The Journey of Anders Sparrman, By Per Wästberg, trans. Tom Geddes

It's hard to know which man has had the more remarkable life. Per Wästberg had his first national newspaper column at 12. He fought apartheid in South Africa, edited Sweden's largest daily newspaper, founded the Swedish section of Amnesty International, and is now chairman of the Nobel Committee for Literature. And he has written more than 50 books – novels, political works, essays and poems.

Anders Sparrman went to the university of Uppsala at the age of nine. The youngest disciple of Swedish botanist Carl Linnaeus, he sailed to China as a ship's doctor at 17. After a spell as a tutor in South Africa, he joined Captain Cook on his expeditions to Antarctica and Tahiti. He wrote books, collected more than 1,300 botanical specimens, curated the Swedish Cabinet of Natural History and campaigned against slavery.

You don't have to ask the Mrs Merton question to see what attracted Wästberg, the questing internationalist humanitarian Swede, to Sparrman. Wästberg first encountered him in 1977 when researching a novel on Cameroon. Bertil Knutson, the son of a Swedish adventurer, showed him a dead man's chest. It was Sparrman's. In it were notebooks, journals, and maps. When Bertil died, Wästberg inherited it. He felt it was "signalling an appeal or squeaking out a challenge... Break Sparrman's silence".

As challenges go, it's quite a big one. Sparrman's life is "full of lacunae". Fiction seemed the only way of filling in the gaps. The Journey of Anders Sparrman is subtitled "a biographical novel". A note explains the sources, followed by notes on "Family Background" and "Childhood Home". Then finally we're there: in Sparrman's childhood, and Wästberg's imagination, in rural Sweden in the 1750s, where grass smells of mint and wormwood, the pine trees of resin and turpentine and white dead-nettles are "flattened to the ground under the weight of the rain".

Wästberg evokes the landscape, and the sights, smells and texture of daily life, both in Sweden and on Sparrman's travels, with lyrical precision. It's a breathtaking endeavour - an attempt not just to capture a life that history reveals only in glimpses, but a whole universe: the world of Lutheranism that Sparrman, radically, challenged; of 18th-century botany, medicine and science; of naval exploration and its mass deaths; of colonisers and colonised, and, devastatingly, the world of slavery. As a historical chronicle, the novel can't be faulted.

As a human chronicle, however, and simply as a story, it falters. Sparrman emerges as a solitary figure, but his imagined relationships are largely unconvincing – perhaps because the characters speak in such clunking dialogue. A prostitute declares that she "must grow accustomed" to "demeaning" herself. Even Lotta, a semi-literate servant girl with whom Sparrman finds love, tells him that "only time can separate us. That, too, is inviolate."

There's also a problem with voice. Not just Sparrman's musings, which, like his fellow-characters', tend towards the abstract, but the uneasy shift between imagined first-person and a third-person which sounds much more like a biographer. The quotations from Sparrman's journals, however, and his letters to fellow-explorer George Forster, are delightful. This, clearly, was a man full of warmth, curiosity and compassion, a man who eschewed pomp. His story is truly fascinating. You can see how it would have haunted Wästberg. You can see how he would want to do it justice. You can see how hard that would be.

He goes a long way towards it, and a long way, in the last quarter of the novel, which is devoted to Sparrman's relationship to Lotta, to getting into his head and heart. "Because love," Wästberg concludes, "is the real journey." And this novel – magnificent, but flawed – is also a journey of love.