There was only one word for it: wow. The 30ft-high rowan tree, the magic tree of Celtic culture, lay on its side at the edge of the Scottish loch, a fat pile of woodchips clustered around the base of the trunk which had been so efficiently cut away. Rowan wood is dense, tough wood – it is used for making walking sticks – but it had been sliced through as if it were cheese.

"They did this one at night," said Jenny Holden, the Scottish Beaver Trial field officer. "We were sitting in the canoe and could hear the male beaver chewing away at something. At the end of the night he swam across the loch dragging a 7ft branch with him, and we came down and found what he'd been doing."

She put her hand on the fallen tree's lichen-encrusted bark with a clear sense of wonderment still, a couple of days after she'd found it. Looking on it freshly, I was astounded. There could be no clearer evidence that a remarkable animal, one with a very high impact in its surroundings, was back in Britain after an absence of hundreds of years.

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Two weeks ago 10 beavers, brought from Norway, were released into the wild in Knapdale Forest, in mid-Argyll. They represented the triumph of an idea first put forward in 1995, that a charismatic mammal, long extinct in the British Isles, might be reintroduced to Scotland in the same way that a captivating bird, the sea eagle, had been successfully brought back.

Once widespread in Britain and most European countries, the European beaver, Castor fiber (a close relation of the North American beaver Castor canadensis) is the continent's largest rodent, much bigger than an otter, weighing as much as 65lb. It was hunted to extinction over most of its range for its fur, its flesh, and also for its scent gland or castoreum, which was used in perfume-making and in early medicine.

Traditionally it has been thought that British beavers died out in the Middle Ages or shortly afterwards but recent research by the archaeologist Bryony Coles has suggested they may have clung on unobserved for much longer, perhaps even as late as 1800.

Professor Coles notes a record of a bounty of two pence paid for "a beaver's head" by the churchwarden at Bolton Percy, a village on the lower River Wharfe in south Yorkshire, at the incredibly late date of 1789 – 600 years after the animal was supposed to have vanished from England. She does not think it can be a mistaken case of an otter, for the same records show a quite separate otter bounty being paid, of one shilling.

Instead, Professor Coles believes the animals may have survived for centuries unobserved on the Humberhead Levels, the great wetland into which the Wharfe flows at the head of the Humber estuary. Going by oral traditions, she similarly feels that beavers in Scotland may have survived much later than once thought, perhaps as late as 1700.

Supporters of the reintroduction feel very much that they are restoring a lost native species, but their enthusiasm has been matched by misgivings in other quarters. For more than virtually any other animal except man, beavers modify their environment. With their hatchet-like incisor teeth they cut down trees for food and for building materials, lots of them, trees bigger even than our rowan; they make wooden lodges to live in, and on rivers less than 30ft wide, they build dams, which create ponds deep enough to dive in to escape predators, and often flood surrounding land.

Opposition to the release has come from farmers, landowners and above all, fishermen, who fear that beaver dams may stop migratory fish such as Atlantic salmon and sea trout returning to their spawning streams. Whether or not these fears are justified, it was clear, looking on the fallen rowan, that the impact on the landscape is going to be an unmistakable one.

The Independent was given the first sighting outside the trial project team of what the beavers had been up to, and in the brief two weeks since their liberation, the three in this particular loch, Loch Linne, had cut down numerous smaller trees such as birches and willows, effortlessly stripping off the bark and the twigs and smaller branches for food.

They will be in Knapdale for five years, while the trial, jointly run by the Royal Zoological Society of Scotland and the Scottish Wildlife Trust, and hosted by the Forestry Commission, carefully evaluates the impact the beavers make on the woodland, and the water quality, and the general ecology of the area. Members of the team firmly believe it will be acceptable. "How much damage do they do? It depends whether you see it as damage." Jenny Holden says.

"Beavers are habitat engineers. They manage wild habitats, doing a lot of the jobs we as habitat managers ourselves do, jobs like coppicing, putting in sluice gates, thinning woodland, rewetting meadows by damming. "The damage they can do pales into insignificance compared to the good they do for the ecosystem, and also the tourist potential for people coming in and watching the wildlife."

Nevertheless, the team stresses that this is an enclosed trial in an isolated area, from which the animals will be prevented from moving, not least into major river systems – they are all radio-tagged – and there are clear criteria for success or failure.

If the project does not meet the criteria, it will be ended, and the beavers taken out of the wild, or if that cannot be done, they will be sterilised so that they will eventually die out.

If it is regarded as a success, the next stage will probably be a trial release onto a major river. The Knapdale team insists beavers and salmon have evolved together and co-existed for millions of years and that Norway, for example, hosts both a superb salmon fishery and thriving beaver numbers.

"It's clear that beavers have very little effect on migratory fish and Atlantic salmon populations," the trial project manager, Simon Jones said.

"But if you want to get an answer to the question, and if the Scottish people and Government decide we should, there's only one way to find out, and that's to try it."

The beavers are in two other lochs besides Loch Linne, Loch Coille-Bharr and Loch Creag Mhor, and the best chance of seeing them is in the evening from the track running around Loch Coille-Bharr from the beaver trial visitor centre at Barnluasgan on the B8025, south of Crinan.