and multiple pairs of legs underneath, like a centipede. These simple creatures were moulded and embellished into a phenomenal variety of forms. Many trilobites sprouted impressive spines and barbs, elegant quills, and devil horns. Some were smooth and rounded, like Bumastus, which looked just like an armadillo if you popped off its head and hid its tail. And like armadillos and pill bugs, most trilobites could curl up into a ball when they were scared.

From their fossilised remains, it’s possible to interpret the ways many trilobites lived their lives. Masses of them scurried across the seabed, leaving footprints as if they had walked over wet cement; these imprints were preserved by rapid burial in sediment and then slowly turned into stone. Fossil trails captured the details of a hunting foray: a line of worm tracks joined by those of a trilobite, and then the trilobite walking off by itself, worm presumably in belly. Cryptolithus evolved to be filter-feeding trilobites that stirred up the sediment by scrabbling the seabed with their forelegs, then straining suspended food particles through their perforated, colander-like heads. Others never set a foot down but chased after prey through the water, aided by hydrodynamic shells. Chunks bitten out of their shells show that trilobites were prey for other animals, such as the giant sea scorpions that also roamed the Palaeozoic seas. Planktonic trilobites floated in great midwater swarms, occupying a pelagic niche similar to the one that krill occupy today. In shallow tropical seas, trilobites were beetling around the world’s first true coral reefs, which had been built in the Ordovician by horn-and honeycomb-shaped corals. Some trilobites ventured between the tides and foraged on exposed tidal flats, but it seemed they never moved into rivers or lakes or made a permanent move onto land.

Uniquely among animals, trilobites’ eyes were made from crystals of the hard mineral calcite, which means they were often exquisitely preserved, and their shapes and arrangements tell us even more about these creatures’ lives. From their inception, Cambrian trilobites had complex, multifaceted eyes, similar in general form to the compound eyes of living insects and crustaceans. Some had eyes on long stalks, which scanned for prey while their bodies lay hidden in the mud. Erbenochile trilobites had columnar eyes that gave them almost 360-degree vision, each eye with a small, overhanging brow that shaded it in bright light. In deep waters of the twilight zone, where sunlight is dim, Cyclopyge trilobites soaked up rare photons with enormous eyes that occupied much of their heads, like the helmet eyes of dragonflies. Deeper still, trilobites evolved to be eyeless and blind, vision serving no purpose in the dark midnight zone.

Some trilobites resembled their nearest living relatives, the horseshoe crabs. Olenellus had a rounded, helmetlike head, rearward-pointing body spines, and a long prong for a tail. Not in fact crustaceans, the trilobites and horseshoe crabs are more closely aligned with spiders.

In all, more than twenty-five thousand species of trilobites are known, and more are constantly being found. (For comparison, there are roughly one thousand named dinosaurs.) They were fossilised in the millions, thanks in part to their tough exoskeletons. Trilobites periodically moulted their outer layer, growing new, bigger ones and tossing the casts into the fossil record, duplicating themselves and increasing the chances of being remembered through the passage of time.

The enormous diversity of species and ecology of trilobites show what very early ocean ecosystems were like, with habitats and food webs that are broadly recognisable in the contemporary ocean. More than five hundred million years ago, although the shape of the global ocean was very different, ocean ecology was already working, in many similar ways, as it does today.

Being so prolific and dotted around the planet, trilobites have also helped scientists reconstruct what the entire global ocean used to look like. For much of the Palaeozoic, the Northern Hemisphere was covered in the huge Panthalassic (meaning “all-sea”) Ocean, and the continents were located mostly in the Southern Hemisphere. The world was warm, with little ice locking up water, and many of the continents were flooded in shallow seas, each home to a unique assortment of trilobites that didn’t cross the deeper ocean in between. Later, when they were long dead and rockbound, fossil trilobites travelled around the planet, pushed by the forces of tectonic drift. Mapping the range of trilobites across modern-day continents is one way palaeontologists have worked out how landmasses moved and the ocean reshaped around them. Through much of the Palaeozoic, a supercontinent, Gondwana, was made up of many of today’s continents and subcontinents all clustered together, including Australia, Antarctica, Africa, India, and Madagascar. Today, their shared trilobite fauna is testament to that earlier convergence. For instance, the same tropical species of trilobites have been chipped out of rocks in western Newfoundland, in New York State, and in the Inner Hebrides archipelago in Scotland, showing these lands were all once part of the same ancient continent.

The stories of trilobites have much to tell us about what the ocean used to be like, and together they refute a wider misconception about evolution. Trilobites are proof that life has not simply been advancing from primitive towards ever more advanced forms. They show that since early times, some organisms have been remarkably specialised and sophisticated. And perhaps the most important message from the trilobites is that their early abundance and diversity weren’t enough to protect them from the changing ocean. Look all through the seas today, and not a single living trilobite is to be found.

After almost three hundred million years of scurrying and swimming, drifting, digging, and rolling up in balls, trilobites went extinct. They were among the species wiped out by the catastrophic Permian extinction event, which drew the Palaeozoic era to a close. This was the most devastating of the five ancient mass extinctions.^a The cause was likely a spell of runaway global warming, triggered by immense volcanic eruptions, which filled the atmosphere with so much carbon dioxide the ocean was cooked, acidified, and sapped of oxygen until most aquatic life suffocated. That was the end of the trilobites, although in fact they had been in decline for much longer.

Trilobite diversity peaked in the late Cambrian and into the early Ordovician, and thereafter this group’s splendour had been fading away. For the rest of the Palaeozoic—through the Silurian, Devonian, Carboniferous, and finally Permian periods—trilobites had been relinquishing their dominance in the ocean. Steadily, their diversity diminished until a single family remained, containing a handful of species that were quite plain and small compared to their predecessors.

We have some clarity as to why trilobites were knocked back. For instance, at the end of the Ordovician, the supercontinent Gondwana drifted over the South Pole and became covered in giant ice sheets, pushing the earth deep into an ice age. Sea levels dropped, and when continental seas dried out, crowds of tropical trilobites lost their habitat and went extinct. Those species that happened to be better able to cope with the cold survived. What continues to mystify is why trilobites didn’t rebound once the ice age was over and conditions on the earth became more agreeable. New trilobite species were still evolving but not fast enough to replace the older species that were going extinct. No doubt the ocean filling up with new predators had an effect, including the first fish and squid, which were busy chasing after trilobites. Another suggestion is that trilobites weren’t very good at shedding their exoskeletons. Fossils show that many injured themselves trying to climb out of their old shells, emerging with damaged eyes and misshapen heads.

Nobody has yet found a convincing, single explanation for the trilobites’ long-term demise, which suggests it was likely a mix of changes and challenges emerging in their world. Whatever the ultimate causes were, from the end of the Ordovician onwards, trilobites suffered repeated setbacks from which they never fully recovered. Their former success did not predict their future survival.

The second great chapter in prehistoric life, the Mesozoic era, got underway around 250 million years ago in the aftermath of the mass extinction that devastated the earth’s biosphere. Trilobites were gone. Huge coral and sponge reefs were gone. So were sea scorpions and spiny sharks called acanthodians. Many other groups of organisms, while not entirely lost, were stripped back to a tiny portion of their former diversity and abundance. In all, fewer than one in ten species survived. For millions of years after the extinction, a disaster fauna, as palaeontologists refer to it, existed on the seabed, made up of species that were just holding on and by no means thriving. The situation was better up in the open water, where shoals of conodonts—eel-like, two-inch-long fish with bulging eyes and no jaws—proliferated. There were also spiral-shelled cephalopods called ammonoids, which looked similar to living chambered nautiluses, as well as an increasing diversity of bony fishes. These animals all became prey for a group of animals whose ancestors had left the ocean more than a hundred million years earlier and in the Mesozoic made a spectacular return to the sea.

Back in the Palaeozoic, a group of fishes had gradually adapted to life beyond the tideline. They already had four legs, which they used while still living at the shallow edges of the sea, and some of them walked out onto land and became the ancestors of all the land-living vertebrates alive today: the amphibians, reptiles, birds, and mammals. Collectively, these vertebrates are known as tetrapods, even the ones that later turned some of their four legs into wings or flippers—and reptiles did both.

By the time the Mesozoic was underway, the reptiles known as dinosaurs^b were famously ruling the land, and pterosaurs had taken to the skies. Meanwhile, the ocean was dominated by different groups of reptiles. Having lost their ancestral fishy gills, these animals drew gulps of air into their lungs and then leapt, slithered, and strutted back into the sea and very soon were well acclimatised to their revamped aquatic life. Within a few million years of the end-Permian mass extinction, reptilian apex predators were swimming through all the seas and making a major impression on the rest of ocean life.

These were the real-life embodiment of mythical sea monsters, with all the ferocity and grandeur we might imagine. Cruising around were sixty-five-foot-long ichthyosaurs, some as long as eighty-five feet. They looked like blue whales with elongate, tooth-filled jaws. Other ichthyosaurs were roughly the size and proportions of bottlenose dolphins. Excalibosaurus had a rapier-like rostrum, as swordfish do today, and presumably used it in a similar way to slash through shoals of fish. Tylosaurs looked like enormous modern-day orcas, up to twice their size, and may have hunted like them too, subduing prey by ramming into it with their bony snouts and then tearing it apart with razor-sharp teeth.

Plesiosaurs looked like archetypal incarnations of the Loch Ness monster, with a streamlined body, tiny head, and two pairs of long flippers, which they paddled in elegant undulations to fly underwater. Many had phenomenally long necks, some measuring more than twenty feet and taking up two-thirds of their body length. It’s tempting to imagine plesiosaurs using their necks to strike out at prey, like a coiled snake, or to grab pterosaurs flying above the waterline. In fact, the plesiosaurs’ abundant neck vertebrae were likely quite stiff and didn’t flex from side to side. These reptiles may have floated horizontally in the water, dipping their necks below their body to rake fish shoals with a snarl of intermeshed teeth or to root out prey in the seabed.

Swimming through Mesozoic seas were reptiles that looked like monitor lizards, others like giant newts, salamanders, or crocodiles, and huge, long sea snakes with little legs and gently bulging bellies. This was also the era when another group of reptiles, the sea turtles, first evolved, including the biggest ever to exist, the two-ton Archelon, which grew to fifteen feet long and would have needed four king-size mattresses to stretch out on.

In all, reptiles retraced their ancestral past and took to the ocean on at least a dozen separate occasions. No one knows for sure what drew them all down to the sea—one idea is that the land was getting crowded with other animals while the ocean offered plenty of space and prey—but clearly reptiles learnt to swim many times over. Their bodies underwent extreme adaptations to enable them to live underwater. Fossils of ichthyosaurs show that their arms gradually became shorter and their hands longer with more fingers—all the better to act as large, swimming flippers. Some ichthyosaurs evolved eyes the size of ten-pin bowling balls, bigger than those of any other animal extinct or alive, which gave them excellent vision as they hunted in the dim waters of the twilight zone. Thalassodraco (“sea dragon” in Greek) looked like a dolphin with a huge ribcage, accommodating enormous lungs that let it take great breaths and stay longer underwater.

Occasional food remains preserved in their stomachs and the arrangement and shapes of their teeth tell us Mesozoic marine reptiles had a varied animal-based diet. The most terrifying of all were those that filled an ecological niche that until then had been empty. Hyper-carnivores—predators that eat other predators—hadn’t existed until the Mesozoic. It’s evident that swimming reptiles were in the habit of eating one another. A fossilised Diandongosaurus, a close relative of plesiosaurs, has been found with its hind left flipper missing, likely bitten clean off by a fellow reptilian predator. And a sixteen-foot ichthyosaur swallowed most of a thirteen-foot thalattosaur, another Mesozoic reptile, shortly before it died, as palaeontologists saw when they found a fossil within a fossil.

Not all the swimming reptiles were spine-chilling carnivores. The oldest known plant-eating marine reptile, Atopodentatus, had a strange hammerhead skull and may have had an unusual two-step mode of feeding. With its chisel-shaped teeth, it likely scraped at seaweeds on the seabed. It also had a row of needlelike teeth, which formed a mesh and could have sieved fragments stirred into the water column.

And not all these swimming reptiles were giants. Many of the early ichthyosaurs were salmon-sized, including one called Cartorhynchus, which had a stubby snout, pebble-shaped teeth for crunching snails and clams, and big flippers with flexible wrists that would have let it move about on land. In life it would have looked rather like a small seal that basked on the shore and dived in the water to feed.

Throughout the 190-million-year span of the Mesozoic, an ebb and flow of marine reptiles occurred, as some went extinct and new forms kept evolving. To begin with, they mostly stayed near the shorelines fringing the coasts of the supercontinent Pangaea, which had formed when other continents collided with Gondwana and spanned both hemispheres. Then, as Pangaea began to break apart, the continents and oceans we know today began taking shape. Volatile seams in the earth’s crust opened, and the new Pacific and Atlantic Oceans were born. Marine reptiles swam along the seaways that opened up between continents and soon were living all across the global ocean.

The Mesozoic was more than just an exciting time for its assortment of swimming reptiles. The reign of sea dragons was part of an oceanic revolution that shaped much of life on earth in ways that continue into the present day. The procession of predators triggered an evolutionary arms race as their prey found means of survival and escape, which in turn caused new life forms and lifestyles to flourish.

A major battleground was the seabed, where reptiles as well as bony fishes, sharks, ammonoids, and crabs were all busy searching for shelled creatures and crushing them in their powerful jaws or claws. The prey responded in many different ways. Clam-like bivalves began a long game of hide-and-seek that still goes on today, as they escaped downwards, burrowing into the seafloor and sprouting long breathing tubes.^c Sea lilies, umbrella-shaped relatives of starfish, escaped the feeding frenzy by drifting off into open waters. Snails evolved thicker, spinier shells; some also took flight from the dangerous seabed, evolving tiny wings and flitting off into the plankton as sea butterflies. Other snails fled the oceans altogether. The Mesozoic reptiles that went back to the sea and evolved into terrifying sea monsters were part of the forces that drove snails into fresh water and ultimately onto land.

Predators evolved countermeasures to keep up with their prey. They developed excellent vision and camouflage, improving their chances of spotting prey from farther away and sneaking up undetected. And predators evolved new ways of breaking through tough-shelled defences. If you ever find a seashell with a neat, round hole, it contained a creature that was killed and eaten via a mode of attack that first evolved in the Mesozoic, when predatory snails gained the ability to drill through the shells of other molluscs and suck out their soft insides.

Ocean food webs had become a scramble of animals eating and getting eaten. In 1977, mollusc expert Geerat Vermeij proposed this ferocious struggle should be called the Mesozoic marine revolution. Since then, signs of the revolution’s effects have been noted in many other animals, but only in 2021 did this transition come into view across the entire fossil record. Earlier fossil studies found obvious signs of the five prehistoric mass extinctions—it’s not too difficult to spot the annihilation of more than 90 per cent of all life. But the influence of mounting predation in the Mesozoic was more gradual and didn’t clearly show up. With advances in computing power and new ways of analysing enormous fossil data sets, researchers can now extract patterns that were previously hidden from human eyes and minds. Scientists at Umeå University in Sweden built what amounts to an interactive digital map of the entirety of ancient ocean life through space and time. It shows that the Mesozoic marine revolution was indeed a life-shaping force equally as powerful as mass extinctions. Dramatic global catastrophes have caused ocean-wide changes in biodiversity, and so too did the prolonged shift that took place under the influence of marine reptiles and all the other predators roaming through Mesozoic seas.

Today the only reptiles that live a fully oceanic life are sea turtles and sea snakes. All the other fish-like, whale-like, and lizard-like Mesozoic lineages went extinct. The last of the ichthyosaurs were gone by around one hundred million years ago, likely because they weren’t evolving quickly enough to adapt to an intense period of climate change. Meanwhile, the plesiosaurs made it to the end of the Mesozoic, then died along with three-quarters of life on earth, extinguished by another mass extinction. That moment, sixty-six million years ago, is the most infamous planetary disaster, one that captures people’s imaginations more than any other and shapes the popular view of extinction. An asteroid hit the earth, the skies darkened, and the dinosaurs’ days were numbered. A story often told is that once dinosaurs were out of the way, mammals had their chance to arise from their shadows. It’s a dusty old theory, challenged by many lines of evidence in the fossil record. And yet, the idea persists that mass extinctions pave the way for new life to rebound—that devastation is a prerequisite for renewal.

Machine learning tools are helping scientists construct another powerful new view of biodiversity through time. By programming computers with artificial intelligence algorithms, biologist Jennifer Cuthill from Essex University (UK) and colleagues have held millions of fossils in their virtual hands and tracked subtle shifts in life from the Cambrian to the present day. Their method detected the same five mass extinctions that everyone else has found, plus seven other major extinction events. They also pinpointed seventeen times in the past when life flourished and evolution surged into action, which they called mass radiations. Critically, they found that mass extinctions and mass radiations don’t tend to pair up in the fossil record. There are only two examples of a notable extinction followed right away by a great radiation. On fifteen other occasions, radiations happened nowhere near an extinction event. Rather, they were often associated with evolutionary innovations, such as the development of eyes or the movement of different forms of life onto land. There is little evidence that mass extinctions are a force of creative destruction.

The pace and rhythm of evolution in deep time is not simply driven by dramatic extinctions; life is far more responsive and complex. A catastrophe isn’t needed to steer evolution down new avenues and sculpt biodiversity in novel ways. A tangle of other elements is constantly in play, as more gradual and less immediately lethal environmental changes take place. And these intricate responses are evident in the ways ocean life changed through the third great chapter of life, the Cenozoic,^d beginning sixty-six million years ago and leading up to now.

When the aftermath of the dinosaur-killing extinction had subsided, roughly ten million years into the Cenozoic, the earth was in the grip of fearsome global warming. There was no ice at the poles. Forests flourished in Antarctica. Flying lemurs leapt through the trees on Ellesmere Island in far northern Canada. The situation suddenly escalated fifty-six million years ago, when the deep sea released a colossal burp of the potent greenhouse gas methane, perhaps because gradual heating had caused seabed sediments to destabilise. Global temperatures spiked by ten degrees Celsius in the ocean surface and five degrees Celsius in the deep sea. The ocean became more acidic, a mass extinction swept through deep-sea ecosystems, species moved towards the poles, and the earth became a tropical hothouse.

Many scientists consider this to be the closest analogue to anthropogenic climate change, and while certainly the most recent such event, it wasn’t as extreme as what’s happening now. In comparison, changes in ocean chemistry and the rate of carbon release are an order of magnitude higher today because of humankind. Back then, temperatures continued to rise until forty-nine million years ago, when the climate dramatically changed course. Carbon dioxide levels in the atmosphere began to fall, thanks in part to the chemical weathering of continental rocks,^e which accelerated when India collided with Asia, building the Himalayan mountain range. Temperatures steadily dropped, and the earth, recovering from its fever, began to descend into an ice house, setting the scene for the climate we live in now.

The world was also beginning to look much like it does now. The continents had drifted into their approximate current locations, except for some subtle but important tectonic shifts that were still to come. Around thirty-four million years ago, the southern tip of South America pulled away and left Antarctica on its own. This allowed an oceanic current to begin swirling endless clockwise loops around the continent, effectively isolating it from warmer waters in the rest of the ocean. It also happened that the Antarctic continent had come to sit right over the South Pole, putting it in the perfect place to get very cold very fast, and it was soon covered in a giant sheet of ice.

Not until much more recently, around 2.7 million years ago, did the seas at the North Pole begin to turn to ice. The story of how this happened is more complex and disputed than Antarctica’s deep freeze. But likewise, continental movements were involved. The gap was gradually closing between North and South America. Where the Central American Seaway had provided a direct connection between the Pacific and Atlantic, now the land bridge of Panama lay in the way, preventing the two oceans from directly mixing. This had profound effects on global ocean circulation and climate, including intensifying the Gulf Stream, which flows from the Caribbean Sea into the northeast Atlantic. The closure was also linked with the strengthening of the global conveyor belt, the ocean circulation that flows around the whole planet today. By pushing moisture-laden air northwards, the Gulf Stream likely played a part in boosting snowfall over Greenland and North America, increasing the size of glaciers that were forming in the Northern Hemisphere as the earth entered a prolonged and volatile ice age.

In the Cenozoic, the ocean settled into its present-day configuration and welcomed the arrival of many life forms that still prevail. A recurring theme during this era was the return of tetrapods to the ocean. Yet again, four-footed land animals gave in to the irresistible lure of the sea, many times over. The main protagonists this time weren’t reptiles, as in the Mesozoic. Instead, it was the turn of mammals to go back to their aquatic roots. Seven different types of mammals dipped their hooves, paws, and feet in the water and evolved ways to survive at sea, in time becoming integral parts of ocean ecosystems.

Around fifty million years ago, an assortment of prehistoric mammals walked the shores of the Tethys Ocean, an ancient water body that used to connect the Atlantic and Indian Oceans across northern Africa. Among them were animals that trotted on long, thin legs and had something of the wolf in them. These were archaeocetes, the beginning of a long animal dynasty that led up to modern cetaceans—the whales and dolphins. They were animals with even-toed hooves, a group that today includes giraffes, camels, antelopes, llamas, sheep, pigs, and cows. Modern cetaceans’ closest living relatives are hippopotamuses.

Proto-whales went through a series of evolutionary stages, as shown by elegant fossils, at first amphibious then increasingly oceanic. They followed parallel lines of evolution leading them to resemble many of the giant marine reptiles that swam before them. Their legs turned into flippers, and their tails sprouted powerful, wide flukes, although they didn’t swing sideways, like the tails of swimming reptiles, but up and down, a throwback to the gait of their terrestrial forebears; contrast a reptilian gecko, which sashays along flexing its spine from side to side, with a cheetah, which bends its back up and down as it runs. It took roughly ten million years for whales to become fully pelagic, with nostrils on the tops of their heads (i.e., blowholes) and inner ear bones adapted to hearing well underwater. But the cetacean lineage really took off a while later.

A key point in the evolution of whales was when Antarctica broke away from South America, and the global climate shifted into an ice house. This was when the two main cetacean lineages—the baleen whales (mysticetes) and the toothed whales (odontocetes)—diverged from each other. With Antarctica on its own, a current now whirled around the continent, which not only sent temperatures falling but also triggered deep mixing of the Southern Ocean, stirring nutrients into upper layers of the sea. Consequently, oceanic ecosystems around the world became enormously productive. Plankton proliferated, and the new group of filter-feeding whales with hairy baleen plates in their mouths had plenty to eat. Around that time, toothed whales began to interrogate the ocean with beams of sound, hunting for prey with their new echolocating powers. From these early odontocetes, a lineage of smaller cetaceans separated and began filling seas and rivers with dolphins, porpoises, narwhals, and orcas.^f

At the time when the proto-whales were starting to wade and swim, a different group of mammals were splashing along the shores of the Tethys Ocean. Sea cows began as pig-size relatives of elephants with stumpy legs that walked on land and wallowed in the shallows. Also known as sirenians, they spread across the planet and evolved into assorted species of dugongs and manatees, which mostly lived along warm and tropical coasts and spent their days chewing on seagrasses. Some moved into the North Pacific rim and fed on the giant kelp forests that started growing there around thirty million years ago as the ocean was cooling. They shared their kelp forest home with yet another gang of aquatic mammals—desmostylians, stocky, hippo-size animals with two pairs of short, goofy tusks sticking out of their mouths. Like many other herbivorous marine mammals, they had thick, heavy bones that acted as ballast to help them sink while they foraged underwater, especially important when their rotund bellies filled up with digestive gases produced by their plant-based diet. Desmostylians existed for only a narrow window of time, between thirty-three and ten million years ago. All the North Pacific sea cows also went extinct, including, most recently, the biggest ever to evolve. Steller’s sea cow grew up to thirty feet long, several times the size of living sea cows. This giant was named after the German explorer Georg Wilhelm Steller, who encountered it in the 1740s while shipwrecked on the Commander Islands, off Kamchatka, Russia. By then the species was already on its way out, driven into decline by the changing climate during the last ice age. When sea levels dropped during glaciations, much of its kelp habitat became fragmented. Human hunters played a part in the Steller’s sea cows’ annihilation, killing the last of them for their oil and for their meat, which apparently tasted like corned beef.

Outliving the kelp-dwelling sea cows and desmostylians are mammals that much more recently took to the ocean. Sea otters evolved in the North Pacific from weasel-like ancestors only in the past two to three million years, and at up to a hundred pounds, they’re the heaviest mustelids but the smallest marine mammals.

Farther south, seagrass meadows along the South American coast were grazed by giant swimming sloths. Five species of Thalassocnus were related to the giant ground sloths that lumbered around the pampas grasslands. These aquatic sloths grew up to eleven feet long from snout to tail, longer than a bison, and they bounded along the seabed, steering with their tails like beavers. Giant sloths swam about for fewer than five million years, a brief moment in the ocean’s history. When the Central American Seaway closed, the changing ocean currents and falling temperatures killed off swaths of seagrasses along the South American coasts. With little to eat and no blubber to keep them warm, the furry, swimming sloths came to an end.

The same climatic shift caused a major stir among the cetaceans too, with the arrival of conditions that favoured outrageously big whales. When the Northern Hemisphere froze over, sea levels dropped, and patches of rich seasonal food developed in the ocean. Smaller baleen whales were confined to the coasts, where the seawater was draining away, and many of them went extinct around three million years ago. Meanwhile, much bigger whales were thriving. They set off on long journeys, cruising between high-latitude feeding and low-latitude, tropical breeding grounds. Today, enormous whales migrate every year to feed in the Arctic or Antarctic, the fifty-foot humpbacks and grey whales, the eighty-foot fin whales, and the largest animals known to have existed, the hundred-foot blue whales.

The Cenozoic also saw several other mammal groups take to the ocean. Pinnipeds originated in the Arctic at least twenty-four million years ago from a similar group of ancestors to the sea otters. Indeed, early on they looked a lot like three-foot-long otters, but their fossilised teeth give away that they were in fact seals. Three groups of pinnipeds have since diverged and moved onto shores around the world: the seals, sea lions, and walruses.^g In the Arctic, walruses and seals haul out on the sea ice to rest and raise their young, all the while watching out for polar bears, the mammals that most recently took to the sea. Polar bears diverged from brown bears and began prowling the Arctic ice within the past half million years.

For seals in Antarctica, there are no polar bears to fear, and they share the Southern Ocean with another gaggle of vertebrates that also dived into the Cenozoic ocean and became Antarctic specialists. Direct ancestors of modern-day penguins evolved twenty million years ago on temperate coasts and islands of Aotearoa (New Zealand)^h and Australia. From there they moved south and took advantage of Antarctica’s circumpolar current to disperse around the continent, gradually adapting along the way to the freezing climate and diverging into new species. First to evolve were the biggest living species, which stand waist-high to a human: the emperor and king penguins. Then came gentoos, chinstraps, and Adélies.ⁱ All of them eat krill, the finger-size swimming crustaceans that evolved at a similar time and became the staple diet of most Southern Ocean animals. Later, around eleven million years ago, the Antarctic current strengthened and propelled penguins northwards to warmer climes, including back to Aotearoa and even as far as the equatorial Galápagos Islands, but they’ve not made it as far as the Arctic. Penguins remain resolutely Southern Hemisphere species.

The many new arrivals in the Cenozoic ocean joined a mix of marine life that had survived from earlier epochs and was busy shifting into a modern guise. During the Cenozoic, modern day coral reefs took shape. Previously, reefs had been built by various kinds of corals as well as sponges and shells. Then, after millions of years of coming and going through multiple mass extinctions, the scleractinian corals finally gained a firm footing in the ocean, and their diversity steadily increased. Individual corals are tiny, flower-like polyps that construct around themselves exoskeletons of limestone, hence their other name, the stony corals. In great colonies, they form the main foundation for tropical reefs today.