I was carrying my five-year-old son on my shoulders, watching Komodo dragons doze in the heat of a tropical afternoon. Covered in knobbly scales and dust and with saliva dripping from their jaws, they are formidable, crocodile-like reptiles, up to 3.5 metres long and weighing as much as ninety kilograms. I walked among them as they lay clustered in groups of three or four, seemingly imperturbable in the scattered shade offered by the scanty trees on Indonesia’s Rinca island. With poor hearing and even worse sight, they seemed like the most hapless of predators. Then I tripped on a cobble, and my son began to cry. Instantly, every dragon sprang to life, their great forked tongues flickering, questing. They turned as one and strode rapidly towards us. But when I quieted my son, the giant lizards fell back into a torpor – as though I had turned off a switch.

Before humans arrived some 45,000 years ago, Australia was home to a diverse megafauna, creatures that weighed more than forty-five kilograms, including the megalania, a gigantic relative of the Komodo dragon that grew up to five metres long and weighed half a tonne, as well as the Komodo dragon itself. The megalania was Australia’s top land predator and almost certainly fed upon the largest marsupials, while the Komodo took smaller megafaunal species.

Long extinct on the Australian mainland, and today surviving only on the island of Flores and its satellites in Nusa Tenggara, Indonesia, Komodo dragons are the last surviving Australian land-based predatory megafauna. Forty-five thousand years ago they roamed northern Australia and islands to the north, all the way to Flores. Today’s Komodos kill goats, deer and water buffalo, all of which have been recently introduced to Flores and other nearby islands. As an Australian palaeontologist obsessed with understanding what my continent was like before extinction robbed it of its megafauna, Komodo dragons have an enduring fascination.

A continent that has lost its megafauna is a profoundly impoverished place. Much of the challenge and excitement of life is gone, as well as the environmental dynamism stimulated by browsing, grazing and predation on a gigantic scale. Just imagine the Australian inland with herds of rhino-sized diprotodon, as well as other gigantic marsupials, being preyed on by marsupial lions and Komodo dragons. Large predators and herbivores are landscape architects, capable of turning forest into woodland or even grassland through their various interactions with other flora and fauna. But in addition to these large-scale changes, they can create intricately varied habitats for smaller species through vegetation disturbance, seed dispersal and the moving of nutrients across landscapes.

The introduction of alien megafauna by the Europeans has been one of many ways in which ecological systems have been degraded, along with colonial farming practices and other extractive industries like logging and mining. And in recent decades, the climate has been changing. As Australians struggle to limit the damage from these changes, it’s vital that we understand what Australia was like before its original megafauna was lost. We may even find that restoring its few banished but surviving megafaunal species, like the Komodo dragon, might help limit degradation of its ecosystems.

My fascination with Australia’s prehistoric past is deep. While still a teenager, I accompanied a scientific expedition into what is referred to as the ‘dead heart’ of Australia – the Lake Eyre basin. We were looking for fossils, and for days we traversed a landscape of muted pastels (so different from its name, the ‘Red Centre’) – sand dunes, parched bushes and salt lakes. The geological memory of a continent is stored in layers of rock. Most continents have very good geological memories, the evidence of the past packed in abundant layers, but Australia is the exception – a forgetful land whose tectonic lethargy has resulted in a great geological amnesia. Yet occasionally, like an elderly relative whose memories of childhood can be startlingly vivid, Australia’s scant geological recall can spring into focus, bringing a long-lost world to life.

In this land without streams, beside a salt lake we found something unusual: rounded pebbles. I set to work with a mattock. After digging down fifty centimetres into the acrid, black mud of the lake’s margin, I encountered a layer of grey clay. I pulled a piece from the mattock blade and sniffed it. The sweet smell of humus – the scent of rainforest – filled my nostrils. Plucking a mummified leaf from the clay, I bent it between my fingers and watched as it hardened and crumbled in the desert air. Ten million years ago, this leaf had fallen to the floor of a vanished rainforest. It was an unforgettable moment. Ever since, I’ve had a thirst for a sensory understanding of Australia as it was during the Ice Age (2.5 million to 45,000 years ago), after it had dried out during the Pliocene, and its interior was home to a host of lumbering marsupials, reptiles and flightless birds.

A few years after my expedition to the Lake Eyre basin, I excavated a deposit of megafaunal bones in western Victoria that was more than 50,000 years old. I remember my trowel slicing into thick black mud and revealing the surface of a massive bone the colour of mahogany. I realised I was the first to see anything of this creature since its demise. Once it was at the lab I discovered it was the thigh bone of a gigantic grey kangaroo that must have weighed 200 kilograms – twice the weight of any grey kangaroo that exists today. Nearby, I found a collarbone from the same species, bearing the tooth marks of a marsupial lion, and felt the sudden, electric thrill of connecting with the drama of life tens of thousands of years ago: my hand was where the mouth of a mysterious predator had been, tearing into its meal.

The marks on the bone took the form of long gouges. Strangely for predators, marsupial lions lacked canines. Instead they had blade-like premolars suitable for slicing flesh from bones. Just how they killed remained a mystery until claw marks found on the walls of caves on the Nullarbor Plain – a vast limestone plateau where the vegetation today grows no higher than the human knee – revealed that they had great, sheathed ‘thumb claws’ on their forelimbs. The stash of bones I uncovered proved to be the remains of a marsupial lion feast, 11 per cent of which bore the lion’s distinctive premolar marks – the highest incidence of remains with these marks yet reported. The discovery showed me that it was possible to put flesh on the bones of Australia’s deep past.

Around sixty species of large to gigantic marsupials, reptiles and birds are classified as Australia’s megafauna. The largest, at around three tonnes in weight, was the wombat-shaped diprotodon. Australia’s equivalent of the much larger African elephant, it roamed the arid inland in herds, consuming the twiggy browse of desert bushes and trees. The diprotodon’s huge head was mostly filled with air (the skull being riddled with great sinuses) and its brain was barely the size of a human fist. It must have been dull-witted and, given its infertile environment, slow to reproduce.

Unique among the continents, Australia had very few species of grazing megafauna. Most of its marsupial giants were browsers, feeding off trees or the kinds of desiccated bushes where I had camped in the Lake Eyre basin as a teenager. Among the most abundant were several dozen kinds of short-faced kangaroos, the largest of which weighed a quarter of a tonne. Most of these creatures were the ecological equivalents of deer and antelope on other continents. But some were very odd indeed, with skulls and teeth superficially similar to those of australopithecines (extinct relatives of humans).
I imagine a furry, three-metre-tall creature with a face like an ape-man, mounted on a kangaroo’s body, with powerful hopping legs and long arms tipped with two elongated, clawed digits used to pull branches within reach of their lips.

Other megafauna were equally odd-looking, including a short-trunked marsupial sloth that may have fed on bark, a hippo-like relative of the wombat that lurked in rivers and lakes, and a carnivorous kangaroo the size of a cougar. Among the most perplexing of the extinct marsupials are the tree-climbing kangaroos of the interior. Today, seventeen species of tree kangaroo survive in the tropical rainforests of New Guinea and Australia’s north-east. They are among the most spectacular of the surviving marsupials, one species resembling a miniature giant panda, while another has blue eyes and bright yellow stripes running down its back. My PhD involved research on kangaroo ankle bones, and I had the privilege of discovering and naming the very first extinct tree kangaroo to become known to science. Bohra paulae’s leg bones, which were the size of a female orangutan’s, had been found in a cave in New South Wales, but had lain unrecognised in a museum drawer for 150 years. I still recall the disbelief of many of my colleagues at the idea that a gigantic tree-climbing kangaroo once inhabited Australia’s dry inland.

More recently, the remains of two smaller species of tree kangaroo have been found in caves on the Nullarbor Plain. Fossilised snails found with these bones suggest that the Nullarbor Plain was as dry as it is today when its tree-climbing kangaroos were alive. Despite its aridity, it seems that the Nullarbor must once have been covered in arid-adapted trees which are now locally, if not entirely, extinct. The find reveals how very little we know about the nature of Australia when its megafauna thrived. Indeed, astonishing new species of extinct Australian megafauna are still coming to light.

Around the time Australia’s marsupial giants became extinct 45,000 years ago, the continent also lost a number of smaller species, including the giant megapode (a relative of the chicken that lays its eggs in mounds of rotting vegetation and leaves them to hatch without brooding) and a small, primitive wombat that might have resembled a porcupine. Other species that also became extinct on the Australian mainland left relatives that survived on offshore islands, including a half-metre-tall, probably flightless coucal (Centropus maximus – a member of the cuckoo family), whose nearest living relative is the splendid sixty-four-centimetre-long violaceous coucal of the Bismarck Archipelago, north of New Guinea.

New Guinea itself provided refuge for another megafaunal giant: the long-beaked echidnas of the genus Zaglossus are the world’s largest egg-laying mammals, reaching a metre in length and weighing up to seventeen kilograms. Australia’s surviving echidna is less than a quarter the weight. The bones of Zaglossus-like creatures have been found in sediments in eastern Australia, but the species vanished there along with the rest of the megafauna 45,000 years ago. Perhaps long-beaked echidnas survived in New Guinea because its dense and extensive rainforests provided a refuge from human hunting.

Meeting a long-beaked echidna face to face is unforgettable. I first encountered them in 1981, high on Mount Albert Edward, on my first expedition to New Guinea when I was still a doctoral student. The chilly alpine grasslands were pitted with holes from where they had inserted their long beaks into the earth to feed on worms, but the creatures remained hidden, being exceedingly difficult to track in the impenetrable scrub that surrounds the grasslands. When I finally saw one I was astonished by its long black fur, which hid the spines. Their beak-like faces are devoid of expression, yet they are highly intelligent, having brains that are the largest and most complex of any Australian land-based vertebrate. After caring for one for weeks, I deposited it in a wildlife sanctuary. When I visited years later it inserted its long snout into my boot and tickled my toes with its tongue. I got the distinct feeling that the creature had fond memories of me.

But the most astonishing survivor is surely the Komodo dragon. Why should this gigantic predator have survived only on small islands at the very north-western limit of its distribution? Flores and its satellites are unusual in that they are also home to a giant rat weighing around two kilograms which, like the Komodo dragon, is something of a living fossil.