One of the few surviving accounts by the Britons of what the Anglo-Saxons did to them is Y Gododdin. It tells the story of what may have been the last stand in England of the Gododdin – the tribes of the Hen Ogledd, or Old North – in 598AD. A force of 300 warriors – the British version of the defenders of Thermopylae – took on a far greater army of Angles at a town named in Brittonic as Catraeth, which was probably Catterick in North Yorkshire. Like the Spartan 300, they fought for three days, during which all but four were killed.

The Anglo-Saxon conquest appears to have crushed the preceding cultures much more decisively than the later suppression of the Anglo-Saxons by the Normans. One indication is the remarkable paucity of Brittonic words in the English language. Even if you accept the most generous derivations, there appear to be no more than a couple of dozen, of which only four are used in daily conversation: dad, gob, beak and basket. It was an obliteration almost as complete as that of the Native American cultures in the United States.

Y Gododdin was written by one of the four survivors of the battle, the poet Aneirin. He tells how the last warriors of the Gododdin gathered in Din Eidyn, the town we now call Edinburgh. (Several Scottish cities, including Glasgow, Aberdeen and Dundee, have Welsh – or, more precisely, Cumbric – names.) They feasted there for a year before marching south, towards certain death in Catraeth. In the middle of Aneirin’s gory saga is something incongruous: a sad and beautiful lullaby called “Pais Dinogad” (Dinogad’s Smock), in which a mother tells her son of his dead father’s mastery of hunting. It names the animals he killed. Most were easy for scholars to identify: pine marten, roe deer, boar, grouse, fox. But for many years one animal was a mystery: llewyn, which looks like a cognate of the modern Welsh word for lion, llew. But what did it mean? Nothing seemed to fit, until 2006, when an animal bone was found in the Kinsey Cave on Giggleswick Scar, 30 miles as the raven flies from Catterick or Catraeth.

Until this discovery, the lynx – a large spotted cat with tasselled ears – was presumed to have died out in Britain at least 6,000 years ago, before the first sod was turned by the first farmer. But this find (and three others in Yorkshire and Scotland) drags its extinction date forward by roughly 5,000 years. It was likely to have been familiar to Aneirin and his people.

However, this is not quite the last glimpse of the animal in British culture. A 9th-century stone cross from the Isle of Eigg shows, alongside the deer, boar and aurochs pursued by a mounted hunter, a speckled cat with tasselled ears. Were it not for the animal’s backside having succumbed to the delapidations of time, we could have made a certain judgement, as the lynx’s stubby tail is unmistakable. But even without the caudal clincher, it’s hard to see what else the creature could have been. The lynx might have clung on in forest remnants – perhaps in the Grampians – for another few hundred years. It was survived by the wolf, whose last certain record, according to the historian of the countryside Oliver Rackham, was the beast killed in Sutherland in 1621. The lynx is now becoming the totemic animal of a movement that is transforming British environmentalism: rewilding.

Rewilding means the mass restoration of damaged ecosystems. It involves letting trees return to places that have been denuded, allowing parts of the seabed to recover from trawling and dredging, permitting rivers to flow freely again. Above all, it means bringing back missing species.

One of the most arresting findings of modern ecology is that ecosystems without large predators behave in completely different ways from those that retain them. Some of them drive dynamic processes – trophic cascades – that resonate through the whole food chain, creating niches for hundreds of species that might otherwise struggle to survive. The killers turn out to be bringers of life.

Such findings present a big challenge to British conservation, which has often selected arbitrary assemblages of plants and animals and sought, at great effort and expense, to prevent them from changing. It has tried to preserve the living world as if it were a jar of pickles, letting nothing in and nothing out, keeping nature in a state of arrested development. But ecosystems are not merely collections of species; they are also the dynamic and ever-shifting relationships between them. And this dynamism often depends on large predators.

It is not only for scientific reasons that many of us now wish to bring back missing species; it is also an attempt to rekindle some of the wonder and enchantment that, in this buttoned-down land, often seems to be missing. Where farming is retreating from barren land, and where people are beginning to question why vast tracts of the uplands should be denuded by deer-stalking and grouse-shooting industries that serve only a tiny elite, there are new opportunities for change.

At sea the potential is even greater: by protecting large areas from commercial fishing, we could once more see what Oliver Goldsmith described in 1776: vast shoals of fish being harried by fin and sperm whales, within sight of the English shore. (This policy would also greatly boost catches in the surrounding seas; the fishing industry’s insistence on scouring every inch of seabed, leaving no breeding reserves, could not be more damaging to its own interests.)

Rewilding is a rare example of positive environmentalism, in which campaigners articulate what they are for rather than only what they are against. One of the reasons why the enthusiasm for rewilding is spreading so quickly in Britain is that it helps to create a more inspiring vision than the usual green promise: “Follow us and the world will be slightly less crap than it would otherwise have been.”

The lynx presents no threat to human beings: there is no known instance of one preying on people. It is a specialist predator of roe deer, a species that has exploded in Britain in recent decades, holding back, by intensive browsing, attempts to re-establish forests. It will also winkle out sika deer: an exotic species that is almost impossible for human beings to control, as it hides in impenetrable plantations of young trees. The attempt to reintroduce this predator marries well with the aim of bringing trees back to parts of our bare and barren uplands.

The lynx requires deep cover, and as such presents little risk to sheep and other livestock, which are supposed, as a condition of farm subsidies, to be kept out of the woods. But the real reason for choosing this species first is that the lynx is magnificent. To know that Dinogad’s father’s quarry, the llewyn in Aneirin’s saga, inhabits the woods through which you walk feels like the shadow that fleets between systole and diastole.

David Hetherington, Britain’s leading expert on the lynx, estimates that at present the Scottish Highlands could support approximately 400, which is likely to be a genetically viable population.

On a recent trip to the Cairngorms, I heard several conservationists suggest that the lynx could be reintroduced there within 20 years. If trees return to the bare hills elsewhere in Britain, the big cats could soon follow. There is nothing extraordinary about these proposals, seen from the perspective of anywhere else in Europe. The lynx has now been reintroduced to the Jura Mountains, the Alps, the Vosges in eastern France and the Harz mountains in Germany, and has re-established itself in many more places. The European population has tripled since 1970 to roughly 10,000.

As with wolves, bears, beavers, boar, bison, moose and many other species, the lynx has been able to spread as farming has left the hills and people discover that it is more lucrative to protect charismatic wildlife than to kill it, as tourists will pay for the chance to see it. Large-scale rewilding is happening almost everywhere – except Britain.

Here, attitudes are just beginning to change. Conservationists are starting to accept that the old preservation-jar model is failing, even on its own terms. Already, projects such as Trees for Life in the Highlands or the transformation of the Knepp estate in West Sussex provide a hint of what might be coming. I am helping to set up an organisation that will seek to catalyse the rewilding of land and sea across Britain. Our aim is to reintroduce that rarest of species to British ecosystems: hope.

George Monbiot’s book “Feral: Rewilding the Land, Sea and Human Life” is newly published in paperback by Penguin (£8.99)