By Ishbel Matheson
BBC correspondent in Nairobi
International experts in early human art are calling for greater protection to save many of these ancient paintings from destruction.
They say rock paintings by early man in Africa are particularly at risk, as human populations grow, and expose sites to vandalism and graffiti.
The rock art survived the harsh climate but is now under threat from people
Near Nairobi this week, experts saw for themselves the problem facing rock art.
On a rocky outcrop, an hour's drive out of the Kenyan capital, Dr Steven Ambrose drew people's attention to some faded white-and-red lines, just visible on a stone-overhang.
Some may be centuries-old, others perhaps dating back thousands of years.
But as Dr Ambrose moves along the rock-face, the style changes, the paintings become more distinct.
"These are... probably made by the Maasai during the course of meat feasting," he says.
"Now we move to the right here, we see a giraffe and a human figure, both in white fine lines."
Some of the art was done by the Maasai
These art forms are not typical of the area.
Plus, they were first recorded in 1995.
Dr Ambrose says it's likely an international film crew working in the area, felt they could improve on the originals, and painted over the ancient lines and whirls.
All over Africa, ancient rock art is under threat.
Exposed on cliff-faces, daubed in caves, or etched onto boulders.
These early works of mankind have survived the continent's harsh weather, but not, increasingly, Africa's burgeoning population.
Martha Mulwa owns the land on which Lukenya paintings are found.
The number of day-trippers to the rocks has grown - and so too has the amount of graffiti scribbled on the boulders. Mrs Mulwa says this is ignorance, born out of poverty.
"People love them, but the struggle for life is tough. That is, I think, the main problem."
At the National Museum in Nairobi, an exhibition stresses the cultural value of these ancient art forms.
One photograph shows floating dream-like figures painted inside a cave in Algeria.
Another shows a mythical creature, like a dinosaur, on a cliff-face in Mauritania.
David Coulson is chairman for Trust for African Rock Art. He says, for the unscrupulous, these paintings are also proving financially valuable.
"Theft is becoming a major problem.
"The worst example we've seen has been in Morocco, where literally hundreds of pieces of art, that might be as much as much as 5,000-6,000 years old, have been hacked out and drilled out of the rock by locals and then sold to middle-men or dealers from Europe."
Experts say this loss is incalculable. Relatively little is known about Africa's rock art.
But some fragments suggest mankind's ancestors on the continent may have been painting as much as 70,000 years ago.
That would make art in Africa older than anywhere else in the world.
"Any destruction of an archaeological site is like ripping off a page from a book," says Mulu Muia, a Kenyan archaeologist.
"Once you destroy it, it disappears forever and you can imagine how frustrating it is to read a book that does not have some chapters or some pages."