Mr Coulson, a Nairobi-based photographer, and his colleague Alec Campbell, former director of Botswana's National Museum, are attempting to record Africa's vast treasure of ancient and often breathtaking rock art, which experts claim could be largely wiped out in less than 50 years without protection.
Their task is awesome, and, as preservation projects go, definitely a first. From Libya to South Africa the pair have begun recording, photographing and campaigning to save the works of people who lived up to tens of thousands of years ago; but they know that the elements, vandals and guerrilla soldiers have every chance of getting there first.
"Rock art is a window on to vanished worlds," says Mr Coulson, the son of a British diplomat who moved to Africa in the Seventies and worked with the late Sir Laurens van der Post on his book on the Kalahari Bushmen. "They document how people danced, loved, lived and thought."
To others, however, the fading paintings of dreaming shamans, bow-and- arrow huntsmen, extinct animals and early European settlers are simply target practice, as Mr Coulson found during a recent field trip to an important rock-art site. There, more recent history had impinged on the ancient past: the paintings had been sprayed with AK-47 bullets.
"The same shelters that were home to early man also provide cover for guerrilla groups and soldiers," says a resigned Mr Coulson, who must still sidestep wars to do his work. He and Mr Campbell were recently delighted to be allowed into Chad, languishing after 30 years of war, but a visit to Algeria's Atlas mountains has had to be abandoned because of the bloody violence there in which hundreds of villagers have been massacred. Their next destination is Libya, if Colonel Gaddafi will let them in.
But tourists can be just as destrutive as the men with machine-guns. Mr Coulson says you need patience to photograph rock art, and can spend all day waiting for the right light to capture the past. But snap-happy tourists, with other sights to see, require speedier results.
"People throw Coke and water on the paintings, or even urinate on them to make the colours stand out," he said. Every time paintings are abused in this way, the art, and the past, fades a little more.
Africa is believed to have the largest and most varied heritage of rock paintings of any continent. There are tens of thousands, usually the only record left by forgotten peoples of the remote past. The oldest - painted 26,000 years ago - can be found in the famous Apollo 11 cave in Namibia (discovered in 1969 at the time of the space mission). They span the centuries right up to Bushmen drawings just 100 years old.
The paintings have their secrets and their puzzles. No one really knows why giant grazing eland dominate South African rock art, when the giraffe is king in the rest of Africa. But while Europe fiercely protects its rock-art heritage - and the secrets it holds - Africa's is deteriorating at an alarming rate.
On a continent racked by poverty and war, protection of rock paintings is understandably a low priority. The political will to preserve the past is lacking, and not simply because of the weight of other, more pressing problems.
Africans are also ambivalent about their ancient past. "There is a tendency among Africans to label rural art as primitive and to dismiss it," says Mr Coulson. "They have picked up the old colonial values."
Without friends in high political places, Mr Coulson and Mr Campbell are relying on the generosity of charities such as the Getty Conservation Institute and businesses such as Anglo American and De Beers, apart from individual donations. Last summer Mr Coulson founded the Trust for African Rock Art (Tara) in Nairobi, spurred on, he says, by Mary Leakey, the renowned palaeontologist who died in December aged 83.
Rock art was one of Dr Leakey's great passions and concerns. She and Mr Coulson planned to work together on a book, but she became too frail to carry on with it. Although Dr Leakey had no money to speak of, she lent the famous Leakey name to the trust. She and the late Sir Laurens van der Post - who also died in December - were founding Tara patrons.
The trust's long-term aim is to persuade governments at least to protect the best examples of rock art. The book Mr Coulson plans will help to publicise his cause, but policing and protecting the heritage of a poverty- ridden continent is not easy.
Namibia's famous White Lady of Brandberg - a curious title given by a European archaeologist to what is in fact a representation of a black man - is now surrounded by metal bars, but that does not stop the tourists wetting the painting to brighten the colours. Paid guides and guards would help, but Africa does not have the money.
Mr Coulson asks other continents to take an interest, arguing that the heritage belongs not just to Africa but the world. Once it is lost, he warns, "there will be no rewinding of the clock".Reuse content