Dramatic discoveries of ancient bead jewellery by archaeologists working in a cave on the southern Cape coast have helped confirm Africa's pre-eminent place in the history of human evolution.

Some 41 ancient shell beads, recovered from Middle Stone Age layers in Blombos Cave near Stilbaai by archaeologist Christopher Henshilwood and colleagues over the past few years, have been accurately dated to 75 000 years ago.

This makes them the oldest dated jewellery yet found on the planet.

The previously oldest known human ornaments are perforated teeth and eggshell beads from Bulgaria and Turkey, aged 41 000 to 43 000 years ago, and 40 000-year-old ostrich-shell beads from Kenya.

And the "new" beads' presence is incontrovertible evidence that humans displaying fully modern behaviour - such as abstract thought, aesthetic appreciation and the ability to communicate through mutually understood symbols - evolved in Africa at least 30 000 years earlier than anywhere else in the world.

A report of this major find appears in today's issue of the prestigious international journal, Science.

Two years ago, Henshilwood and fellow researchers - some of them also involved in this newly reported find - made an equally dramatic discovery at Blombos Cave, about three hours' drive from Cape Town, when they uncovered two small pieces of ochre exquisitely engraved with an abstract pattern.

Also dated at 75 000 years - within the Middle Stone Age (MSA) period - the ochre was the earliest firm evidence of abstract thought and hence of modern human behaviour.

Until this discovery, the dominant theory of behavioural evolution was that while humans were anatomically modern at least 160 000 years ago, they didn't develop critical modern behaviours until about 40 000 or 50 000 years ago, and then not in Africa.

But Henshilwood and his colleagues - who include Francesco d'Errico and Marian Vanhaeren of the University of Bordeaux in France and Karen van Niekerk of the University of Bergen in Norway - argued that the ochre and sophisticated bone tools of the same era found in Blombos showed that modern behaviour, such as the use of external symbols, developed gradually throughout the MSA and not suddenly when our early human ancestors spread from Africa to Eurasia.

The bead find strongly supports this thesis.

The early Blombos "jewellers" manufactured the perforated beads by boring holes in the tiny shells of a small gastropod (snail) found in estuaries about 20km from the cave.

The common form of the holes and the similar patterns of wear clearly define the shells as beads, the researchers argue in their article.

The beads were found in seven separate clusters over a period of several years, and each cluster could represent a different jewellery item, they believe.

Henshilwood, one of South Africa's most prominent archaeologists who was recently knighted by the French government for his services to science, explained that the shells are called tick shells because they are approximately the same size and shape as ticks.

"They really are small, less than one centimetre across, and they're all whitish to ivory in colour, quite lustrous and very attractive, despite their small size."

Shells of the same species, Nassarius kraussianus, are still found in the estuary of the Goukou River at Stilbaai, he added.

All the beads, which appear to have been selected for size, had clearly been brought into Blombos cave because no non-bored shells of this species had been found there in the MSA layers.

Traces of red ochre indicated that either the shell beads themselves or the surfaces against which they had been worn were coated with this widely used iron oxide pigment.

"And each cluster of beads looks slightly different, so they are definitely not from just one necklace," Henshilwood said.

The first of the shells had been found as early as 1997, although he had only examined them seriously three or four years ago.

"I wasn't absolutely convinced at first that they were in fact beads, simply because I didn't have the right equipment to look at them closely," he said. "But during the past year or so we've been able to examine them through the latest scanning electron microscope at the CSIR in Pretoria, courtesy of Professor Coetzee, and that's given us some marvellous pictures.

"The 'wear' facets on the beads show up pretty clearly, and also the wear around the keyhole fractures."

So it's clear that the shells were used as beads that had been strung together, the researchers say in their report.

But they are not sure whether the beads would have been strung for use as a necklace - they may have been used in bracelets or possibly even as decorations on clothes.

"We simply don't know," Henshilwood said.

But what was clear was that the beads suggested a common practice of using jewellery, and that was hugely significant, he added.

Some archaeologists have argued that the Blombos ochre find is not conclusive proof of abstract thought and behaviour, he explained.

"There was a dispute as to whether it was art and whether the intention to create the abstract pattern was deliberate - there was the suggestion that it could just have been 'idle doodling', although I think that is a separate argument.

"But no researcher will dispute that beads are symbolic, and this find (of the beads) shows that the people who lived in Blombos Cave had the capacity to symbolise - their behaviour was mediated by symbolism.

"And that conferred an enormous advantage on these people 75 000 years ago."

Together, the beads and the ochre artwork suggest the existence of a language capable of sharing and transmitting the symbolic meanings of these objects, the authors say in their article.

"Being able to store something outside of the brain is the basis of everything in a modern, material culture - it means you can put out a message and someone else you personally haven't even spoken to previously can understand it," Henshilwood added.

The discovery of the beads reinforces their argument that the ochre engraving and bone tools are proof of modern human behaviour at Blombos Cave, meaning such behaviour evolved in southern Africa well over 70 000 years ago.

"This is real, 'home-grown' evidence that people here were absolutely modern in their behaviour, and that was at least 30 000 years before anywhere else in the world," Henshilwood said.

This was the type of evidence which could help Africa shed its "step-daughter" image of being backward and incapable, he suggested.

"I think that's pretty important for us."

The beads will be on display at the Decade of Democracy exhibition at the Castle from next Wednesday.