For sale: the letter that sent an explorer in search of Livingstone

Quentin Keynes, the great-grandson of Charles Darwin, could be said to have the spirit of exploration in the blood.

So when he heard that a handwritten letter from the explorer and missionary David Livingstone wasfor sale, he scraped together the funds to pay for it. Then he set off to recreate Livingstone's voyage down the Zambezi and visit the place where the letter had been left - in a bottle.

Nearly 50 years later, the story of his boy's own adventure has come to light as the "letter in a bottle" is put on sale again.

Livingstone's plea for provisions will be auctioned at Christie's in London on 7 and 8 April, along with the extraordinary £3m collection of travel and natural history books Keynes acquired before he died last year, aged 82.

And lot 431, Livingstone's "letter in a bottle", is not just a rare memento of the adventures of one of Africa's most famous missionaries. It is a footnote in the story of Keynes.

While working as a wildlife photographer and film maker, Keynes accumulated books, including a copy of a work about African missionaries signed by Livingstone and Henry Stanley, the man who went to find Livingstone when he was feared missing. And he always kept his Livingstone letter.

It was written in May 1859 when Livingstone, as consul of the British colonies in East Africa, was leading an exploration of the Zambezi. He had discovered Lake Shirwa and was optimistic that it offered the prospects for a prosperous British colony. His plea for "salt provisions" came after his second trip up the river. "Several members of the expedition have suffered from fever, but not in its severest form," he wrote. The letter echoes his diary and Margaret Ford, a director of Christie's book department, said that was one of the reasons it was important. "It is a living document of things we know about from the journals," she said.

It appears to have been found by a member of the crew of a Royal Navy vessel and handed to the ship's surgeon, whose family kept it until the auction in 1957.

In a book of essays on Livingstone published in 1973, Keynes described his excitement at discovering that the document was to be put up for sale. "Romantic that I am, I had always longed to own an original manuscript written by Livingstone during one of his expeditions in Africa," he wrote. "From my reading of boys' adventure books, I had always understood that explorers were in the habit of leaving their letters in bottles."

Having beaten off "stiff and expensive competition", the letter was his. And the next step was to visit the Zambesi delta, where Livingstone had buried it 100 years earlier.

Accompanied by two friends, Tarquin Olivier, the son of Sir Laurence, and David Coughlin, an American student, Keynes set off. They were disappointed to find no sign of the point where the bottle-letter was buried; but they made a discovery of a different kind.

A local man whose grandfather had met Livingstone pointed them to a wide baobab tree, where the intertwined initials D and L were carved in the same handwriting as the letter. "It appears to be the last mark left on the trees of Africa which can definitely be attributed to Livingstone," Keynes said. The government of Mozambique subsequently declared the baobab tree an historic monument.