4
THE LOST WORLDS OF THE GIANTS

Walk into a natural history museum anywhere in the world and the visions that greet you are nearly always of giants. In Washington DC, Tyrannosaurus rex is locked in a death struggle with a Triceratops. In London, the skeleton of the largest animal that has ever lived—the blue whale—seems to glide towards you, suspended from the roof, while the Dinosaur Park at Beijing’s Natural History Museum has no fewer than 23 reconstructed behemoths roaring, waving armoured tails, and (for extra realism) panting and blinking.

It seems the human psyche is drawn towards the awesome, the spectacular, the terrifying, even the monstrous. This attraction, seemingly at odds with our instinct towards self-preservation, was present long before the age of modern, well-stocked museums. In the cabinets of curiosities that could be found in the mansions of the aristocracy of the seventeenth and eighteenth centuries, fossil mammoth teeth and tusks were ‘must-have’ items. A particularly celebrated, indeed notorious, prize was a magnificent, wickedly toothed mosasaur skull discovered in Maastricht in 1770, that became a prime target for Napoleon’s forces (on Bonaparte’s direct orders) when they laid siege to that city in 1794. With the help of 600 bottles of wine as a bribe—as some stories say—it was duly carted off as war booty to the Natural History Museum of Paris, where it still remains (Maastricht would like it back but has not succeeded in extricating it yet). Even further back in human history, in Greek and Roman times, the bones of giant extinct mammals were avidly sought after, and sometimes sparked a ‘bone rush’, as these were seen, then, as precious relics of the ogres and heroes of legend.¹

Within the labyrinthine corridors and spaces that generally lie behind the grand museum galleries, the rest of the more humdrum petrified life is stored. These storehouses are in their own way just as spectacular, just as breathtaking as the dinosaurs on display for the public. One such storehouse is the British Geological Survey’s museum at its main headquarters, just to the south of Nottingham, where fragments of life that populated the seas and land millions of years ago fill its many storage cupboards. This is the oldest geological survey in the world, and since 1835 its scientists have been cataloguing the nation’s rocks, and the riches that lie within them, including the wealth of fossils they contain. There are now some 3 million fossils in its collections, in racks of purpose-built wooden trays four levels high. The wooden trays contain smaller cardboard boxes in which the fossil-bearing slabs are carefully stacked and numbered, each slab separated by a protective strip of paper from its neighbour. Some palaeontologists, conscious of space, packed these slabs in so tightly that an incautious attempt to pull one slab out can cause a whole boxful to spring out into the air, in a small palaeontological explosion. It is a goldmine of extinct life that seems endless (though it is dwarfed in turn by the 40 million fossils of Washington DC’s Natural History Museum).

In some of the larger storage cupboards are the bones of hyena, rhinoceros, elephant, and hippopotamus that walked the landscape of Britain just a short while ago geologically, most being from the last-before-present warm climate interval, about 120,000 years ago, and from the 100,000-year interval after that when ice or permafrost mostly covered the land. Some of these bones—hyena bones, notably—come from the caves of Creswell Crags which lie in a narrow gorge that straddles the county boundary of Derbyshire and Nottinghamshire. Here, on the northern habitable edge of Ice Age Europe, people and animals vied for the life-protecting cave space, leaving behind their remains over a period of nearly 50,000 years. Artefacts from these caves include stone tools made from flint, engravings on the walls, bone implements including sewing needles, and the bones of animals that once lived here, including those of humans. On the northern side of the gorge, on its sunny south-facing slope, is the labyrinthine Robin Hood’s Cave, where that legendary and probably mythical outlaw is said to have hidden away from the Sheriff of Nottingham’s men. Immediately opposite, on the southern side of the gorge, is the long narrow cave called ‘Church Hole’. It shows the faintest traces of bas relief cave markings, perhaps once painted with ochre, but now often covered by a thin layer of limestone that has grown over the cave’s surface from thousands of years of dripping water. In the dim light of the cave, deer, bison, auroch, and other animals emerge as ghostly outlines on the walls. At the end of the last ice age many giant animals, like the woolly mammoth and woolly rhinoceros, still occupied the British landscape—but they soon disappeared. It was a pattern that was being played out across the world.

Demise of the giants

The caves of Cresswell Crags are among the latest part of a long historical record left as fragments of bone and scattered stone-tool artefacts across the Old World. For many millennia, ancient humans existed as a small and ephemeral part of a landscape which they shared with many other animals and plants. Even so, as early as 2 million years ago they had developed the ability to kill and process large animals like antelopes and horses.² Archaic humans had also developed a command of fire, perhaps as early as 1.5 million years ago,³ and over time they gradually evolved stone tool kits of increasing complexity and utility. The first signs that our ancestors, in the form of Homo erectus, might have begun to change the animal landscape came in the form of modest extinctions of sabre-tooth cats and some elephant species in Africa, that cradle of humanity, about a million years ago, while these animals continued to thrive elsewhere.

Early human species travelled: one of their number, known to us as Homo antecessor, had reached Britain some 900,000 years ago, leaving implements and footprints by the cliffs of what is now the village of Happisburgh in Norfolk. Members of our own species, Homo sapiens, were living in North Africa by 300,000 years ago, but still with mostly negligible effects on other species. That pattern began to change from about 70,000 years ago, as cognitively modern humans left their mark on Africa, Europe, and Asia, and then moved on into new lands.

Australia was one of these new lands, where no humans had trod prior to our arrival 65,000 years ago. People crossed from South East Asia during the last Ice Age when sea level was lower, and more land was exposed. This journey must nevertheless have required remarkable navigation skills as they travelled, perhaps via the Timor Sea, or via island-hopping from Sulawesi to Papua New Guinea. At its narrowest stretch, these early mariners must have traversed some 90 kilometres of sea, with any hint of land being far beyond their gaze. What kinds of boats these people used, or how they made the crossing, is unknown. The oldest preserved boats are much younger than this, dugout canoes dating to about 8,000 years ago. Today, it is very hard to imagine anyone paddling across a large expanse of open water, such as the Timor Sea, in such a crude boat. By whatever means these settlers arrived, over succeeding millennia they dispersed across Australia.

The biological landscape that these ancient humans explored, and proceeded to transform, was remarkable. Even the remnants that had survived by the time the first European explorers arrived were enough to provoke amazement, with bizarre creatures, to their eyes, such as the kangaroo, koala bear, and potoroo. Charles Darwin, who set foot in Australia as a 26-year-old in 1836 during the return leg voyage of the Beagle, was deeply struck by the sight of platypuses playing in a river on the slope of the Blue Mountains, by Wallerawang.⁴ Here were creatures that were occupying the niche of a European water rat, with similar adaptations—but, as mammals that possessed duck-like bills and laid eggs, were almost absurdly different biologically. If species had been fashioned by any sensible Creator, why would not such a deity have simply put the water rat in Australia as well as Europe? That would seem to be much more straightforward than designing a whole new and very different organism. Darwin noted that ‘an unbeliever in everything beyond his own reason’ might be prompted to cry out, ‘Surely, two distinct Creators must have been at work!’ It has been called Darwin’s ‘platypus moment’⁵: a significant step that set him on track to questioning divine creation, and towards building his theory of ‘descent with modification’ by natural selection.

What might Darwin and his colleagues have thought if they had alighted on to the Australia of 65,000 years ago? They would have encountered beasts that were not only bizarre, but impressively bulky too. There were such as the Diprotodon, the ‘giant wombat’—a marsupial 2 metres tall at the shoulder and weighing nearly 3 tonnes; Procoptodon, a giant kangaroo up to 3 metres high; Propleopus, a 70 kg carnivorous or carrion-feeding kangaroo; and Zaglossus, an echidna the size of a sheep.

By 40,000 years ago, human populations had reached Western Australia, and as far south-east as New South Wales. The giant kangaroos and wombats were by then extinct, likely killed by a combination of hunting and vegetation burning. These skilled hunters left clues as to their tastes—for birds’ eggs, for instance. Fragments of eggshells attributed to a bird that looked like a large turkey belonging to a group called the ‘megapodes’—‘birds with big feet’⁶—commonly show signs of burning and cooking in a fire. This kind of pattern, of a spread of humans and demise of giant animals, was to intensify around the world in the coming millennia.

The Americas lie even more geographically remote from the origins of modern humans than Australia, but at the time of the last Ice Age there was a direct chain of geographical connection through north-east Asia and onwards via the icy wastes of the Beringia land bridge that joined easternmost Siberia to Alaska. This land bridge gradually disappeared as meltwater from receding icesheets forced sea level to rise at the end of the last Ice Age. People had probably been living in this landscape for 5,000 years when the rising waters dispersed some back to Asia, while others made the journey into the Americas. Before humans arrived there, the biological landscape of North America was very different from what it is now.

Here lived true giants, aptly named ‘megafauna’. Along with the more familiar Ice Age animals, such as mastodons and mammoths, there were giant beavers 2 metres long with formidable 15 cm-long incisors, and Camelops, a North American relative of the camel that stood 2 metres tall at the shoulder, while the formidable American lion was up to 2.5 metres long. Out-gunning all of these for sheer size was the giant short-faced bear, standing 1.8 metres high at the shoulder, and rearing up to be 3 metres tall when standing on its hind legs. Likely a voracious predator, it must have been a fearsome sight even for a hardy human hunter from Beringia. Some of these animals, like the giant beaver, appear to have been in decline before the arrival of humans, perhaps due to climate change at the end of the last Ice Age. But others show clear evidence that the human invaders were hunting them. Amongst these, one group of megafauna had grown to colossal proportions: the ground sloths.

Ground sloths first evolved more than 30 million years ago on the then-isolated continent of South America, where some of their living relatives, tree sloths and anteaters, still live. As the two Americas became joined through the formation of the Isthmus of Panama about 3 million years ago, ground sloths migrated into North America, eventually reaching as far north as Alaska. They also reached into the islands of the Caribbean. Some of these ancient sloths reached elephantine proportions. Eremotherium, which was present in North and South America, could grow to 6 metres long and weigh 3 tonnes.

Humans were clearly hunting and butchering these gigantic animals. Near the surface of the playa lake called Alkali Flat in the White Sands National Monument of New Mexico, there are superbly preserved fossilized footprints that are at least 10,000 years old.⁷ These show close interaction between sloths, sometimes apparently flailing around for protection, and human pursuers. So close is the connection that the human footprints sometimes stand within those of the sloths. Whether people really were actively hunting them is difficult to discern, because there are no preserved carcasses at the site, and no evidence for butchery. Elsewhere, in South America, there is direct evidence of sloth kills.

At Campo Laborde, in the Pampas of Argentina, is a 12,000-year-old record of the largest of all ground sloths being butchered, a Megatherium—the name literally means ‘great beast’—which reached 4 tonnes in weight. Many other bones of mammals are found at the site, but only Megatherium and the Patagonian hare—a species still living today on the Pampas—show signs of being butchered. There are cut marks on a sloth rib bone that indicate the removal of its flesh, while those on the leg bone of the hare suggest skinning.⁸ Although such finds of butchery are rare in the Americas, the rapid disappearance of ground sloths within about 2,000 years of the arrival of humans suggests that hunting drove their demise. Another of these rare sites is La Moderna, also on the Pampas, where a large glyptodont called Doedicurus was butchered on the margins of a swamp some 7,500 years ago. This was a heavily armoured animal a little like its modern mammal relative the armadillo, but one that possessed a long tail that bore a spiky club at its termination, like that of the dinosaur Ankylosaurus. This club was probably of no use in defending it from humans, as the animal had poor rear vision (more likely, it was used in conflicts with other glyptodonts). It and many megafauna disappeared from American landscapes. The last few ground sloths clung on in the islands of the Caribbean until about 4,000 years ago.

Our fascination with these rare, gigantic animals is not entirely frivolous, nor just a way for museums to boost their incomes. The sheer scale that these leviathans could reach tells us something of the limits of biology, in different times of the Earth’s past and in different conditions, on land and in the sea, in good times and bad. These giants fuelled their bulk by consuming the tissues of very many plants, or of many other animals. The apex predators—and the apex herbivores that on land could often reach even bigger proportions—were entirely dependent upon the overall health of the meshwork of life, among which they formed such spectacular outliers. These giants also exerted a control on that landscape which is hard for us to imagine in these days of impoverished wildlife. But let us take stock of the scale of the change that has taken place on Earth, over those thousands of years.

The scale of loss

To assess the scale of loss, we first need to define some categories. What are the megafauna? One simple and commonly used category is just to lump together all moderately to large-sized animals, and have a modest cut-off point, at 100 pounds weight (45.3 kgs). But there is an ecologically based classification that more clearly separates the giants from the rest. Megaherbivores, here, are understood as plant-eaters weighing more than 1,000 kg, while megacarnivores make the grade at 100 kg.⁹ Why the order-of-magnitude difference? Well, one good defence against predation is simply to grow too big to sensibly attack, and this has been a constant strategy for herbivores since animal life developed on land.

Once having reached that size, an animal becomes more than an impressive addition to the scenery: it shapes that scenery. The megaherbivores change the balance between grassland and forest by prodigious eating, by trampling, by uprooting trees on an enormous scale. Within a forest, they can change tree community structure by more easily destroying saplings, the surviving trees then forming extensive stands of mature forest. The enormous volumes of vegetation eaten and then excreted fertilize the soil—especially as the herbivore guts help break down the often tough and resistant plant matter. Herbivore guts, too, are a good vehicle for dispersing plant seeds, and many fruiting plants have co-evolved with herbivores over geological time, becoming as dependent on them as many flowers are on pollinating bees. These giant herbivores, especially when young or sick, are not altogether immune from the megacarnivores, of course, as these in turn shape the biology around them through the ‘landscape of fear’ they generate.¹⁰

That—with only geologically brief pauses (such as just after the dinosaur-killing impact of the asteroid that brought the Mesozoic Era to an end)—has been the state of the terrestrial landscape for hundreds of millions of years. That kind of long-term global functioning of the terrestrial biosphere has, in the geological blink of an eye, come to a halt, as the landscape-shapers have been decimated. Before the main phase of human-driven megafaunal extinctions began, some 50,000 years ago, there were some 50 megaherbivore species in the world. Now just nine remain, all in Africa and Asia, as the still-existing (if severely depleted) elephant, hippopotamus, and rhinoceros species. The Americas have fared much worse, losing all of their 27 megaherbivore species. The 10 largest species of that original 50 have all become extinct, so that even the remaining megaherbivores are modestly sized, as giants go. The consequences of this loss are now difficult for us to envisage, or understand at a visceral level, given the little that is left of this kind of world that we can directly experience. But, these consequences may run deep.

One consequence—particularly noted in North America, where the megafaunal crash was acute, was a conversion from a patchwork of forest and park-like grassland, sustained by all that trampling and uprooting, to more continuous forest cover. And that in turn commonly seemed to have led to more widespread and frequent fires, with the loss of natural firebreaks as the giant animals declined, to produce a landscape tilted towards a ‘black world’ (frequently fire-ravaged) scenario, and away from ‘green world’ (forested) or ‘brown world’ (herbivore-maintained grassland) conditions. Even within a forest, the megaherbivores are thought to help encourage the growth of old, mature trees, grown too large to easily uproot, by thinning out the more vulnerable samplings. This kind of landscape change wasn’t just restricted to the tropical and temperate parts of the Earth. At high latitudes, in cold northern Europe and Asia, a constant feature of warm intervals in Ice Age times were ‘mammoth steppes’ with dry fertile soils that supported a rich animal fauna and a diversity of plants. When the mammoths were hunted to extinction, these conditions of plenty disappeared, to be replaced by waterlogged, much more biologically impoverished tundra-like soils. The key to the mammoth steppes seems to have been the abundant megafauna that, by constant grazing, encouraged the growth of deep-rooted grasses that helped cycle both nutrients and water.¹¹ With this ecological function thousands of years ago, this ‘natural’ (to living human memory) landscape of Siberia and kindred regions already represents a heavily modified—and poorer—kind of world.

These wide continental landscapes, thus, have long (on human timescales, that is) been transformed. But some parts of the world long remained essentially pristine and untouched, protected by their geographical isolation. Islands in the middle of great oceans have formed little isolated worlds of their own since the early days of our planet, each one a laboratory for biological evolution for the few castaway species that landed there. But even those refuges for animals and plants large and small, could not escape the constantly spreading web of human impact. In such places, at least, there is recorded human witness for the transformation.

Fragile islands

There are many places on Earth, on the land and in the sea, where animal and plant species have already made their last stands. On others, they are now in the midst of seemingly terminal struggles.

The island of Rodrigues in the Indian Ocean is one such place. It is terribly isolated, lying about 560 kilometres (350 miles) to the east of Madagascar, its total surface area only about 108 square kilometres (40 square miles). Still, this small island has a wide range of terrains rising to nearly 400 metres above the sea on Mount Limon, and before humans arrived, its heavily forested landscape supported a diverse island flora and fauna. Here, until the eighteenth century, lived the Rodrigues Solitaire, a less famous cousin of the dodo. The bird approached the size of a swan, though with a pigeon-like body with long legs and neck, hooked beak, and tiny wings. No longer useful for flight, the wings were used in combat, and also for communication, perhaps to police their territories; the sound from their flapping wings was said to carry for some 200 metres. The first person to describe the solitaire was the naturalist François Leguat, a French Huguenot escaping persecution in France, who was marooned on the island for two years in 1691. He seems to have been quite taken with the birds’ appearance, which he described as moving with grace and stateliness. This, though, did not save the birds.

The Rodrigues Solitaire, a solitary bird on a solitary island, had nowhere to escape to when the invaders came. Its forest habitats were destroyed, the birds were hunted for food, and their eggs were probably consumed by pigs and cats. By the middle of the eighteenth century, a few decades after the first sighting of the birds, they were gone. The Rodrigues Solitaire was not the only casualty.

Before humans arrived on Rodrigues there may have been tens or even hundreds of thousands of giant tortoises. These were hunted to near extinction during the mid-eighteenth century for food and fuel. There are still giant tortoises on Rodrigues now, but these are introduced Aldabra tortoises, another giant species of the Indian Ocean, and one that is important for engineering ecologies for other organisms to live in, called tortoise turf. The indigenous tortoises of Rodrigues may have filled a similar function, dispersing seeds and making clearings in which diverse plants could thrive. There were two native species, the giant ‘saddlebacks’, which lived on the taller vegetation, and the smaller ‘domed tortoise’, which was adapted for feeding on undergrowth. By the beginning of the nineteenth century only a few surviving tortoises could be found, but too few for a breeding population.

Other Rodrigues animals suffered the same fate, as the island was cleared for agriculture by burning its vegetation, and by the combined onslaught of cats, rats, and pigs. The vanished animal species include the Rodrigues starling, an elegant bird with white plumage and a yellow beak, which seems to have clung on in some tiny islets that surround the main island, probably until rats swam across from the mainland and set about eating its eggs. Gone too is the beautiful green-backed ‘Rodrigues day gecko’, which lived amongst the trees where it fed on insects and nectar. Five of these animals were pickled in museum collections for posterity, an ignominious end to a lizard that reached a quarter of a metre long. What happened on Rodrigues is typical of the islands of the Indian Ocean, and those of the Pacific and Atlantic too, many of which have lost thousands of species.

Islands beneath the sea

The Earth’s wide landscapes have become biologically diminished since human written records began—and long before. Even what were once considered wild, pristine forests and savannahs were fundamentally altered as a new top predator, Homo sapiens, reconstructed the apex of the terrestrial ecosystem, and set in motion a cascade of ecological consequences. This is a large part of what makes the current interglacial phase quite distinct from the fifty-odd preceding interglacial phases of the 2.6 million year-long Quaternary Ice Age, and helps justify setting it apart formally as its own epoch, the Holocene, on the Geological Time Scale.

In the oceans, however, things were different. For a long time, these deep, dangerous, and seemingly endless expanses of water excluded any kind of fundamental human influence. Humans did cross them, with courage and skill, to reach new lands, and they did fish, though mainly in shallow and coastal waters. But, for most of recorded history, they barely scratched the surface of the main bulk of the oceans, and the richness of life they contained.

A few centuries ago, serious onslaughts on this realm began, as fishing boats were developed that could make longer voyages, and tackle bigger prey, up to the oceanic megafauna of whales, dolphins, and sharks. The decimation of these charismatic beasts, and of the smaller fry lower in the food chain, peaking with the industrialized fishing fleets of the nineteenth and twentieth centuries, is—unlike the ancient disappearance of the land giants—documented with statistics and eyewitness accounts, and vividly chronicled.¹²

In this short time, the deep oceans have been transformed from seemingly endless and inexhaustible resources to being, now, in many places essentially fished out. The reach of technologized humanity now extends through the increasingly impoverished waters, and is set to extend to another set of islands, those that lie beneath the sea.

Out of sight, far beneath the surface waves of the ocean, these other ‘islands’ have their own specific ecosystems, often harbouring unique forms of life. These are the islands made by ancient and long-dead volcanoes that form underwater seamounts. They too are now threatened, by deep-sea fishing and mining. There may be as many as 100,000 of these submerged islands with their biodiverse communities, that together cover a huge area of the oceans.¹³ Although their physical distribution has been generally mapped by geophysical surveys, only a few hundred of them have been studied biologically in any detail.¹⁴ Many of these seamounts lie beyond the jurisdictions of individual countries and thus are threatened by overfishing and exploitation in the high seas.

One such area of seamounts is the Walters Shoals that lie about 850 kilometres to the south of Madagascar in the southern Indian Ocean at latitude 33° 12' 0" S, longitude 43° 54' 0" E. Geographically these seamounts are part of the Madagascar Ridge, that lies to the north of the south-west Indian Ocean ridge, a 6,000 kilometre (3,700 mile)-long gash in the Earth’s surface where new ocean crust is being formed by volcanic activity. The Walters Shoals are mountains that rise more than 4,000 metres from the seabed and are at some points just 15 to 18 metres below the sea surface. Farther down, at 500 metres depth, the flat-topped surface of the shoals covers an area of about 400 square kilometres, or four times the area of Rodrigues. The Walters Shoals has a unique ‘island’ fauna of sea lilies, sponges, crabs, shrimps, corals, lobsters, and fish. And because the upper reaches of the shoals are in the zone that is penetrated by light, there are pink reefs of coralline algae—communities of small unicellular organisms that are capable of making large limestone structures.¹⁵ Many species of these have so far been identified on the shoals.¹⁶ The seamounts also provide a habitat for whales, whilst seabirds gather here to forage. In a story that shows parallels with the destruction of Rodrigues’ biodiversity, the Walters Shoals have subsequently been trawled and fished. Not discovered until 1962, the shoals once supported a large population of Galapagos sharks, but these were rapidly fished out.

The Walters Shoals are just one part of a much bigger problem facing ocean islands below the sea. They are now seen not only as a source of fish, but also of precious metals like cobalt and tellurium. Many deep-sea mining companies are keen to prospect for these metals, which form the basic components of batteries for electric cars and solar panels, a supposed solution to our over-consumption of fossil fuels. But the collection of these metals from the seabed involves the use of giant suction devices, or continuous chains of buckets, that rip through the delicate ecosystems. It is part of the dilemma we face. Does our generation continue to consume the Earth’s resources in this kind of way, and degrade the rest of nature, making a mass extinction inevitable? In such a scenario, the oceans and land will have an impoverished assemblage of organisms for (at least) hundreds of thousands of years into the future. Or are there ways to live with nature and preserve its biodiversity? The nature of past mass extinctions, and the millions of years it took for life on Earth to recover after each of these events, shows what is at stake.

The day of the dead

On 2 November each year Mexicans celebrate the ‘Dia de Muertos’. The festival is an old one with its origins in the Aztec goddess Mictecacihuatl, the keeper of bones in the underworld of Mictlan. On this night in November a blurring occurs between this world and that of the dead, and the spirits of the deceased are able to join the world of the living again. For most of Mexican history this festival was mainly celebrated in the south of the country, while today in Mexico it has become a national holiday. Its fame has been enhanced in the most contemporary of ways. The 2015 James Bond film Spectre featured an (entirely mythical) Dia de Muertos procession in Mexico City. This magnified the festival’s notoriety worldwide, and the government saw it as an opportunity to highlight indigenous traditions. Since then, an annual procession has taken place for real through the streets of the capital.

The Dia de Muertos, though—even without its recent association with a licence to kill—is not a celebration of death, but one of life, in invoking the spirits of the past to join in with the lives of their living relatives. For palaeontologists, all animals, plants, fungi, and microbes that have ever lived also fall within the scope of such a view of the day of the dead, as ever more ingenious attempts are made to try to bring their fossilized remains back to life within our imaginations. Some of these organisms, like the giant Tyrannosaurus rex, are remembered from their petrified skeletons. Others may leave impressions, footprints, burrows, or trails, while yet others have been transmuted into coal, oil, and gas, and now join us in our lives in the most literal of ways. Very many—indeed most—of these organisms have not yet been detected, especially if they have left no physical trace of their own bodies. Yet, scientists still try to resurrect some echo of their activity: a virus might leave no direct trace of itself in strata, for instance, but its activities might be inferred from patterns in the fossilized remains of the animals it infected.

At the heart of these painstaking reconstructions is the desire to understand the grand patterns of life and death on Earth. Life and death, of course, can be understood in different ways. We usually think of it in relation to individual lives. No organism has lived forever on planet Earth, though some can have a good innings. Creme Puff, the Texan cat, lived for 38 years, for instance.¹⁷ Her owner put this down to a healthy lifestyle and a good diet of broccoli, turkey, bacon, eggs, and coffee with cream, with a snifter of red wine. Thirty-eight years is a long time for a cat, though far outdone by Ming the 507 years old Atlantic mollusc, unceremoniously frozen to death (by humans) in 2006. Poor Ming,¹⁸ in turn, continues to be far outlived by Methuselah, a 4,852-year-old (as we write) bristlecone pine growing in the White Mountains of California. It first sprouted at about the time that cities emerged in Mesopotamia, and before the Pyramids were constructed. However, it too is a mere youngster compared to the tiny bacteria occupying the ancient ice of Antarctica that may have been alive—if in a state of near complete suspended animation, since the Pleistocene.¹⁹

The longevity of some individuals, at least, can approach geological timescales. But what about the longevity of individual species, like rabbits or dogs? Here we are in deep waters from the beginning, for a rule-of-thumb test for a modern species—organisms that can interbreed to produce fertile offspring—cannot be recognized for fossils. So, palaeontologists use ‘morphospecies’, defined by distinct and consistent shapes of fossilizable parts like bones or shells—a definition which usually requires a lot of careful observation and measuring in practice. With that proviso, what might the geological record of a Nile crocodile be, for example? Its fossil record extends back to the Pliocene Epoch about 3.5 million years ago,²⁰ and so one might regard it as a long-lived species. Other species had briefer sojourns on Earth: many species of the beautifully coiled ammonites that lived in the seas of Jurassic and Cretaceous times lasted for only a few hundreds of thousands (and some seemingly just a few tens of thousands) of years: their early demise in those times now makes them hugely useful to geologists, as precise time-markers for the strata that they are found in. At the opposite end of the spectrum of both size and time, the tiny single-celled foraminiferan Trilobatus sacculifer seems to have originated in the Miocene Epoch, some 20 million years ago.²¹ It has left a worldwide fossil record, because it lives in the ocean waters and its tiny calcareous skeletons sink to the seabed on death. The Nile crocodile and this foraminiferan continue to thrive, but one day they too will join the long-vanished ammonites. Species thrive because they have evolved to be well adapted to their environment. If this is large, like the global tropical oceans, they can thrive for a very long time. But if their habitat is small, like a tiny island, then their chances of becoming obsolete are very much greater. Other species thrive because they are generalists, like cockroaches, and so can adapt to many different landscapes, including those strongly modified by humans.

Many palaeontologists have tried to come up with a figure for the average durations of different species, for groups like mammals or molluscs. Species longevity seems to vary considerably depending on the group, and the quality of its fossil record. But a figure of somewhere between 0.5 to 5 million years is thought to be the typical duration of most species. These calculations allow us to estimate the rates of extinction over time, called the ‘background extinction’. By such estimates it would be expected that nine vertebrate species would have gone extinct since 1900.²² But background extinction rate is not a constant and is influenced by factors such as climate change, super-volcanoes, and asteroid strikes. Sometimes these factors can have a colossal influence, and trigger mass extinctions of species.

The killing times

Mictecacihuatl has stacked up the bones of most of the species that have ever lived on Earth, and which are now extinct, and she has assembled these for us in the fossil record. Animals and plants eventually go extinct because other organisms become more competitive in their environments, or because the environment itself changes and they cannot adapt. Species with narrow environmental ranges, like many island species, are more susceptible to extinction, and conversely this is why cockroaches, rats, and jellyfish are good survivors. The rate of extinction of plants and animals is worked out from the average timespans of species, based on observational data, and from a reading of the fossil record. From such information it is possible to make an estimate of background extinction rate, and this suggests we would expect to lose one mammal species every 200 years, and one bird species every 400 years. During periods of mass extinction this changes dramatically, and millions of species may be lost in a short time—some in the space of a single day, as when the Cretaceous world came to an end.

There have been five such episodes of mass extinction in the past 500 million years, and if we reach back a little further to encompass the loss of the strange Ediacaran organisms about 539 million years ago, one might add a sixth. Each time the Earth has lost something of the order of 75 per cent or more of all its species within a geologically brief interval (‘geologically brief’ meaning anything from several million stressful years to perhaps just a few months at the end of the Cretaceous). These extinctions, and many lesser but still important ones, punctuate the fossil record. Each major extinction pulse has played out over different frames of time and space, and different kill mechanisms, too—albeit with some common factors.

The more ancient major mass extinction events played out almost exclusively in the seas, for the simple reason that there was little then in the way of life on land. The ‘pre-Big Five’ event at the end of the Precambrian that saw the loss of many Ediacaran organisms may have been the result of evolution. For the first time on Earth, an array of mobile, muscular, skeleton-bearing predators emerged for which the Ediacaran organisms were easy prey. These new mobile animals also burrowed through the microbial mats that many Ediacarans relied on. The Ediacarans may have been the first victims of the arms race between hunter and hunted that goes on to this day.

That first of the ‘Big Five’ mass extinction events within the Phanerozoic took place at the very end of the Ordovician Period, some 445 million years ago. It was a puzzling and unique event in a number of ways—but there are also ways in which it eerily resembled our own contemporary crisis. It seems to have been an episode of mass death propelled not by mighty volcanic outbursts or catastrophic meteorite impact but rather by an extreme oscillation of climate. Within the space of a million years, an enormous ice sheet grew over much of South America and southern Africa, then conjoined and positioned over the South Pole. It abstracted so much water from the oceans that much of the continental shelves went from being shallow seas to being dry land. The ice sheet was short-lived, though, and collapsed as a pulse of global warming led to flooding of oxygen-poor water back across the shelf areas. Long interpreted as a kind of double whammy for the biosphere, it has now been suggested that the initial climate shock did most of the damage, with the abrupt reflooding simply complicating the recovery process.²³

This would make sense. The shallow sunlit waters of the continental shelves were the main cradle for the abundant and diverse life that was developing in the late Ordovician, as the land was still largely barren except for some mosses growing in moist places, while the ocean depths were often starved of oxygen. The sharp reduction in the extent of continental shelves as the ice sheet grew simply left less room for life. In the firing line were most of the coral species then living, and their disappearance led to one of the first ‘reef gaps’ in Earth’s history.

There is a sense also of a loss of the most flamboyant and sophisticated forms of life. This was the heyday of the charismatic, carapace-bearing trilobites. Not all of these succumbed, but among those that did were many of the more elaborately constructed forms. Late in Ordovician times, for instance, there thrived many species which developed an elaborate fringe, marked by complex arrangements of holes, around their heads. These abundant and mysterious animals (palaeontologists still scratch their heads over the purpose of the bizarre fringe) all became extinct as the ice grew. There were also trilobites which had left the sea floor to join the plankton, with light, thin armour and huge eyes that extended all around their heads, so they could see things below them as well as to the sides and above: the ideal animals to survive an extinction that reduced sea floor space, one might have thought. But no, these disappeared too (and never re-evolved), in company with other specialized forms of zooplankton. There was clearly an array of kill mechanisms at work, reverberating through the whole ocean system as continental shelf space was squeezed. The survivors tended to be small and simple forms, the kinds of animals that one can loosely interpret as generalists. Through the bad times, these hung on. Some even thrived, and have been termed ‘disaster species’ for the effectiveness with which they coped with conditions that killed most other species. Even in the worst of times, there have been winners as well as losers.

Later in Earth history, once life spread in force to the land, mass extinctions had a wider theatre in which to operate, and other sets of factors came into play. Since that time, curiously, there has not been another major mass extinction triggered by a glaciation. This might have been because the End-Ordovician glacial pulse was particularly fierce. But it may reflect change to the Earth’s biological fabric too. Life on land is not quite so sensitive to sea level change as regards living space (it can migrate downhill as sea level falls, for instance), and it is significantly less sensitive to temperature change, having to cope with daily and seasonal air temperature rises and falls greater than those in the sea. Subsequent onsets of glaciations have seen some groups of animals and plants suffer—but nothing on the scale of the end-Ordovician calamity.

The more recent ‘great dyings’, which affected both marine and terrestrial life, have had different triggers. By far the most notorious one is that at the end of the Cretaceous Period 66 million years ago, when the dinosaurs and much else perished. This was long among the greatest mysteries in geology, until the discovery by the father-and-son team of Luis and Walter Alvarez of a thin layer of iridium-rich dust at the exact stratal layer where the extinction event is clearly signalled by a drastic change in the kinds of marine microfossils present. The iridium layer was later found in rocks worldwide, and also contains frozen droplets of melted rock and intensely shocked mineral fragments. It most plausibly came from a giant meteorite impact, and a 200 kilometres-wide crater of that age was later found beneath the Gulf of Mexico.²⁴

A clear case of biosphere collapse by catastrophic impact, one might think. And yet, there was (and remains in some quarters) considerable resistance to the idea. In some places the pattern of extinction seemed gradual rather than sudden, which might suggest a gradual kill mechanism, such as a powerful series of volcanic eruptions—which did occur around that time, in what is now the Deccan region of India. And, there is strong evidence that such extraordinary volcanism can cause mass extinction events, as the gases released poison the air and sea, and change climate. That is now clear for the greatest mass extinction event known, at the end of the Permian Period 251 million years ago, when more than 90 per cent of species died out, and coral reefs were lost from the oceans for several million years, and for another mass extinction event at the end of the Triassic Period, 200 million years ago. So why not the end-Cretaceous event too?

It has taken a great deal of detective work to show that the end-Cretaceous meteorite likely was the main culprit. In regions where a more gradual extinction had been surmised (as with dinosaurs on land), this was often shown to be due to the imperfections of the fossil record; where fossils are rare in any case, as with dinosaurs, it is hard to demonstrate that they disappear simultaneously. The pattern of the Deccan volcanism, closely examined, did not fit the pattern of extinction. And, drilling into the crater itself has shown the extent of the mayhem wreaked at the impact site, and made the global repercussions more plausible.²⁵

For the past 66 million years there has been no mass extinction. Now, though, the rate of extinction loss is far higher than the background level—and far higher also, with far wider effects, than during the megafaunal extinctions likely caused by our Stone Age ancestors. More than 50 mammal species have been lost since 1900,²⁶ and these include iconic animals like the Caribbean Monk Seal (last sighted in 1952) and the Yangtze Dolphin (last seen in 2002). If this trend continues, we might expect to see 75 per cent of species lost within a few hundred to a few thousand years,²⁷ and we would then be in a sixth Phanerozoic mass extinction event. This calamity is not caused by volcanic eruptions, asteroid strikes, or rapid climate change, but by human activities, and these are now so extensive that the whole of the world is threatened, in effect behaving as a single if gigantic island, like a far larger version of Rodrigues, when humans first arrived in the seventeenth century.

We cannot board Noah’s Ark and escape this flood of environmental devastation. There are no other nearby islands to which we can flee. Our closest planetary neighbour Venus is a Hadean world with surface temperatures hot enough to melt lead, and an atmosphere so heavy that it would the crush the life out of any Earthly plant or animal. Mars too is an inhospitable place, with no oxygen in its atmosphere and almost no water at the surface to sustain life. As Carl Sagan said, the Earth is where we must make our stand.

The last of the velvet worms?

Despite the periodic setbacks of mass extinction, the Earth has been an ark for biodiversity for hundreds of millions of years, nurturing the major animal groups that are alive today, from sponges to sea urchins, to animals with backbones. And although the geological record shows evidence of five Phanerozoic mass extinction events, at no point has a major animal group—a phylum—been deleted from nature’s inventory of life. This has been pivotal to the success of the biosphere, allowing nature to restore and recolonize from the greatest range of types of body plan. So how might the current human-driven biodiversity loss presage a change that is worse than these earlier extinction events?

Velvet worms are small unobtrusive animals that, unlike their probable marine ancestors, are entirely land-living with a body a few centimetres long, bearing a few tens of pairs of legs.²⁸ They mostly live within moist forest environments, though some live underground, and they capture their prey by squirting a sticky fluid upon them and hunt everything from spiders to cockroaches. Fewer than 200 species of velvet worm have been documented, but they have wide geographical distribution in the tropics and Southern hemisphere.²⁹ This small number of species is in stark contrast to the possibly more than 5 million insect species recognized, or even the 100,000 types of tiny unicellular diatoms. In a sense, velvet worms are island species too, each living in a small rainforest habitat. And because of this they are highly vulnerable to deforestation, and to extinction.³⁰

Velvet worms are one of nature’s special types of body. On the animal ‘tree of life’ they form a major branch, like that of chordates (the group that includes animals with backbones) or molluscs (with snails, clams, squid, and others), and like these two groups they have a deep geological history. Cambrian rocks from Canada to China bear fossils of a loosely defined animal group called the lobopods. These were mostly soft-bodied, though some bore small armour-platelets embedded in their skin, and in most fossil deposits it is only these tiny, disarticulated platelets that remain. Occasionally, the whole animal is preserved as a fossil, revealing a worm-like body carried above many pairs of stubby legs. Some of these lobopods bear a strong resemblance to living velvet worms.³¹

Velvet worms have been part of the tree of life and its environments over hundreds of millions of years, and they are the main branch from which other branches—insects, shrimps, and scorpions sprang. No one knows quite how velvet worms have survived past mass extinctions events, because being mostly soft-bodied, their fossil record is very poor. But they have been a success, and long ago made the transition from being a group of seafaring animals to one that lives on land. Humans are fast destroying the environments in which velvet worms live, as the rapid loss of rainforest in South America and elsewhere shows. Not all velvet worms are classed as threatened. For now, some live within protected areas, while others might have the capacity to adapt to human-changed ecologies, like banana plantations. But of all the main branches of life, this is the ³² one that now seems most vulnerable to being lost altogether.³³

The velvet worms are a message of caution from the biosphere. Their loss from life’s tapestry would, from an evolutionary sense, be more final than the extinction of the trilobites, dinosaurs, or ammonites, whose arthropod, vertebrate, and mollusc relatives live on. In the coming decades, keeping a close watch on the fortunes of the velvet worms will tell us a good deal about the planetary severity of the biodiversity crisis that is unfolding.

Restoring great beasts

How might we mitigate the impact of an unfolding biological disaster at the global level, one which might leave poor Mictecacihuatl with no more bones to tend? One good test of the survivability of land-based ecosystems is their ability to support the large and dynamic beasts with which we began this chapter.

One of these beasts is the white rhino, an animal so big that it has no natural predators and whose population size is controlled by the available vegetation where it lives. There are about 20,000 white rhinos in the wilds of Africa, but these are now found only in the south of that continent. The ‘northern’ species—which occupied savannah regions of central Africa—is essentially gone from the wild. Being up to 4 metres long, white rhinos are the fourth-largest terrestrial mammals, with only the three living species of elephants standing before them. Despite their size, they are not aggressive, and they have suffered terribly from the dual ignominy of being hunted by white colonialists for sport, and now by organized crime rings who kill them for the horns to use in Asian medicines, even though they have no medicinal value. In South Africa’s Kruger National Park, white rhinos were extirpated by white hunters more than a century ago. But they were reintroduced in the 1960s, and now number in the thousands, though poachers continue to threaten them. As they graze, a little like gigantic lawn mowers, they have a significant impact on the ecology of the savannah, increasing the diversity of short-grass habitats as a small echo of the megafaunal terraforming of past times.³⁴ In Kruger National Park the density of the rhinos is still too low for this to provide widespread living places for other animals, but elsewhere, ‘rhino-mowing’ provides areas for birds, mammals, and insects.

White rhinos are one example of how rewilding through the introduction of a large mammal can have a positive impact on the whole ecology. A similar pattern was seen with the reintroduction of grey wolves to Yellowstone National Park in the USA during the 1990s. They too had been absent from their natural landscape for some 60 years and were returned to the park to help check the growing population of elk. This produced what ecologists call a cascade effect, like the lawn-mowing rhino on the African plains. The wolves hunted the elk and reduced their numbers, and this curtailed the grazing pressure on willow, aspen, and cottonwood. Willow, which is particularly prevalent along the streams in the park, is very important for the livelihoods of beavers,³⁵ who use the willow to build their dams. And beaver dams provide small, cool pools for fish, while increased willow provides a habitat for birds.

This is all well and good for national parks, where the density of people is very low and where much of the original ecosystem may still be intact, but such parks represent a small proportion of the Earth’s landmass. What happens when wildlife is allowed to develop in more densely populated regions, where it may come into direct conflict with humans? One example of the complex problems that arise from rewilding in places where people also live is the Oostvardersplassen area to the east of Amsterdam. Originally under the sea, the area was drained in the 1950s and 1960s to be used as an area for industry. A rich wetland emerged, but by the 1980s, woodland was taking hold, and the wetlands with their birds were beginning to be threatened. Dutch ecologists planned a cascade effect, hoping that by allowing red deer, horses, and cattle to roam freely and graze, the wetlands would be preserved. But this was an imperfect ecosystem that lacked large predators like wolves, and the numbers of grazers increased dramatically, devastating the plant and bird populations. And because the area is fenced in to prevent conflict with neighbouring farmers, the animals were not able to migrate to new pasture.³⁶ Trapped in a landscape where their numbers increased dramatically, as did their pressure on the plants in the reserve, all came to a head in the severe winter of 2018, when many animals starved to death. The proximity of this tragedy to a large urban population caused widespread anger at the suffering of the emaciated horses, deer, and cattle. Now the number of grazers in the reserve is carefully managed, but by people and not by natural predators like wolves. And the future development of these reserves may involve connecting several such areas through western Europe, so that the animals can migrate. Oostvardersplassen shows that reintroductions of wildlife into areas that are densely populated by humans take time to get right and require the tolerance and engagement of the local people when they go wrong.

A question of scale

This tolerance of, and support for, wildlife can extend to much smaller scales too, into the potential wild spaces of our farms and individual homes, where for decades we have worked to obliterate the ‘pests’ that eat our crops and garden plants. In any garden centre or hardware store there will be shelves full of insect-killing chemicals in colourful and appealing plastic bottles. These will carry a warning not to inhale or drink the contents, but no warning to say that the impact on insect biodiversity of your garden may be catastrophic. Something of the order of two-fifths of the world’s insect species may be threatened with extinction within a few decades; they are being widely exterminated across both urban and agricultural landscapes, and are decimated by pollution in aquatic settings.³⁷ There may be more than 5 million species of insects,³⁸ and their origins go back 400 million years to the Devonian Period. They serve many complex roles in ecosystems and, having co-evolved with flowers for well over 100 million years, they are important pollinators. Bees alone are one-third of all pollinators of flowers. But insects from dragonflies to beetles have many other functions too, such as controlling the numbers of insects like mosquitoes, while termites are essential for breaking down and recycling the many nutrients locked up in fallen trees. Some such relationships have evolved remarkable complexity, and certain termites are farmers, growing their own cultivated fungi to help decay plant matter.

Many insects also provide an important food supply to animals like bats, birds, and even fish. The North American mosquitofish has even been used in pest control. But where these fish have been introduced as a non-native species, they have had damaging impacts on wildlife, not least because they eat a range of other insects as well as mosquitoes. Because insects are deeply embedded in the functioning of Earth’s ecosystems, a major loss to their numbers and diversity would have incalculable effects; indeed, would likely cause wholesale collapse to ecosystems, including those that sustain us.

All these ecosystems are essentially pathways for energy—and matters of energy are also central in our disruption of them, as we explore next.