When the foundations of Trafalgar Square were dug in the 1830s, builders exposed river gravels crammed with the bones of hippopotami, straight-tusked elephants, giant deer, giant aurochs and lions. Also compacted into this archaeological nougat were the fossilised faeces of spotted hyenas. Other sites in London have yielded the remains of woolly mammoth (the Strand), reindeer (South Kensington tube station), woolly rhinoceros (Battersea power station) and giant ox (Knightsbridge). Some of these beasts, survivors of the Ice Age, are thought to have roamed the land beneath our feet until as recently as 11,000 years ago, when climate change, disease and humans finally did for them.

You don't need to dig to uncover traces of prehistoric fauna and flora. If you know what to look for, the ecological biography of nearly half a million years ago can still be read in today's landscape. Many of our trees and shrubs, including oak, ash, beech, lime, sycamore, alder and willow, can re-grow from the point at which the stem is broken. This resprouting, or coppicing, could be an evolutionary response to attacks by elephants, who habitually snap or uproot trees. The ability of some trees to survive the removal of much of their bark is also suggestive of elephant-proofing. In another adaptation, the robin may have turned to land disturbed by humans, following the demise of the wild boar, a notoriously messy eater.

An optimist might find something to celebrate here: the remarkable capacity of nature to adjust to change and its enviable escape from what Nietzsche termed "the malady of history". The tree does not sigh for the elephant, the robin does not fret for the boar; they are not made melancholy or sleepless by the pastness of the past. Animals may have memory, they may even have consciousness, but unlike humans they don't strain to make urgent meaning out of life. Humanity is an existential prejudice of which they, like the trees, are free.

In Feral, we find George Monbiot in gloomy mood, mourning the loss of the improbable bestiary that lies under Nelson's Column, and with it a world that was once rugged and wild and big. Hemmed in by JG Ballard's "drowsy villas, sheltered by benevolent shopping malls", life feels shrunken, "cautious, constrained, meticulous". We are ruled by "prohibitive decencies", our passions are sublimated, we have had to learn "to press our roaring blood into quieter channels". The closest we get to nature is feeding ducks in the park, and "the greatest trial of strength and ingenuity we face is opening a badly designed packet of nuts". In short, civilisation has squeezed the wildness out of our environment and out of us. Unable to flex our Paleolithic muscles, we have withered, aetiolated, gone to seed.

Monbiot's personal solution to this problem of physical and spiritual uselessness is to move out of the city to a cottage in rural Wales. There, he hopes to balance his social duties – as father, taxpayer, journalist and campaigner – with "a richer, rawer life" of the kind that has eluded him since his youthful experience in the rainforests of southern Brazil reporting on an illegal community of goldminers.

The miners' activities threatened to drive the local tribe to extinction; Monbiot hated what he saw (Joseph Conrad was in that jungle with him), and yet he was drawn to their feral, lawless behaviour, where "conflicts were resolved not through legal instruments or on the sofas of television studios, but by shoot-outs in the forest". He returns to the memory of this and other adventures in the tropics – encompassing cerebral malaria and all manner of foulness – and finds that it compares favourably with his "small and shuffling life back home, where … there is no remaining moral space for the exercise of physical courage". To Wales, then. Let the Hunger Games commence.

Initially, Monbiot's craving for the wilder life and its "high note of exaltation" is at least partially satisfied. There are many briny accounts of his exploits in a kayak off the Welsh coast, where we find him dragged "almost beyond will" into a furious sea ("white horses reared up from nowhere and came down upon my shoulders with a clatter of hooves"), only finding safe passage back to shore thanks to the timely appearance of a dolphin. When he free-dives for the spider crab he reaches "two and a half fathoms down" with no flippers, and wrestles his "monstrous" catch to the surface; he swims towards the horizon (no, he is not swimming, but "porpoising through the water"), he spots another crab, hangs above it, "feeling like a hawk about to swoop on its prey".

On land there are more feats to admire. Where lesser mortals drag themselves over fences, wondering how to get that troublesome second leg to follow the first, Monbiot simply "vaults" or "jumps" the obstacle. He traverses a ravine, clinging by his fingertips, a river roaring beneath: if he loses his footing he could slide down the gorge to his death; suddenly a salmon leaps, "as if I had summoned it". Stalking up a tidal channel with a spear over his shoulder, he is "as flexed and focused as a heron". After lifting a dead muntjac on to his shoulders, his lungs fill with air: "This, my body told me, was why I was here. This was what I was for. Civilisation slid off as easily as a bathrobe." As the genetic memory of a simpler, wilder past courses through his veins, he is "transported" into the full, unmediated moment. He is "freed from knowing", he has escaped history.

The feral bliss Monbiot describes is not a destination. It is an abandonment, a forgetfulness, and therefore most elusive when we search for it. It is not in the gift of civilisation (rioters or devotees of the January sales may beg to differ), but of nature – raw, untamed, unpredictable. In Monbiot's dream, everybody would have the right to access the wilderness and sing the body electric and, yes, possibly be killed by a wolverine or a bear, though the chances of this, he hastens to add, are less than death by collapsing deckchair. He's not talking rambling but huge swathes of really wild places where "absent" plants and animals have been reintroduced – hence "rewilding" – and left to find their own way; places that are "self-willed, governed not by human management but by their own processes". Nature, not man, will decide what's what.

There's nothing ignoble about Monbiot's vision of reinstating ecosystems in which man's power to dominate is consciously withheld. It is a vision fed by his growing disenchantment with the landscape that surrounds him in Wales, by the "hessian emptiness" of the Cambrian Desert that lies at his doorstep, an area of 460 square miles that he learns to loathe as a vast tract of manmade ecological declension. Where once this land was part of a great Atlantic rainforest (he stumbles across a tiny remnant, "a pocket of canopied jungle", that serves as evidence), it is now open, treeless, a bare waste of "sheep-scraped misery". It is not a "natural" landscape, but the "aftermath of an ecological disaster". The sheep, Monbiot claims, have caused "more extensive environmental damage in this country than all the building that has ever taken place here". It's not even an indigene, but a ruminant from Mesopotamia, yet we "protect" the land it has damaged and impoverished by continuing to graze it so that other "invasive" (ie native) species do not take over.

The Cambrian Desert is often extolled for its bleakness, but at a time when austerity, for many people, is more real than aesthetic, its rewilding along the lines Monbiot advocates becomes an attractive proposal, a hopeful metaphor for something over nothing. He also makes a compelling case for other sizable protected areas in the UK, all "sheep-wrecked", that might similarly be released from the "conservation prison". But he is on less firm ground when he campaigns for the rewilding of upland areas that have been husbanded by generations of hill farmers. Among the last frontier people in Europe, working steep, wind-chafed contours that are largely inaccessible to heavy machinery, these Welsh and Scottish farmers do not regard unworked land as their ideal. Their history, from which they seek no escape, is written into the ground and they are not inclined to surrender it to a fantasy of ecological throwback.

Monbiot insists that rewilding should not take place without consensus. There would be no forced evictions, no clearances – the right of kings to enclose land has passed. But hill farming, he maintains, is financially and environmentally unsustainable. Rewilding upland areas and reintroducing missing species (boar, wolves, bison, lynx, elk, even elephants) would revive the land and the people who are part of it: as "wildlife-watching" becomes a more important industry, hill farmers need only re-skill to find themselves "in high demand". They could become wardens and guides, run B&Bs, farm shops (what, with no farms?), clay-pigeon shoots, bicycle hire, horse riding, lake fishing, falconry, archery.

In his enthusiasm for this scheme, Monbiot seems oblivious to its whiff of social engineering (perhaps the hill farmers could open a few casinos and sell cheap cigarettes?). It is not made clear, but presumably these recreational activities and the infrastructures needed to support egalitarian mass tourism would be placed outside of the actual wilderness area. How would the presence of humans be regulated once they are inside it? Would there be a permit system, as in American national parks, and/or zones for day-trippers? (In Yosemite, the valley floor, which is reserved for short-term visits, is as crowded as any mall.) On these and other questions about how to calibrate human demands with the ideology of the wilderness, Monbiot is silent.

Perhaps the wilderness is itself a human concept, a sentimental idea about the ultimate "authentic" landscape where nature's ethical influence is experienced as the revival of the self. Could we ever imagine a wilderness from which we are excluded, which owes us nothing? Probably not. To take humans out of nature we'd first have to take the nature out of humans.

• Frances Stonor Saunders's The Woman Who Shot Mussolini is out from Faber.