‘I know this bear. He knows me. We’ve met several times.” Our guide for the day points to a damaged sign in Sutjeska national park, at the beginning of the trail that descends to the forest of Perućica in south-east Bosnia. The wooden post is covered in scratches from large claws. “Bears are the sharks of the land, because they have the keenest sense of smell on the mountain. They are highly intelligent. I’m deeply persuaded that they know who is a friend and who is a foe. I come often to the forest, so this guy knows my smell. But there was one incident, a hunter who came here to kill, and a bear peeled off his face like an orange.”

With that image, Dejan Elez commands our full attention. A Bosnian Serb law graduate turned ranger and now mountain guide, he is a born storyteller and raconteur. My travel companion, Chris, and I are rapt as he describes the famous battle that was fought near here, when Yugoslav partisans broke through a German encirclement in 1943, taking the Wehrmacht by surprise under cover of a violent storm – “the wind was rising and the lightning was like a strobe” – but after that, Dejan’s narrative leads much further back in time, into the depths of one of Europe’s most ancient forests.

Scientists estimate Perućica – which spreads across the slopes of a canyon in Republika Srpska, the autonomous Serb-majority region of Bosnia and Herzegovina – has grown without human interference for 20,000 years. Along with Białowieża, which straddles Poland and Belarus, it is considered the last true remnant of the primeval wildwood that once covered the continent. But Perućica, says Dejan, is much better preserved. It has never been inhabited, and rough terrain and precipitous slopes have saved its trees from logging. Its 1,434 hectares (3,543 acres) are now under strict protection – no one can enter without a guide – and the site’s importance is recognised by Unesco.

From a viewpoint on a rocky ridge, dense greenery spreads below, clinging to the sheer canyon walls above a river. The river is fed by Skakavac, a 75-metre waterfall thundering into a mist of spray, and far above shines the white summit of a mountain. Originally, Chris and I had hoped to climb Maglić – at 2,386 metres, the highest peak in Bosnia – but it snowed a couple of weeks ago and we have been warned that conditions in early spring are perilous. We’ll save going up for another time. Today we are going down.

Dejan leads the way along the winding trail through groves of mixed beech, fir, spruce, pine and maple. He admits he does not know the names of all the trees, but what he does know, intimately, are the tracks of animals. The forest, he says, is “legible”, and he reads it like a book. Circular patches where the humus has been grubbed away are made by chamois foraging nutritious roots. Roe deer do the same, but their holes are precise and deep. “Look” – Dejan points to a wet log – “those scratch marks were made by a canine, either fox or wolf. But there, on the same log, something even more exciting.” The wider, deeper scratches were made by a passing brown bear. Nearby is a larger hole where a bear has raided a honeybee hive. Of course, sightings are never guaranteed, but they are always possible at this time of year, when bears emerge from hibernation to gorge on wild garlic – known in many Balkan languages as “bear’s garlic”. “If I stop, you stop! Now we are in stealth mode.”

Every few minutes of descent seems to bring us to a different realm as the temperate rainforest grows taller and more tangled. Deadwood lies everywhere – paradoxically a sign of health. Lichen, moss and fungus drip from the branches. The astonishing biodiversity protects Perućica from epidemics such as spruce bark beetle infestation, which has devastated old-growth forests elsewhere. With about 170 species of tree and shrub and more than 1,000 plants, it’s little wonder that our guide doesn’t know them all.

During the last ice age, Perućica escaped the freeze, acting as a refugium, an isolated region in which many species were preserved. From here, the trees expanded their range northwards when it thawed. The forest’s name, Dejan believes, is connected with Perun, the pre-Christian and pre-Islamic Slavic god of lightning. Certainly, it feels as if we are wandering in a temple.

He has packed sandwiches – cheese and ham in thick white buns – which we eat on a grassy ledge overlooking the waterfall. Chamois clearly dine here too, as their droppings are everywhere. We don’t see the herd – neither do we see the wildcats, lynx or wolves that also call the forest their home – but the knowledge that they are here, perhaps even very close to us, brings a sensation I can only describe as reverence. Each of us goes quiet with awe. Maglić and the other mountains disappear behind white cloud and it begins to rain. The wetness doesn’t matter.

With his legal background, Dejan is clear-eyed about the threats. The ancient forest may be safe, but hunters come to the wider national park and the rangers are sometimes bribed to look the other way. Most visitors to Sutjeska stay in the village of Tjentište, a scattering of guesthouses and modest restaurants along the main road, but increasingly there is encroachment at the borders of the park. Even our cosy timber-frame cabin near the entrance gate is, in his opinion, too close.

It’s not that people shouldn’t come here, he tells us as we walk back up. What matters is how they come, as respectful guests. We are not the owners of the forest, not even the owners of this trail, which is walked by and shared with many other feet. He stoops to brush aside some leaves and identifies flecks of bone that have passed through a wolf’s digestive tract, and further on, wolf excrement filled with chamois hair. “This was left on the trail deliberately to tell us it’s their territory. They don’t do anything by accident. Everything has a meaning.”

After almost five hours, we emerge on the gravel road that leads towards Tjentište. The absence of so much tangled life is vaguely shocking. Both of us feel changed by our glimpse into Europe’s wild past – and perhaps we have been subtly changed on a biological level, too. Dejan announces when we part: “You two gentlemen have been exposed to a universe of micro-organisms you will never find in England. This forest gets inside you.” Perućica clings to us as we leave the national park and return to Sarajevo, a two and a half hour drive, going north as the trees once did when the ice age ended.

Guided tours of Perućica with Outdoor Bosnia or Wild Balkan Trails from £50pp. Mountain View, Tjentište, sleeps two, from £44 a night. Alternatively, Apartmani Šarović, also in Tjentište, sleeps two, from £47 a night