By Jonathan Amos
Science reporter, BBC News, Norwich
Evidence is emerging from Africa that colours were being used in a symbolic way perhaps 200,000 years ago, a UK scientist working in the region claims.
This ochre has a groove where the powder has been rubbed out
Lawrence Barham has been studying tools and other artefacts left by ancient humans at a site in Zambia.
He says the range of mineral pigments, or ochres, found there hints at the use of paint, perhaps to mark the body.
If correct, it would push back the earliest known example of abstract thinking by at least 100,000 years.
Being able to conceptualise - the ability to let one thing represent another - was a giant leap in human evolution.
It was the mental activity that would eventually permit the development of sophisticated language and maths.
Shells from Israel that were strung as beads into a necklace or bracelet are widely accepted to be the oldest unequivocal evidence for such behaviour in humans.
But Dr Barham said it would be hard to accept that humans were not engaged in such activity much earlier in the archaeological record.
"As an archaeologist I am interested to find out where colour symbolism first appears because for colour symbolism to work it must be attached to language," the Liverpool University researcher said.
"Colour symbolism is an abstraction and we cannot work this abstraction without language; so this is a proxy for trying to find in the archaeological record real echoes for the emergence of language," Dr Barham told the British Association's Science Festival.
Dr Barham's work over the past 10 years has majored on a site known as Twin Rivers, an old cave complex in the south of Zambia.
It was occupied by humans some 170,000-300,000 years ago. Which type of human is not clear, however; there is a fragment of bone which could belong to Homo heidelbergensis, the ancestor of modern humans, which like us had a large brain.
Dr Barham said the tools found at Twin Rivers showed evidence of increasing sophistication, with simple handaxes giving way to small blades and flakes that had to be attached to handles.
The emergence of this "composite" technology coincided with the systematic use of ochres. Ochre is a soft stone that contains iron oxides; it comes in a range of colours.
At Twin Rivers there are red, yellow, brown, pink, black and even purple ochres.
If they are scraped, they will produce a powder which can be mixed with animal fat, for example, and used as a paint.
Dr Barham wonders if ancient humans in Zambia wiped these pigments on their bodies, using them in rituals - just as paints are still used in some cultures today, to mark the passage of warrior to elder, or the coming of age of boys and girls.
The problem is that there are purely functional uses for this material as well, to preserve hides and as a glue to bind stone blades to their shafts.
"If you were to argue that these oxides were purely functional and have no symbolic value, you have to explain away the range of colours that are being selected from different places in the landscape," said Lawrence Barham.
"Because if it was just for the iron element, any of them would do - the red, or the yellow. Some are closer to the site than others, so it seems that people were deliberately selecting the material for the colour property. That's my argument anyway."
A number of claims have previously been made for conceptual thinking in humans hundreds of thousands of years ago. Many of these claims relate to pieces of rock that are said to represent the human form.
The Zambian ochres could have been used to paint the skin
The Berekhat Ram figure from Israel and the Tan-Tan figure from Morocco, for example, have been presented as the work of Homo erectus.
But many sceptical researchers believe these items are merely accidents of nature; they are objects that have been moulded into human form through chance geological processes.
And Dr Barham knows he has some way to go to convince colleagues of his case.
"Archaeologists are a very cautious group. We set high standards for accepting an interpretation based on symbolism," he told BBC News.